Available for the Kindle and also in the traditional paper back. It will make a great gift for the runners you know, and helps support our work here on The Science of Sport.
Exercise and weight loss Part 3: Fat
Fueled by fat: Fat burning 101
There are many reasons to want to burn fat during exercise. For obvious reasons, in the context of the series I'm currently doing, people want to burn fat during exercise to lose weight. In that regard, one must emphasize that as much as we talk about weight loss, fat burning (or rather, a change in body composition) is the priority for most people who commit to exercise and diet to lose weight.
I must emphasize (and this is a late addition to the post) that the principles we've been speaking about in our previous three posts in this series do not suddenly cease to exist - in other words, the fundamental issue is still total calorie balance, not necessarily fat burned. And so this post looks at fat use during exercise, but I don't want to overplay the concept that you can burn fat to accelerate fat loss. In fact, in the long term, it's the creation of the calorie deficit that is needed, as we discussed in Part 2 A and B of this series. If in doubt, read the whole series as one "book" and it should be clearer...
For those who are more interested in performance, fat burning is equally important. Ever since the 1970s, when the muscle biopsy technique first allowed scientists to "look inside" our muscles during exercise, we have understood that fatigue and performance are associated with a depletion in the body's glycogen stores. Delaying this depletion of your body's carbohydrate stores is seen as an important adaptation to training, and we know that highly trained endurance athletes have a much greater capacity to use fat. This is also the reason why science has looked at all kinds of strategies, like caffeine use, fat-loading, high fat diets and fat during exercise to try to delay carbohydrate depletion.
So, having made the case for fat burning, let's look at how it plays out.
A car with multiple fuel tanks: Fat vs carbs
You'll know that our body is capable of using numerous sources of energy during exercise - think of your body as a car with multiple fuel tanks. The first tank is glycogen, the storage form of carbs in the muscle and liver. The second is fat (strictly speaking, it is free fatty acids), which is stored in the muscle and in adipose tissue as triglyceride. This is summarized in the diagram below.
The problem we face is that our body's glycogen stores are finite - muscle and liver glycogen supply energy for exercise for about two hours (depending, of course, on the intensity, level of training and habitual diet, among other factors), and once they are depleted, hypoglycemia (from liver glycogen depletion) and fatigue are the result.
On the other hand, much to most people's dismay, fat stores are not limited. "Infinite" is not the correct word, because they're not, but in terms of physiology, you have more than enough fat to power exercise for much longer than you can ever exercise, even if you're an elite athlete. Most of us are not, and so our objective is to cut down these already abundant fat stores into something a little more "acceptable"!
For now, I won't go into the concept of "preferred fuels" and how the heart can use lactate, while the brain is able to use ketone bodies. I've also oversimplified by leaving protein out, but that's because I want to focus on fat vs carbs today.
Burning fat - how fast, how long?
Perhaps the best approach to this topic is to ask when each of the fuel stores is used? In otherwords, how does your body decide when to use fats versus carbs as a source of fuel, and to 'fuel' the discussion by starting with some of the theories around fat burning.
The first theory is that you burn more fat when you exercise at a low intensity. That is, walking is better than running, and cycling slowly is better than cycling fast, and so on. The idea is that your body uses carbs more and more as exercise intensity rises - this is the basis for target heart zones and the fat burning zone you see in books and on gym equipment.
Let's look at that in more detail. Below is a diagram that shows an example of how the use of fat and carbs are related as exercise intensity rises.
So, on the left, in the pink shaded box, you can see that at low intensities (walking, slow cycling), about 80% of your energy comes from fat, while only 20% comes from that carbohydrate (CHO) "tank". As the exercise intensity rises, the relative contribution of fat falls while CHO rises. The result is that at about 60%, shown by the green zone, the contribution is equal - 50% from fat, 50% from CHO. This point, at which the contribution from CHO becomes greater than that from fat, was called the Cross-over point by George Brooks, a very famous exercise physiologist, who wrote a great textbook on the field (the book that was prescribed to us in my studies, in fact).
Then, as exercise intensity rises even more, the contribution from fat declines, so that by the time you're exercising vigorously, between 80 and 90% of the energy comes from CHO, and only about 15% from fat.
Just to comment on how you can measure intensity, my value is calculated as a percentage of VO2max, which is not accessible to many people. It's impossible to say exactly what heart rate that corresponds to, because it is very much dependent on the person and the context of exercise. For some, it's a one-to-one relationship, so that 50% VO2max is 50% HRmax, whereas for others, heart rate will be higher than VO2max - 50% of VO2max would correspond to say 60% of heart rate max. I wouldn't get too hung up about this, for reasons explained below. I know it sounds simple, but if you're wondering what 60% corresponds to, it's 6 out of 10 on a scale of exertion, where 10 is maximum effort.
Note also that the cross-over point, and in fact the whole pattern of the two lines is quite 'malleable' in that training, diet and genetic differences affect the relative percentage. For example, a highly trained athlete burns fat more efficiently than an untrained person, and so they tend to use more fat at a given intensity. People who habitually eat high fat diets are also shifted to the right, meaning that they burn more fat at a given intensity. Sympathetic nervous system activity does the reverse, shifting the lines to the left, so that you burn more CHO and less fat at any intensity. So the point is, the above graph is an example, not an absolute guide.
Low intensity is better, right?
So, what you're probably thinking is that theory that low intensity exercise is better if you want to burn fat is correct. Well, think again. It is true that at low intensity, when you walk, most of your energy comes from fat, and that as you increase the intensity, less and less comes from fat.
But what is missing in this picture is the TOTAL amount of energy. Let me phrase it this way: Would you rather have 20% of the money in Jonathan's bank account, or 80% of the money in my bank account?
Your answer of course, should be that you don't know. And what you should be asking is "How much money is in your bank accounts?". The reason is, you'd be pretty annoyed if you took 80% of my money, because my bank account might be empty. Jonathan, on the other hand, might be loaded, and 20% of his money sees you retiring at 40! (Truth is, neither of us retiring early, which is why your donations are welcome! Kidding...)
So, look at the graph again. You should not be too pre-occupied with the percentage, but rather the total amount of fat that you are burning, because that 50% that comes from fat in the green zone might be more than the 20% that comes from fat in the pink zone. For answer to that, we look at a study done many years ago by Romijn.
Below is a graph that I've redrawn with some calculated figures, based on Romijn's study.
So again, at low intensities (25% VO2max), you'll see again that most of the energy comes from fat (75% in this case), with only 25% from CHO. Jump to 60% and the fat contribution falls to 48% and then at 85%, fat provides only 20% of the total energy. This confirms what we saw in the first graph.
But, the key is that the TOTAL energy, shown here on the y-axis, rises as well, and so the 48% of energy from fat that you get at moderate intensity actually adds up to more TOTAL fat use than the 25% did at low intensity. Think of the y-axis (total energy) as the size of the bank account, if you will. The green boxes above each bar show the total amount of fat burned in an hour - 24g, 37g and 23g per hour at the three intensities.
The moral of the story, which is shown further in the graph below, is that if your objective is to maximize fat burning, low intensity is not necessarily the best option. Rather, moderate intensity burns more fat per hour, before a decline in fat use as the intensity rises beyond about 60%. The reasons for these shifts, incidentally, include increased sympathetic response to exercise, activation of different muscles as intensity increases, local regulation of metabolism in the muscle, and also the exponential nature of physiological responses to an increase in intensity. That's a post of its own!
