The Science of Sport: Boston Marathon: if you can't stand the heat...

The heat and fear doctrine of the Boston Marathon

Later today, Geoffrey Mutai will attempt to address the Ethiopian dominance of marathons so far in 2012 when he defends his title in Boston. Last year, aided by a following wind for most of the race, Mutai and Moses Mosop stunned the world when they scared the 2:03-barrier in Boston, Mutai eventually winning in the astonishing time of 2:03:02.

The other factor that enabled Mutai's incredible time last year was the ideal temperature for the race - 50F. This year, it will be a little different. The forecast for Boston is temperatures in the 80s (that's 25 to 28 celsius), and it's caused a real panic among organizers, media and runners.

The Boston Marathon organization have recently issued a statement which has advised runners to consider their decision to participate. That is, they are so worried about the heat (all 27 degrees of it) that they are telling people rather to stay away, especially if they are not heat adapted or accustomed to running in the heat (which is physiologically valid advice).

Now, I can partly appreciate this - as we've seen many times with things like footwear and hydration, common sense is sadly uncommon, and the race organizers would usually bear the brunt of any mishaps that occur as a result of the heat.

And if you read the statement issued by the medical directors of the Boston Marathon, it is well balanced and addresses the key issues. It emphasizes the most important point, which is that this will not be a day for record times or personal bests, whether you are Geoffrey Mutai or a runner trying to break 3 hours, or even 3:30. In fact, these slower runners will probably be more affected than the elites, as the statement acknowledges. Therefore, common sense dictates that anyone running in Boston accept the heat as a factor they can't control, adjusts their pace and still finishes safely.

Easy solution. No need to panic.

The day after tomorrow

However, the reaction from elsewhere is a little less sensible. It reminds me of a Hollywood blockbuster like "The day after tomorrow" where the high temperatures are closing in and the citizens are running for the hills because of the "death sentence" that the heat must be.

Some of the advice being given to runners is unnecessary and will only over-emphasize the risk of the conditions, when there is a really simple set of guidelines that runners should heed. The biggest concern will be around dehydration and its supposed link to heatstroke. People are freaking out that they'll lose so much water that their bodies will incinerate them from within, as is the general perception, I have to say, within the USA.

The advice that will have been thrown around at the Boston Expo, in the local papers, and among runners and their coaches is that it's essential to "drink as much as you can", and to "drink early, drink often, and drink plenty". This is the most dangerous advice that can be given.

Replacing one problem with a more serious one

The problem with this fear of dehydration is that all it has done is to create a new problem, far more dangerous and lethal for runners. This problem is called hyponatremia, and it happens because people overdrink during exercise. They replace more than they sweat, and the result is that they dilute their body's sodium level (hypo = low, natremia = sodium). This condition, if severe enough, leads to coma and death, and has claimed far more lives than any dehydration-related condition ever will.

Most of these deaths, incidentally, are in US marathons because that's where the pre-occupation with dehydration exists. About a decade ago, an Ironman race in New Zealand suffered from an incredibly high number of hyponatremia cases. In response, organizers did a little research, discovered that the cause was overdrinking, and the next year, they cut back on the number of water tables available, advised runners NOT to drink unless they were thirsty, and the result was no hyponatremias and no hospitalizations.

Boston has produced perhaps the most famous case of hyponatremia in Cynthia Lucero, who died in 2002 as a result of overdrinking, after she followed the advice given to her through all manner of sources, including the race, the magazines, the experts.

Sadly, it's such an avoidable condition, because all it takes to avoid the risk is drinking to thirst. If you drink to thirst, you cannot possibly develop hyponatremia during exercise, whereas if you attempt to force hydration, or to follow a schedule, then you put yourself at risk. And the problem for Boston 2012, is that all the advice being given to runners is to drink, drink, drink, and it will create a problem for their medical team that is far greater than any risk of dehydration was going to be.

Dehydration and heatstroke - no evidence, and dehydration is normal

A note on dehydration - there's no evidence at all that links dehydration to heatstroke. In fact, there is very little evidence on heatstroke either. There are some documented cases of heatstroke, but they point towards unnaturally high rates of heat production, and not dehydration, as the cause. Other cases of heatstroke have been found to occur in cool conditions, very early on during races, and therefore not linked to the environment or dehydration in any way.