Remember also that these are graphs based on calculations and measurements for a typical person - the actual values may vary for different people depending on training, diet and sympathetic activity, as mentioned. One also has to make some assumptions, such as VO2max values, the RER during exercise and how it changes with intensity. But I want to highlight the principle, not the values, so bear with those assumptions.
What does this all mean? The role of time and putting it into perspective with diet
So, what this means is that if your goal is maximum fat use, then lower intensities may mean a greater relative contribution, but it is moderate intensity that gives you greater fat use per unit time.
Time is key though, as many of you are no doubt thinking. If you do a low intensity session, walking (25% VO2max in the above graphs), then while your total fat use per hour is not as high as at say 60%, you might be able to do this exercise for three hours, compared to only one hour at a moderate intensity. Three hours with 24 g per hour (see figure) beats one hour with 37g. So duration plays an equally important role. What I am emphasizing is that for a given time, you will be better off at a moderate intensity somewhere between 50% and 65%.
The next point, before we get too carried away, is that the actual differences are pretty small. Not to discourage you too much, because exercise is vital for weight management and health, but one rule of thumb is that you can estimate total energy use during running by saying that you burn 1 kcal per kilometer per kilogram.
So, taking our 80kg man, who runs 8km (5 miles), he'll burn about 640 kcal. It doesn't matter how fast he runs, thid amount is the same (the difference, of course, will be how much time he spends burning it). Now, assume that half his energy comes from fat and half from carbohydrates. That means 320 kCal from fat, corresponding to 35 g (this is about the same as in the graph, since I based that graph on this kind of calculation).
So, 35 g of fat in 8 km or 5 miles. To burn 1 kg of fat, he therefore has to run 230 km (142 miles). Not very encouraging. There is an additional 'after exercise' effect, which one can't ignore, but I want to make the point that actually burning fat is not as simple as we often assume. In the words of Bengt Kayser, who pointed out this principle, it puts into perspective the resolution that says "I want to burn off 10kg of fat by running".
You can work out a similar thing for cycling. If you ride for an hour at 200W, your energy use is probably going to be about 800 kcal. If 50% of this comes from fat, then you've got 400kCal, or about 44g of fat. That is about the same rate of fat use as your 8km run gives you, and a lot of cycling to burn those 10kg of fat.
The value is in energy, not necessarily one fuel
And then perhaps most significantly, the key is still to create a calorie deficit, which means that you need not worry too much about whether your energy use is coming from fat or carbs - the key is to create that deficit, because in the long run, the energy will have to be provided and you will achieve similar results regardless. Again, I'm oversimplifying a little, because we have discussed how efficiency changes, and there are some issues as to how the nutrients are oxidized as they are ingested, but the principle is not necessarily to burn the most fat, but to create an energy deficit.
And this is where diet comes into the picture. If you want to create the energy deficit in order to lose weight, and notwithstanding that this is an oversimplification as we described in Part 2B of this series, then the combination of diet and exercise the way to do it, because to actually burn energy directly is a lot more difficult than people think. This is why the combination, and the long-term approach is so important. Things do not happen overnight in physiology, particularly when it comes to weight loss.
The progress can be slow, and invariably, when it is not, it's not sustainable. Crash dieting, starvation might lead to rapid results, but they also lead to subsequent rebounds, and health problems that we won't go into now.
But for the purposes of this post, which I'll wrap up because it's been rather heavy on numbers, the key point is that burning fat during exercise happens right across the range, and the ideal intensity, if you are looking to make the most of your time, is a moderate intensity, and not the low intensities that you'll often hear recommended.
Next time, we'll look a little more at energy use during exercise and the role of diet.
Ross
58 Comments:
Nice article. Can it be estimated what it means 50% intensity in terms of heart rate? My guess would be that if I take resting HR of 60, max at 200, then 0% intensity is at 60, 50% intensity at 130 and the "ideal" 60% at 144. Is it a good estimate? Thanks
Hi Mike
Good question. I've actually gone back and added a paragraph, because it is important, so thank you for raising it.
My paragraph was:
"My value is calculated as a percentage of VO2max, which is not accessible to many people. It's impossible to say exactly what heart rate that corresponds to, because it is very much dependent on the person and the context of exercise. For some, it's a one-to-one relationship, so that 50% VO2max is 50% HRmax, whereas for others, heart rate will be higher than VO2max - 50% of VO2max would correspond to say 60% of heart rate max. I wouldn't get too hung up about this, for reasons explained below. I know it sounds simple, but if you're wondering what 60% corresponds to, it's 6 out of 10 on a scale of exertion, where 10 is maximum effort.
I would not try to play the numbers game too much here - there is too much individual variation in heart rate. Some people would barely be walking at 144 bpm, for others it might be correct. Heart rate and exercise is another issue altogether...
Ross
A great topic that I am very interested in.
Something I have always wondered about though is: when you talk about burning fat, what does that mean?
Dietary Fat?
Fat stored in muscles?
Adipose Fat?
Hi Trev
All of the above.
If you look at the graph in this post of the bar charts showing total energy and different intensities, you'll see the breakdown - some fat comes from muscle triglycerides (stored in the muscle), and some comes from the adipose tissue (the FFA in the diagram).
So it's both. Dietary fat is less important, but remember that it is stored in the muscle and adipose tissue anyway, so I guess it's included in the above categories that way.
Regards
Ross
Thanks for this wonderful explanation! What is your opinion about the highly advertised weight loss strategy now of doing highly intense exercises for a short period of time, i.e., less than 30 mins, and doing that on a max of 3 times a week?Does that really increase your resting metabolic rate and make you burn fat even while you're resting? I'm trying that now and just making sure I'm on the right track.
Thanks!
HI Russell
Also a good question. 30 minutes of high intensity exercise would not, during the 30 minutes, burn a lot of fat, even if energy use is very high. If you refer to the graph in this post, then at about 80% of max (which is what most people could sustain for 30 min continuously), you'd burn about 25 g/hour, or maybe 13 grams in the bout of exercise.
If you do the 30 minutes as an interval session, where you go harder, say up to 90%, then it's even less.
The difference, however, is that the post exercise metabolic rate will be greater and will stay elevated for longer after high intensity exercise. So it's misleading to look only at the 30 minute exercise time, and this is likely the basis for the promoted method.
I don't know the specific values though. I don't know how much the metabolic rate is raised or for how long, so I'll have to look this up and then give you the actual numbers.
In order for it to be better, the post exercise fuel use would have to make up for the fact that if you do higher intensity training, you burn less fat during the session. I doubt that it would - I just can't see metabolic rate being elevated that much more after 30 minutes of hard exercise compared to 60 minutes of moderate.
Even if your resting metabolic rate is raised by 500% for an hour after exercise, that contributes a further 30 g of fat use in that hour. If you did moderate intensity exercise, at it was raised by 300%, you'd get an extra 20 g, so the difference is pretty minimal. But as I said, I'll look into it.
Incidentally, a study has just come out looking at exercise and fat loss, and it found that the number of days a week was the key. It took four days a week to achieve significant fat loss, and three did NOT have the same effect. Then again, this was low intensity exercise. I'll post on this in the future!
Ross
I'll just raise this question now, since it has come up several times. Is there really clear evidence of significantly raised metabolic rates after exercise? There seemed to be some serious question about the methodology of the early studies that showed this. I thought that follow-up studies seemed to lean against any signifant effect. Perhaps I've missed out on some recent studies reinforcing the evidence.
This is just excellent! The last graph shows that about 60% is ideal. For me, that would mean about 112 BPM which is I would consider very slow but doable. Currently, I'm trying to run for 2 hours per workout at about 65% MHR so I'm close (using HR as a very rough estimate of VO2 max). Years ago, I read a study which purported to show that 2 hours/run was the ideal length, beyond which endurance benefits dropped off markedly. The most beneficial part of the 2 hour run was the last 15 mins. I cant remember where I read it. I understand that your article was about fat burning and not about HR and exercise but I think "whats good for the goose may be good for the gander" i.e. one can lose fat and train for endurance with the same model: long, slow distance.
The other issue is about the storage of fat in muscle and adipose tissue as triglyceride. Are distance runners more likely to have higher triglyceride levels than normal as a result of storing higher levels of fat in adipose tissue and muscle? My levels are usually higher than normal. Thanks for the great article.
Suppose two people exercise the same amount (say, 1000 kJ) every day, one super-high intensity (100% CHO burning) the other low intensity (0% fat burning).
#2 burns all fat. If he supports base-line metabolism, all other things constant, he loses 1000 kJ of fat per day.
#1 burns no fat. But then he sucks 1000 kJ of CHO directly out of his diet to replace lost glycogen. He thus has less CHO left to produce body fat. He also then loses 1000 kJ worth of fat per day.
Sure, there's differences in metabolic efficiencies which I'm ignoring, but in the end of the day, assuming the same amount of exercise and the same amount of eating, who cares if it's "fat-burning" or not? Accounts need to balance in the end.
Sure, fat-burning may yield better endurance than CHO-burning (which leads to bonking). But I'm assuming exercise amount is fixed.
That's exactly what I was going to ask, djconnel... To give a speculative answer to my own question, I'm guessing it is hard to get weight loss in the real world solely from burning carbs, because being short on carbs/glycogen probably makes our athlete #1 ravenously hungry, whereas #2 doesn't have this problem because he took the energy directly out of fat store. I look forward to an explanation from an expert, though!
Great post guys. I would love to see you put together an article or link to one previously published on nutrition strategies for endurance athletes. I have read a ton on the strategies for the ratio of carbs/protein during let's say Iron distance events, but I would love for the two of you to break it down in your easy to understand manner.
Hey guys,
When you're working in the zone that mixes a combination of fat and carbs for fuel, how does this effect post-training refueling ?
thanks
Thanks for the great post!
I've got a question about refueling though. I've heard that when you eat after exercise those calories replace whatever calories that you've just burned (be it carbohydrates or fat). If so, then it doesn't matter if you burn fat or carbs; it only matters the net amount burned. Any thoughts on this?
I have the same question as djconnel.
If two different excercise intensities result in the same overall energy deficit at the end of the day, does it result in the same fat loss even though the lower intensity excerise burned more fat up-front?
Hi DJ, AT, Marie and Jimmy
That's a very provocative question, one which I'm going to look into a little more!
Of course, it is hypothetical, because burning ALL the energy from fat is very unlikely, especially during exercise. Even as you sit reading this, you'll be making use of carbs, though in smaller amounts. If you are eating a muffin, then it's higher because you use what is available! You can burn all the energy from carbs, but after exercise, there is still increased use of fat, so even that is a stretch.
However, let's take the example:
You're correct that balancing is required, but the complexities underlying that balance make it impossible to predict how this 1000kcal amount would play out, whether it is in excess (in the absence of exercise) or a deficiency created by exercise.
If you assume that in the absence of exercise, the person would have stored all 1000 kCal worth of energy as fat, then creating a deficit by exercising would cause fat loss, yes. As I understand your example, this person used exercise to bring energy balance about, otherwise there would have been no fat production.
However, if those 1000kCal that would ordinarily be in excess are not all stored, then suddenly only part of this deficit is 'saved', so to speak.
And the use of ingested nutrients is quite complex - if you eat more, you burn more, particularly with carbs, because of that thermic effect of feeding that I mentioned in the very first post. So some of the ingested 1000kCal excess would normally be oxidized for energy, some would be excreted and some would be metabolised for other body functions like repair and regeneration (since the body knows that energy is available).
As a result, of that 1000kCal that would normally have been stored as fat, only 700kcal actually was. The other 300kcal was used as a result of ingesting it. So you don't quite get the full amount "sucked out" of storage. Some, yes, but not all. What it comes down to is a kind of reverse of the energy-conserving metabolic response to a decrease in energy intake.
I don't know a figure though, I'll try to find out and will post more on this, it's a good question.
Ross
Ross: I'm a bit concerned in the same way I believe djconnel and jimmy are. By discussing this topic--fat burning vs carb burning--in a weight-loss context, you sort of imply that it makes a difference. But does it? Won't someone lose essentially as much weight, and fat, with an XYZ calorie deficit accumulated through carb burning as the same XYZ deficit accumulated through fat burning? Usually the analysis you've presented here is placed in the context of, "Don't place any importance on those 'walking burns more fat calories' arguments, because they don't have any real-world relevance."
I think one element worth noting is that the use of FFA is a wholly aerobic activity, and the way to raise the total amount and proportion of FFA used when exercising at a given intensity is by improving one's aerobic capabilities.
Doing this means one should also include a reasonable amount of time spent at high(er) intensities, which ironically may well be primarily fuelled by CHO, since performing exercise at these higher intensities is an excellent way to stimulate the physiological adaptations which improve aerobic abilities that will support a greater use of FFA at any given intensity during the bulk of one's training.
Secondary thought:
In terms of the fuel substrate utilisation and weight loss - if exercise time is limited (which is common for most non-athletes), why limit oneself to an "optimal fat usage zone"? I mean total calorie balance still rules in these scenarios.
So one should really go as hard as they can *sustain*, for a given workout and also sustain over the medium-longer term (IOW - not jeapordise the quality of a workout today because yesterday was too hard, or not be able to keep it up over the course of weeks/months), provided that such exercise isn't also accompanied by an unecessary lift in calorie intake.
Hi Amby, Alex
Thanks for the comment. Your question is valid, and I must confess that I debated whether to dwell on the fat burning argument in a post, and in the end I decided I wanted to emphasize that the low intensity is better argument doesn't necessarily hold theoretical water, and I've ended up creating a debate about energy burning vs fat burning. So it seems I may have overplayed the theoretical in this post, without that context. Maybe my thinking was too isolated from the first three posts in the series, where I did discuss energy and not specific fuels, and assumed it had been covered already.
I do believe the issue is quite complex, and maybe this is further highlighting the complexity of energy in vs energy out, because they change dynamically. But you're right, the creation of an XYZ deficit from high intensity exercise would produce the same weight loss as an XYZ deficit from lower intensity exercise (assuming of course that duration is the same).
And to Alex, the potential downside is injury and overdoing it, of course. But in theory, yes, true.