The reality is that it is absolutely safe for humans to lose fluid during exercise. We have an adaptation that allows this, because when our survival depended on our ability to hunt, we did not have the option of stopping every 10 minutes to drink a Gatorade, and so we are delayed drinkers. We tolerate losing fluid very safely, and then we replace it later on. Our research from Ironmans, Comrades and Two Oceans Ultra Marathons (90km and 56km, respectively - long enough to see problems if there were any), has shown that the vast majority, and we're talking 90% here, of finishers will lose between 0% and 3% of their body mass during a race.

And these people, I must emphasize, are absolutely fine. They've started a race weighing 180 lbs and ended it at 176 lbs, for example. By the evening, they've replaced that fluid and are safe. The ones we worried about were those who had not lost weight, or even gained. These were the people who had taken on too much fluid, and needed to be hospitalized because they were seriously ill.

But again, to stress - dehydration is normal, it is safe to lose 2% of your body weight, or even 4%, 6%. In fact, the race winners will have lost the most weight. There are cases of race winners losing between 6% and 8% of their body mass, and they are absolutely fine, no complications other than being tired from the exertion.

The ultimate message here is that if we drink to thirst, we may lose fluid over the course of the race, but this is normal, and it is safe. What is unsafe is forcing fluid intake, developing a schedule that doesn't allow for the impact of intensity and environmental conditions on our sweat rate. Or taking generic advice that says, for example "Your body needs at least 1,200 ml of fluid per hour". That kind of advice, however well-intentioned, could be a recipe for disaster, and the concern for Boston 2012 is that this is the message that people are now hearing.

The pace - expect a slower race, whoever you are

Then the other key thing for Boston is the pace, and the impact that higher temperatures will have on it. It's really simple - when it is hot, you will run slower. The elite athletes will get this right - they will start the race at a slower pace, and you won't see a halfway split of 61:30.

We know that this happens because our body is smart enough to anticipate the future physiological consequences of our "actions", and so when it is 27 degrees and we run at 2:58/km, we generate heat but fail to lose it, and the body is able to work out that this is not going to produce an optimal result. Why not? Because once our body temperature hits about 40 degrees celsius, we stop. Our brain, once that hot, doesn't recruit as much muscle, and the pace would be significantly reduced as a result.

Over-riding regulation and running to the clock - it's all about perception

The problems happen when this regulation is over-ridden. And this is why the Boston Marathon Association are warning people, because a lot of people will run "to their watch". They'll have worked out that they can run 3:20 if they hit certain targets along the way, and they'll try to do this, regardless of conditions.

It is these people, the ones who are inflexible and who race to a schedule, who run into trouble. They don't get heatstroke is 99.9% of the cases, but they feel terrible. They feel incredibly hot, because their body temperature has probably been driven up towards 39 or 40 degrees celsius by the 25km mark of the race, and their perception of effort is so high by that stage that they think they're close to death! (read this review for full discussion of why the perception of fatigue is so crucial)

It's this perception that knocks them out, and may put them into the medical tent. For all but a very tiny minority, that's the end of it, and all they need is rest and some cooling and they're fine, because there's actually nothing wrong with them apart from prematurely having a body temperature of 39 degrees Celsius (which is safe, by the way). This is often over-interpreted and called heatstroke, when all they are is hot and tired too early in the race (race winners will be close to 40 degrees, by the finish line, and therefore "acceptably" hot and tired!)

But the point is, the pace must drop on a hot day. These two articles explain the physics of heat loss and how pace has to be adjusted on a hot day:

That second example also shows how being smaller is an advantage during exercise in the heat, and this is one of the reasons that the elite runners are less affected by the hot conditions than those running 3h30. They tend to be 60kg, tiny runners, whereas those at the back are often 50% heavier. That, plus the adaptation to the heat as a result of where they train, means that the elite will problem be slowed by 2 to 3 minutes, whereas those at the back could lose 10 to 20 minutes on a hot day.

Bottom line - the body knows. So listen!

The bottom line, however, is not to panic about the heat. Yes, it makes things more challenging, and yes, the risk goes up. But only if the runner ignores the obvious, disobeys common sense, and disobeys their own body, in two important aspects - hydration and pace.

So the only guidelines you need listen to are the following:

Run within yourself, not to the clock; and
Drink to thirst

Simple as that. So by all means, if you're not able or willing to adjust the goal from a 2:59 to a 3:10, and if you've been training early in the morning in temperatures of 4 or 5 degrees, rather than 27 degrees, then this is a Boston race to miss.

What it is not, however, is a race that "could kill you", and the heat is not a death sentence that should be avoided at all costs. And whatever you do, don't drink everything in sight!

Ross