Ross
My experience from many years of coaching and my own exercise adventures lead me to believe that one's dominant muscle type is a factor. There are overweight people who exercise at moderate intensities regularly and lose significant amounts of weight, while there are others who, doing the same, lose almost nothing. I am the latter. At 6'3" and 250 pounds, regular running and cycling did absolutely nothing for me in terms of weight loss. However, interval running and high intensity weightlifting, paradoxically, melts pounds off me. I have come to believe that for those with more of a fast-twitch body composition, higher intensity exercise leads to endocrine system changes that in turn lead to weight loss, whereas with slow-twitch individuals, aerobic activity is the key. Am I barking up the wrong tree?
it'd be sweet if you guys provided more pubmed sources for us nerds out there. particularly i was interested in the bold statements from this paragraph:
For example, there is evidence that if you delay eating after exercise for about an hour, you burn more fat than if you eat right away. This has to do with keeping insulin levels down, and insulin is a hormone that drives carbohydrate use, while "tuning down" fat use. The problem is, if you delay eating, you may compromise your recovery, which means you can't sustain high quality training day after day. You also can't cut carbohydrates out, and you certainly can't under eat - there is compelling evidence that the biggest risk factor for becoming sick during intensive training is an energy deficit.
for that 2nd bold statement, i think the main concern there is higher glycogen synthase activity immediately following exercise, but for those not doing double days, i've read that so long as you get the carbs over the next 24 hours, you'll be fine when your next exercise bout rolls around.
glad you brought in some of rosenbaum's work. i think it's important, albeit discouraging, to point out that EE per kg lean mass may be reduced for even a year after weight maintenance. then we may get into issues of how it is important to not pack on the lbs in the first place. i'd be interested on your take of a weight set point and how that relates to rosenbaum's research, which would suggest that it is incredibly difficult to 'reset' that point once it's raised via weight gain.
for the 40-40-30 60-30-10 business i can't help but think about modelling macronutrient percentages on those found in some paleolithic/hunter-gatherer/fringe populations, who tend to be quite healthy (and injury free during minimally shod running!) until exposed to western diets. i took a class on paleolithic nutrition at colorado state and think the evolutionary nutrition perspective is worthwhile (Paleo Diet for Athletes by Cordain and Friel might be worth mentioning).
finally, I'd be very interested if you guys could provide some basic physiology on what does happen for weight loss if the caloric deficit is substantially carbs. i've always understood the degree of gluconeogenesis to be relatively limited, particularly since you can't get fat-derived acetyl-CoA into glucose since PDH is irreversible. so if you're exercising mostly at 80% vo2 max (granted a recipe for injury), and we're saying the main recipe for weight loss is simple calorie deficit, can your body sufficiently replace the burned carbs if the diet isn't quite high in CHO?
thanks for what you do, love the site.
-mark
Hi Mark
Thanks for the comments and questions.
Re the references, I have sometimes done that - I hate to use this as an excuse, but time is often the limiting factor. To search on Pubmed, find the relevant article and the provide the link often takes 5 to 10 minutes, and I refer to three or four papers in one post, it would add considerable time to the posting. And honestly, I squeeze these posts in on the fly - often I write them in about 8 shifts, interrupted by meetings and other work, so that's a poor reason, but it's what happens because of the obligation to work. I wish I could write this site full-time and live off it, then I'd be able to compile articles full of references, with proper "rigor".
So what happens is that I usually refer to things I've read or discussed previously, with links only when I know the reference offhand. But I'll try refer more in future.
I'll try to find that one about fat burning and insulin levels in relation to diet.
Re the carbs, those studies, most of which looked at scandinavian cross country skiers, found that chronic low carbohydrate intake was a major factor causing immune suppression. So once off, yes, you're quite right, but I was referring to people who habitually don't replace the carbs that are required. I will also try to find those studies - i lectured on them a few years ago, don't know the references offhand.
And then to answer your questions, I am certainly going to try to cover that paleolithic diet issue and also the issue of body weight set point.
That's not my area of expertise though, it's too specialized, and I'm afraid I don't have the time to spend a week looking it up, so I'm hoping to do that section as a Q & A with an expert in the field.
So hopefully that happens soon!
Thanks again!
Ross
"And to Alex, the potential downside is injury and overdoing it, of course. But in theory, yes, true."
which is of course why I used the word "sustain" for the level of effort in exercise performed.
Exercise that has a high chance of leading to injury is not sustainable and hence is a factor in determining how hard one can go (or sustain, within a workout and in the short, medium and longer terms).
And how hard one can go of course can increase with sensible progression.
Anyway, it's one of those "deceptively simple" topics. Always good to get some sensible thoughts down on it 'cause there is so much rubbish written about it.
I think one issue with using just metabolic zoning (such as fat burning zone or carb burning zone) is that it does not take into account what the other physiological systems in your body is doing. But the answer isn't to look at just VO2 max either. For example, everyone keeps talking about VO2 max in the literature as the most important criteria in zoning. But it has been proven that VO2 max isn't the best measure of zoning. Someone who is unfit and exercising at 60% or 65% VO2 max could clearly be exercising at an intensity that is equivalent to their 1-hour pace, while someone who is well trained may be able to use STF fibres and keep going for hours.
In addition, you can burn a total, absolute amount of calories from fat as a fuel source by going slightly harder, but if you also are burning through massive amounts of glycogen and/or blood glucose, how does a training session like that benefit you overall if your event lasts longer than 2-3 hours (marathon, long course triathlon)? Obviously there is a time and place for these training sessions but if every long training day involves burning through all your sugar stores, you might want to revisit that training plan. Going slow has its benefits too.
There is no "best pace" for everyone. Depending on everyone's goal races and their individual reactions to their workouts, they will need to make adjustments, simply because everyone's body is different.
My personal experience is that if we're dealing explicitly with weight loss (take it from someone who went from 22% BF to 8% BF in 6 months), you should focus on weight loss specifically in the off season. When it's time to start training, be properly fueled and do the sessions right. I don't find that exercising does much to help body composition. The best way for me to improve body comp is to focus on a Paleo-style diet for 4-months in the off season (Nov, Dec, Jan, Feb).
I wonder if you are reading too much into the fat vs carb burning during exercise. If one exercises at any intensity for any duration & then remains in a calorie defecit diet, wouldn't fat be used for regular metabolism, & hence one would slowy lose excess fat.
Hi Farhad
Sure, I responded to that in the comments already - the above two posts.
I also added a paragraph or two to clarify the issue and the purpose of this particular post, which I hope doesn't make it seem that the previous three posts were irrelevant. If you read all four together, then it doesn't change the fundamental point. So you're right, but it's not a question of reading too much into anything.
Ross
Can you comment on this:
http://alancouzens.blogspot.com/2008/06/mummy-returns.html
Is it possible to improve fat burning with this method? Maybe you can cover this topic as a follow up in reference to paleo diet.
Cheers
George
For cyclists training with power, might you define "moderate intensity" as a percentage of functional threshold power (FTP)?
This is a great series, and I'm enjoying it very much, but there is something seems a bit odd to me.
In this post you seem to have quite convinvingly proved that to lose 10kg you'd need to run 2300km at optimal fat burning intensity - basically something that nobody would ever do. However, we all know heaps of people that have successfully lost that much weight with far les effort, and far less science than that.
So what is going on?
To me it seems that actual difference is being made the bit you don't talk about - the raised metabolism after exercise, and possibly the improvements to diet and lifestyle that consistent exercise often inspires in people. If your analysis is correct (and I can't fault it), then the amount of energy used during exercise is far too small to account for any significant weight lost.
If that is true, then doesn't it make this whole discussion a bit like the guy who loses his wallet on a dark street, but then looks for it under a street lamp because that's where the light is?
Amby, djconnel and jimmy have hit the nail right on the head. I still see professionals who should know better (not Ross and Jon), making points about 'targeting' fat burning as a way to lose weight.
It's a sliding scale with fat and glucose as fuel. One hour of exercise a day is only one out of 24. Burning glucose as fuel during exercise lowers blood, muscle and liver glucose/glycogen. After you stop, allowing for some refueling, fat burning will be enhanced while these storage depots top up. It's part of EPOC.
The system swaps glucose and fat burning as appropriate according to substrate status at any time over 24 hours and fat gets used, one way or another, with sufficient energy expenditure. You don't have to target fat burning to lose weight, only energy expenditure and energy intake.
Hi b
Thanks for the comment, fair question. I hope I can provide an answer.
First, I must point out that when I wrote 2300km, that is to burn FAT during exercise only. You must remember that the energy deficit you create from exercise is still the crucial variable, because even if some (or all) of that energy comes from carbohydrates, it still contributes to weight loss, as we've discussed in the comments section here. I fear I've confused the issue a little by not making it clear enough that this post was really intended as an illustration of how the body burns fat, not necessarily overall weight loss.
The take home point is that if you burn say 1000kCal, even if some of that energy comes from carbohydrates, it's still part of overall weight loss.
Perhaps even more important than this is diet. One rule of thumb used by dieticians I know is to say that 1kg of weight loss requires about 7000kCal energy deficit. So that is equal to 1000kCal per day (which is a lot of running - that works out to about 12 km a day for an 80km man, for example).
That deficit can be created by exercise alone (you'd have to run a whole lot to do it this way!), or by diet alone, or by a combination.
And the combination works most effectively, for a number of reasons, but to illustrate very simply: You can exercise for 45 minutes a day (not unreasonable amount) and burn 500 kcal, and you can cut out certain foods, or cut portion sizes by 20%, and cut out 500kCal. That adds up to 1000kCal, and the weight loss is the result.
So the answer to your question is that much more rapid weight loss is possible because:
a) People burn more energy than simply energy from fat, which I perhaps over-focused on in this post, and
b) People use diet to create a much larger energy deficit.
I'm going to try to cover some of this in a kind of "wrap up" of the series post next week some time!
Ross
Is it possible for an endurance athlete to preferentially burn fat during exercise vs carbo loading for fuel? There maybe a difference in efficiency (or effectiveness) between the weekend warrior and the professional triathlete or cyclist or marathoner. I am curious, as I think about the results of an athlete like Dean Karnazes (sp?) vs any top American marathoner, as well as all those Tour de France cyclists. We weekend warriors, those who might fare well in our age group in local events, have been pushed to carbo load for fuel, and I wonder if it's really worth it. I hate most of the gels, powders & bars out there, and would really prefer a ham and cheese sandwich. But, it is impractical on a bicycle, I concede.
Thoughts?
Hi Cheryl
Yes, definitely. That's a big part of adapting to training. And it's a topic I'll definitely cover in the future. In this series, though, I committed to covering it from a non-competitive/elite perspective, so I didn't touch that aspect at all.
And time (and my energy levels) don't allow me to go into it right now, but I'll definitely get onto it some time in the future.
Short version - training improves the ability to burn fat, without question. And diet and other strategies to drive fat burning may offer a performance advantage. The evidence on that is not yet clear. Stick with carbs for now, and train hard!
Ross
Ah yes, see how easily 'we' are drawn to complexity and silver-bullets - in nearly complete disregard of your caution about this in your first post on Exercise and Weight Loss. Quite predictable.
Regardless of the endless curiosity about the details, I think most of responders (and maybe even yourselves Ross & Jonathan) have lost track of the central point of this installment - which is you don't burn many calories, from any source, during exercise.
For normal folks, eating less is the only significant way to affect weight. With that in mind, the most important exercise - pushing the plate away.... Repeated consistently.
Exercise for fitness, for fun, for the sense of accomplishment, not for weight loss.
Hi Ron
You're right about the key point about exercise, but wrong about losing sight of the central point. This is a series, remember, and there's more to come. The goal here was always to write about exercise and weight. And had I done a series on diet and weight loss, then perhaps I'd have written more about diet.
There's more to come, though, and a post I'd like to do is to look at diet vs exercise and do it properly. And what is the role of exercise? It's not just fitness, but that's a post that I still have to do. So while you're right, give the series a chance to develop before deciding that it's lost!
Ross
Oh, I'm absolutely reading every word and learning a lot. And very much appreciate all that you do. My comment was more on the = what about this detail, or that angle, what if I do this, or that = aspect of the discussion. Also, realize that I'm being repetitive about the KISS point and will leave it alone from here on.
I like the way Hunter Allen splits this topic here (http://blog.trainingpeaks.com/2009/08/maximize-your-ability-to-burn-fat-as-fuel-by-hunter-allen.html).
Situation 1 is clearly for those interested in losing weight.
Situation 2 keeps a sharp focus on endurance performance.
I'm only interested in the latter. In an extention of that, can you comment on low glycemic training? This article (http://www.bicycling.com/article/0,6610,s1-4-22-16252-1-P,00.html) recommends *occasionally* going hard/long to deplete CHO stores on day one, intentionally not replenishing CHO, then ride w/o CHO the next day. I'm really interested in improving my non-CHO systems. Might this be a good way to do so quickly?
awesome series, thanks.
i have a request for an article topic: that of muscle mass loss. much is written about muscle hypertrophy and strength but little about the antithesis.
there is some mention of 'muscle melt-down' in marathon runners etc, but what is the physiological process?
apologies if you've covered it.
cheers, dave
Ron wolf said "For normal folks, eating less is the only significant way to affect weight. With that in mind, the most important exercise - pushing the plate away.... Repeated consistently."
I don't think that's true, and I don't see how Ross' article supports that. Figures above show how an 'average' male might expect to burn 640 kCal by running 5 miles. Following through Ross' calcs then, 6-7 runs like that equates to around 1lb worth of fat (35g of fat for 50% of the 5 miles, 35g of fat to replace the carbo energy store depleted for the other 50%, = 70g x 6-7).
It's perfectly reasonable for someone to run 5 miles 3 times per week (or the equivalent in their chosen form of activity). No change in diet, and you could expect to lose 1lb per fortnight. Sounds like a healthy, sustainable, rate of weight reduction to me.
The problem isn't that the sums for exercise due to weight loss don't work. The problem is that an exercise programme like this (3 x 1hr per week) is seen as beyond normal people.
Ross & Jonathan - first time commenter, but avid reader over a few years now. Keep up the great work!
Paul.
"...The problem isn't that the sums for exercise due to weight loss don't work...."
should of course read:
"....weight loss due to exercise...."
Linda K. Hibbard - regarding your shameless and silly product plug - sorry, you are 100 years too late. for a good laugh check out U.K. patent application 15,269 "Improvements in Abdominal-fat Reducing Apparatus" on October 27, 1910 at:
http://www.wwp.de/en/kuriositaeten_detail.php?id=15
Paul the Builder - right, exercise helps. problem is the 640 calories you mention is easily made up for by eating, for instance, one pastry at Starbucks. but, ok, both then, workout, and eat less.
I don't see a way to contact you directly, but just to be sure you saw this article in today's NYT:
http://www.nytimes.com/2010/02/04/fashion/04best.html?hpw
No real news, but it does seem appropos of the exercise and weight loss, insofar as you digress into discussing athletic training.
The article still seems to promote this idea of fat burning to lose fat mass when the issue is not substrate usage but creating a deficit.
Usain Bolt does very little "fat burning" exercise yet is quite lean as are his fellow competitors. They may not be as lean as a Kenyan distance runner, but I am thinking most men would not mind being 10% or so.
If a person is time limited (as most people are) then a good strategy is to increase the intensity of one or two exercise sessions a week to create a larger energy deficit (and avoid overeating to eliminate that deficit completely).
One of the good things about the studies in TIME mag was that it helped drive home the point that exercise does not give you free rein to eat all you want.
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Great article. How does one calculate calories burned based on time in heart rate zones 1-5? I've used the following:
zone 1 - 4 cal/min
zone 2 - 7
zone 3 - 10
zone 4 - 15
zone 5 - 20
where bottom of zone 5 is LTHR.
Great series!
Quick question. For folks who are following the general advice here and losing weight through a combination of ramping up training and reducing caloric intake, is there a "science" to the optimal target body fat (say, for an elite, but no pro, cyclist who specializes in hillclimbs)?
It's sometimes said not to go below 5% body fat -- is there a sound basis for this? Is it a reasonable lower limit?
Thanks,
Jim Dannis
Would love to read the next instalment in this series, wondering when you will write the next part?
Fellows, I recommend you read Fein & Feinman's research, titled: Thermodynamics of Weight Loss Diets - in order to more completely grasp the big picture - more precisely, the effect of isocaloric diets on body mass/weight loss.
http://www.nutritionandmetabolism.com/content/1/1/15
Great job stressing awareness of principles in a contextual view over minutiae and quantification.
That said, I think you missed the opportunity to clarify a few points. It’s more precise to state that glycogen (carbohydrate) becomes the preferred fuel at the point where high intensity exercise begins. This is of course at the “6” on the scale of 10 mark that makes it easy for people to get – and more precisely yet… in the specific ‘working muscle’. And that is what the Romjin graph demonstrates – what happens in a muscle – not the body– and points to why this leads to an assumption that this “local effect” carries over to the body systemically – which it pretty much does when we working the legs. Hence using a different set of muscles – say arms over legs – or switching to a particular exercise at which one is more efficient may not effect the body systemically speaking – and so like you said… correlating heart rate with V02 max isn’t really wise - since this relationship varies from person to person.
People need a sense of scale… and though you hit on it, you need to refine it. That said - it’s not as precise to say that fat substrate use is maximized during moderate exercise – but is maximum at the point where high intensity begins – for those set of working muscles. And again, this is point is felt systemically… as a ‘6’ out of ten in terms of exertion (when working the legs mainly) – and correlates with the feeling where ‘the burn’ occurs, i.e. at the lactate threshold when the removal of lactic acid from the muscle no longer exceeds the production. (Hence… this type of training stimulates the production of more mitochondrial enzymes to oxidize more fat – at certain intensity levels – and spare the glycogen.
Romjin's graphs - and more precise examination of the fuel use in sport can be found on Montana State's University site:
http://btc.montana.edu/Olympics/nutrition/fuel03.html
Also a further contextual refinement. Low rates of glycogen depletion in either a single exercise bout - or over time, say weeks or months – tantamount to the need to refuel with less carbs and hence by defacto, a greater percentage of fat and protein (and yes, overall caloric intake must be relatively lower just like the bank account analogy. (Or vice versa with high depletion rates via high intensity exercise) Thus, balancing – once again – health and performance – it would better serve your readers to simply say that matching carb intake – in principle, against the relative depletion - would optimize both repleting glycogen stores in muscles and the health of the body. Overstocking carbs (remember… finite storage) overloads the liver and converts the carbs to triglycerides.
Some people fail to understand that fat is the main fuel substrate at rest or during low intensity exercise and so will simply overeat carbs – which can’t be stored in their already overstuffed muscles and livers. Moderate exercise has no meaning until a person 'feels' a shift to a higher intensity level. ‘They’ – like the road cyclist who commented on fatty acids - often thinks fatty acids only burn when he’s riding his bike. No dude.. this is happening at rest! He still didn't get the relationship you have worked hard to elucidate.
You hit it right on otherwise! Asking WHEN fuel substrate burns is the best way to examine energy use. But at this point you still need to present the second law of thermodynamics as part of the picture.
Applying the second law of thermodynamics – relative to human physiology - will clarify the misunderstandings that are prevalent in this post.
A simplified energy balance “accounting practice”, i.e., balancing “total energy out” against “total calories ingested” - indicates that we restrict our observation of weight loss within the confines of the first law of thermodynamics, and that we ignore the second law of thermodynamics. I will explain the problem with this soon, below.)
You have created a series of posts that lead people to think (and create scenarios in their comments) the following belief:
Balancing ‘energy intake’ equally against ‘energy output’ tantamounts to maintaining a constant body mass. This belief is mistaken, shortsighted, and again… it ignores the second law of thermodynamics.
For those of you thinking I am violating the principles of energy balance and the first law of thermodynamics – and feel like barking back right now - I suggest you pause and learn how the ‘second law’ applies to the physiology of the human body; read the two links below… and if you still feel like trying to debate what I am saying, then feel free to have at it.
http://www.nutritionandmetabolism.com/content/1/1/15
http://www.proteinpower.com/drmike/metabolism/thermodynamics-and-weight-loss/
Also, a lot of you will appreciate Montana State University’s adaptation of Romjin’s graphs and the excellent graphic displays that are also found in this blog’s postings.
http://btc.montana.edu/Olympics/nutrition/
Fellows, I have also used the Romjin information in my lectures, but I think you have missed some opportunities to present the information more clearly and effectively. I implore you to seriously question and examine how the ‘simple-balance-accounting practice’ observes ONLY the first law of thermodynamics, ignores the ‘second law’, and so prevents awareness of basic exercise physiological principles. What precipitated from this series/post are mathematical and conceptual nightmares – which you have tried to address, sensing the muddled waters.
In a nutshell, all the mathematical quantification and ‘ energy balance scenarios’ in the comments indicate a total deficiency of understanding the big picture of how the body partitions and converts the total amount energy from food into both mass and heat. (At this point, I am purposely – but ONLY momentarily – omitting exercise, training adaptations, and substrate utilization). A simple examination of the ‘second law’ and the concomitant effects of isocaloric diets on the human physiology will indisputably reveal that the popular notion: “a calorie is a calorie is a calorie” represents a falsehood.
Only then will people develop a more discrete sense of how the body loses mass (either fat or lean mass). I am ignoring the conversion of fuel substrate into mechanical energy ONLY FOR A MOMENT for clarification purposes. So hang in there.
At nay rate, learning how the “internal partitioning” of energy occurs (how the ‘second law’ applies to weight loss/fat loss/storage of energy) represents the flip side of the coin (from substrate utilization) and will only complement the knowledge you have presented, i.e. how the body selectively burns fuel substrate relative to intensity level. Again read Fein and Feinman for the details.
In principle – and indisputably in the world of physics - the second law tells us how energy is used and distributed – either stored and/or converted into different forms of energy. Here’s the real take home message: If you ‘waste’ more calories from ingested food – i.e. lose them as heat – then the body conserves less mass, i.e. you lose weight – and we do NOT violate the first law of thermodynamics. To get this… you must examine isocaloric diets. Read the links.
- continued next comment -
Whether a human loses mass or not, energy is still conserved. Dependent on macronutrient mix, the grand total energy ingested within in an open system is expressed into different forms – like in a light bulb, either into light or heat. It’s wise to examine this BEFORE considering how food energy converts to mechanical energy – which is to say how and WHEN the body depletes fuel substrate.
Providing a more complete picture is the fundamental issue – and if you say it’s all about calorie balance (which it is) then by your own logic - we must examine the distribution of energy and NOT make the mistake by thinking that ‘xyz’ calories ingested (in terms of balancing out and maintaining an unchanged body mass) will replace all the ‘xyz’ calories in the body and ‘stay put’ as mass! This works for a closed system… like a bomb calorimeter – but the body is not a closed system.
Thus, most people mistakenly think the body works with food like a closed system and that a calorie is a calorie, is a calorie – like in a bomb calorimeter – which can only convert energy from into one form, and that is heat. A light bulb serves as a better ‘body metaphor’; energy may convert into light AND heat. Florescent bulbs partition more energy into light than heat compared to an incandescent bulb – and in the cold Minnesota winters – the new police car flashers do not melt the snow readily like the old lights did! Hah! Efficiency increases… but into what form!?
Hence, with the human, body mass is a third variable to consider in the equation of how energy may be used and partitioned. Read Fein and Feinman as well as Michael R. Eades.
More confusion arises from this statement: “The take home point is that if you burn say 1000kCal, even if some of that energy comes from carbohydrates, it's still part of overall weight loss”.
This is misleading; Restocking glycogen will nullify this temporary weight loss – and in the case of many athletes and people who train and eat plenty of carbohydrates for performance or weight loss/fat loss – the final result is ADDED weight in muscle and the whole body – due to glycogen storage, water, and the increased density of mitochondrial enzymes. Over time – those who train for performance or want to get leaner – do NOT want to lose glycogen in their muscles anyway! People who want to look lean still want their muscles to be firm – full of glycogen, water, and mitochondrial enzymes, thus, fat mass may drop quite a bit while total body mass may not change as appreciably as one hopes for – on a scale.
At any rate, reducing weight loss to a SINGLE BOUT in the form of glycogen is – when you think about it – completely inane. Lost energy from carbohydrates does NOT represent “part of overall weight loss” OVER TIME… especially in anyone who eats plenty of carbs to restock glycogen. Face it… the issue is fat loss – which is synonymous with burning fat and synonymous with observing the whole body losing fat over time – as indicated by the title of the post/series and your subsequent presentation.
Thus, humans do not really “burn fat to accelerate fat loss” as you have written in your first paragraph but rather - burning fat IS fat loss – which happens to occur at different rates absolutely and different ratios relative to intensity level and THEN… we eat food - and the energy in food may be directed into different compartments for storage or transformed into heat or mechanical energy.
Ignoring the post burn effect – the events of burning fat and removing it from the body are as separate from one another as people imagine. Yes, we could look at burning FFA’s and muscle triglycerides versus depleting stored lipids, another issue of time – but my point is that you have distorted the picture by saying glycogen depletion represents weight loss. It’s harder to get at the stored lipid – time wise, but that should be discussed after getting all this other stuff straight and clear. But all this is small potatoes in this muddled scenario.
Continued next comment
The eating part is where most people go completely astray. The specificity applied to substrate utilization must be applied on the other side of the energy equation. Again…the simplified “accounting practice” simply just provides an incomplete and inaccurate picture – and will continue to confuse people until the issue is clarified!
You help everyone down this road of confusion by saying, “the fundamental issue is still calorie balance”. The fundamental issue is – instead - how the energy is directed, transformed, and partitioned – as you have detailed accurately ONLY in terms of substrate utilization. The confusion from this incomplete presentation is found in virtually all the scenarios/comments in the posts.
There is a need to now clean up a mess and you need to take the next step past the ‘simple calorie balance’ concept. If your intention was, as you have written, “intended as an illustration of how the body burns fat, not necessarily overall weight loss” - (via exercise, illustrated by substrate utilization graphs) – then why would you say two sentences later, “Perhaps even more important than this is diet.” You guys opened a Pandora’s box.
Examine these words, “that the principles we've been speaking about in our previous three posts in this series do not suddenly cease to exist”.
Indeed, fat burning NEVER ceases, many people still think it happens only during “aerobic activity” – as reflected by the avid cyclist/commenter – and you have also presented information in a graph that leads people to think it (burning fat substrate) ceases to exist on the extreme high end of intensity level. I’ll get to this – and explain my use of the word NEVER below – where I explain how the said graph does not represent reality – but represents only an extrapolation into an extreme that can’t be obtained by a human over time, including Usain Bolt.
Thus, you need to make clear that the body is - practically speaking - ALWAYS burning fat – as a normal physiological condition – and that this condition merely ‘speeds up’ as intensity level increases – as you have detailed already. Further refinement: Carb intake should be precisely adjusted against glycogen depletion – in order to: 1) not overfill glycogen stores and 2) simultaneously prevent excessive insulin production due to an increased glycemic load.
It takes awhile for people to comprehend that exercising ‘moderately’ or at ‘lower intensities’ are synonymous with lower rates of glycogen depletion – and hence, fat and protein intake – by defacto - must comprise a larger relative amount of total calories ingested – albeit in a lower absolute amount. Again, another missed opportunity – (even though you presented the science on substrate utilization) – to present the dietary adjustments needed to refine the energy balance equation.
The take home message is about energy and specificity! I suggest you consider rewording your phrase, “in other words, the fundamental issue is still total calorie balance” – into a more precise and accurate form by saying, “the fundamental issue of calorie balance is learning to precisely adjust macronutrient intake against fuel substrate utilization, i.e. selective burning.
You need to hammer home that the fundamental issue is to eat energy according to selective burning! This is the paramount value of the Romjin graphs. Besides… people who eat either for performance or weight loss – will benefit and hopefully become less confused. Eliminating confusion, even better.
At any rate - for practical and clarification purposes – it is now crucial to present the physiological effects that arise from isocaloric diets (eating different ratios of macronutrients that supply the same total amount of calories) in addition to explaining substrate utilization and efficiency changes that are due to exercise intensity and training adaptations.
continued in my last, next comment!
Alluding to and examining secondary factors such as post exercise metabolic rate and tertiary factors such as changes in leptin output or how the heart uses lactate sounds impressive, but will only add more confusion on top of the confusing scenario that already has been established.
By clarifying the science of substrate utilization – you have revealed how to more precisely adjust carbs and fat due to selective burning, i.e. substrate utilization at any given intensity level…at any moment or over time. Bring it on home! I ignore protein like you do for the sake of simplicity. We humans - whether we train for performance or weight loss - don’t prefer too burn the engine, i.e. muscle - unless we are under-caloried, severely stressed, sick or fighting infections.
Moreover, it is not a gross oversimplification to ignore protein, but rather a necessity for teaching clearly.
Now, for my NEVER statement. Your graph that shows Absolute Fat Use in grams/hour deviates from reality at the extreme high end. Think Usain Bolt.
Nobody, not even Bolt, can maintain max intensity for more than approximately 10 seconds. Fat use approaches zero (key word; approaches) – but only for a relatively super brief explosive moment. Thus, in reality - pertaining to MOST people who train ‘hard’ often, weekly, and monthly, consistently OVER TIME (an element of reality that you stress in your post, i.e. “Time is key though, as many of you are no doubt thinking.”) – the absolute total fat burned still remains very high – even if the percentage drops relatively low compared to glycogen use.
In other words, unless you’re in the Tour de France, absolute/total fat substrate depletion rarely drops to the degree as your graph indicates - especially over time. Thus, even still – guys like Lance Armstrong and Usain Bolt are utilizing HUGE amounts of fat substrate – especially when exercising below lactate threshold – which is usually insanely high in elites.
Only elite endurance athletes really push that curve to the extreme, but even they do not reduce it zero - EVER. A ‘commenter’ on this posting presented a mathematical scenario of extremes – as if the body could literally utilize either 100% fat or 100% carbs. Using extremes is a useful tool for getting points across; I use them too. But they deviate from reality and practice, especially over time. Most of the posts created various energy balance scenarios and vainly tried to sort things out based on good science – but none of it works or becomes clear if we ignore the second law. I have made use of Romjin's graph for years, I respect your work, but I can’t help but critiquing it.
Further still. By truly and more precisely balancing carb intake against the selective depletion of glycogen – we can more precisely stock – without overfilling – muscle and liver stores. THEN…. ingesting a relatively higher intake of fat – [which should be higher… percentage wise within the energetic whole for people who exercise at ‘low’ to ‘moderate’ intensity levels - and therefore do not deplete glycogen levels greatly] --- will ensure that the body continues to burn fat at its optimum rate – thereby reducing the body fat without sacrificing either muscle glycogen or performance. Now we optimize three elements – performance, weight loss (body composition really) and health!
Finally…the efficiency and partitioning of ‘energetic flow’ that occurs from ingesting various ratios of macronutrients – IN ISOCALORIC DIETS - must be examined discretely from the efficiency changes and selective burning shifts that arise from physical training adaptations.
Examining the physiological effects of isocaloric diets and how this rules the big picture will clarify all the ‘accounting’ errors that arise from applying only the first law of thermodynamics to the issue of mass loss and fat loss. Again – see Fein and Feinman for the “biochemical formulas” and a more detailed look at this.
Ross - When are you guys going to finish this series?
Hi Stewart
Good question - the answer is as soon as I have enough time...
The problem with these posts is that they take longer to do than normal posting, because of the level of technical information. Maybe two hours to write a decent post, whereas commenting on the Olympics and mental performance takes 30 minutes, and the running shoe posts I'm doing now take 5 minutes, because all I'm doing is cutting and pasting from an interview done earlier this year.
So time is the issue - the series was interrupted by travel with the Springbok Sevens team, and I'm now traveling again. Hopefully in April I will have time to get to it.
Ciao
Ross
Hey there!
I read all the parts, and being a recreational endurance sports enthusiast-athlete with at least a little bit more information I have a question.
The energy values listed for foods are measured by burning them right? So the kcal listed is the bonding energy in the molecules and some bonding energy among molecules.
However, when we calculate the "work done" on a workout we usually calculate the mechanical work which is J=F.x J:joules , F:Force in N , x:distance in meters
Suppose a cyclist is cruising and he has a power meter which reads 200W. He is in fact burning much more than 200j per second due to his muscles not being 100% efficient. In fact I think I have read in an article that cyclists are at most 20, 25% efficient.
The rest of the energy is transformed to heat and hopefully dissipated.
The thing is if we assume the cyclist is 20% efficient than in fact he's burning 1000j per second. And he should do the math using the calorie burned value.
There are a lot of tables which tells us how much calories we burn in an hour for a given activity. Does these tables really include calories burned or do they list amount of work done?
Thanks
Ross and Jonathan,
I'd like to hear your thoughts re the training "supplements" that purport to encourage the body to preferentially use fat as fuel. They seem to contain proprietary mixes of extracts from a certain species of hornet....
Melani
Hi Ron
You're right about the key point about exercise, but wrong about losing sight of the central point. This is a series, remember, and there's more to come. The goal here was always to write about exercise and weight. And had I done a series on diet and weight loss, then perhaps I'd have written more about diet.
There's more to come, though, and a post I'd like to do is to look at diet vs exercise and do it properly. And what is the role of exercise? It's not just fitness, but that's a post that I still have to do. So while you're right, give the series a chance to develop before deciding that it's lost!
Ross
it'd be sweet if you guys provided more pubmed sources for us nerds out there. particularly i was interested in the bold statements from this paragraph:
For example, there is evidence that if you delay eating after exercise for about an hour, you burn more fat than if you eat right away. This has to do with keeping insulin levels down, and insulin is a hormone that drives carbohydrate use, while "tuning down" fat use. The problem is, if you delay eating, you may compromise your recovery, which means you can't sustain high quality training day after day. You also can't cut carbohydrates out, and you certainly can't under eat - there is compelling evidence that the biggest risk factor for becoming sick during intensive training is an energy deficit.
for that 2nd bold statement, i think the main concern there is higher glycogen synthase activity immediately following exercise, but for those not doing double days, i've read that so long as you get the carbs over the next 24 hours, you'll be fine when your next exercise bout rolls around.
glad you brought in some of rosenbaum's work. i think it's important, albeit discouraging, to point out that EE per kg lean mass may be reduced for even a year after weight maintenance. then we may get into issues of how it is important to not pack on the lbs in the first place. i'd be interested on your take of a weight set point and how that relates to rosenbaum's research, which would suggest that it is incredibly difficult to 'reset' that point once it's raised via weight gain.
for the 40-40-30 60-30-10 business i can't help but think about modelling macronutrient percentages on those found in some paleolithic/hunter-gatherer/fringe populations, who tend to be quite healthy (and injury free during minimally shod running!) until exposed to western diets. i took a class on paleolithic nutrition at colorado state and think the evolutionary nutrition perspective is worthwhile (Paleo Diet for Athletes by Cordain and Friel might be worth mentioning).
finally, I'd be very interested if you guys could provide some basic physiology on what does happen for weight loss if the caloric deficit is substantially carbs. i've always understood the degree of gluconeogenesis to be relatively limited, particularly since you can't get fat-derived acetyl-CoA into glucose since PDH is irreversible. so if you're exercising mostly at 80% vo2 max (granted a recipe for injury), and we're saying the main recipe for weight loss is simple calorie deficit, can your body sufficiently replace the burned carbs if the diet isn't quite high in CHO?
thanks for what you do, love the site.
-mark
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