What the body says and how the brain lies: heat and hydration

Strategies on how to cope with running in stifling heat, humidity and smog.

Summer is certainly upon us in many parts of Canada. A perennial topic in running circles is “what to do about training in the heat.” It seems that, like so many other basic and fundamental questions about physiology, there is now more confusion than clarity regarding how our bodies respond to heat and what we should do about hydration.

What follows is a longish, meandering reconsideration of what has long been held as the gospel on the matter, as well as many newer and perhaps more legitimate findings regarding how our bodies function under stress, including the stress of heat and looming dehydration.

When one thinks about it, it’s actually quite simple: running requires the body and the brain to work together in order to perform for a given period of time at a given level under given circumstances. Changes in any one of those variables just requires some form of compensation elsewhere. The one wildcard in all of this is the push-pull between the old adage “listen to your body” and a somewhat newer idea that the brain perhaps shouldn’t be listened to so intently.

Warning: reading the following may lead to fuel belt and GPS neglect this summer.

The thirst mechanism

For something that is so automatic, so engrained in our lives otherwise, the debate about how thirst plays into running performance, especially during long periods of exertion or in extreme temperature situations seems to still rage on without a definitive answer.

For many years, long distance athletes assumed that fluid or nutritional intake was pointless during a race (the same logic was no doubt applied to training runs as well, but there is little information in that regard).

Then, with the advent of sports drink and the research surrounding dehydration (as well as the marketing surrounding said drinks), opinions drastically changed. In 1996 the American College of Sports Medicine indicated that “runners should be encouraged to replace their sweat losses or consume 150 to 300 ml every 10 to 15 minutes (600 to 1200 ml per hour). Until very recently, this has been held as the gospel regarding electrolyte and fluid loss, suggesting that massive fluid intake is necessary in order to stave off dehydration.

Of course, there are two problems with the ACSM figures: it was not clear what form of running activity and conditions necessitate this amount of fluid intake (and, moreover, we’ve seen come to realize that sweat rates vary wildly), and secondly that consuming that quantity of fluid is seemingly unnecessary under most circumstances.

Without sounding paranoid, there was of course a ‘sports drink explosion’ during the 1990’s and much of the research that was done, according to Russ Tucker, PhD and Jonathan Dugas, PhD of the respected industry blog The Science of Sport, was driven by sports drink companies (many of the findings came from the Gatorade Sports Science Institute). The consensus at the time, not surprisingly, was to hydrate like mad.

What came out of this, not surprisingly, was contrary research, including quite a bit of work done by famed South African physiologist Tim Noakes regarding hyponatremia, which is an electrolyte deficiency caused by overhydration. What started to happen was that athletes were taking hydration far too seriously and were often massively flooding their systems with fluids that threw their sodium levels out of whack. In extreme instances this condition was fatal.

Thus, we find ourselves today in a grey zone, where there is still quite a bit of conflicting data about what to do and not do when training, or when running races of varying distances at varying effort levels.

One common sense approach remains: listen to your thirst. If you truly feel thirsty, respond to that physical trigger. For many, that doesn’t necessitate carting around a water bottle during most runs, even on a warm summer day. More practically, it might just require that you know where public water fountains are, just in case.

Also, an important means of monitoring how your body is responding to the heat is to weigh yourself before and after runs regularly. This will give you a good idea as to how much water weight you’ve shed on a particular run, and also allow you to become more familiar with how your body reacts to varying conditions and under varying types of stress.

Just as with other aspects of running, such as gait analysis, an approximate VO2 max level or establishing one’s resting heart rate and target heart rate zones, you can have your sweat rate calculated. For those that don’t have the extra time and money to explore such details, it’s still worth keeping data regarding before-and-after weight-ins, especially on long runs or harder workouts of 90 minutes or more.

One guideline typically has been to avoid losing more than two per cent of your body weight from loss of fluids while racing or putting in an intense, long term effort such as a long run of 30K or more. But a significant study published in the British Journal of Sports Medicine in 2011 shows that the faster the marathon finishing time, the more fluid weight was lost. Runners finishing under 3:00 lost on average 3.1% of their weight during the marathon, whereas those that finished over 4:00 lost less than 1.8%. This would suggest that perhaps the two-per-cent-rule is playing it a touch too safe and that, with improved training, the body can shed more fluid than previously thought during a long, intense run.

Fluid balance vs replacement

In truth, our concern with fluid intake during hot days, especially during races, is more about performance than it is about staying hydrated.

When you think about it this way, even during the coldest, slowest death-march in the dead of winter, you are still becoming dehydrated bit by bit as you carry on through your run. Under those circumstances, we seldom even bother to carry fluids (as they often freeze by mid-run anyhow).

So it’s not really about fluid replacement as much as it is about maintaining a balance. What we are really trying to do is make sure that the cells don’t slowly slip into a situation where performance begins to become adversely affected by a variable that we could have otherwise managed with a bit of fluid intake. Seen that way, many would probably find themselves trusting how they feel much more so than the typical sort of overcompensation that many indulge in even on easy runs.

When racing, and even when training with intensity on hot summer days, think about maintaining a balance more so than just forcing in fluids for the sake of topping oneself off. The body is designed to take some abuse and shed fluid throughout the day and yet still function at a high level, especially after a period of adaptation.

The consensus now seems to be to use common sense. Drink when you find yourself feeling thirsty during a race. If you desire the safety blanket of carrying fluids with you during a moderate effort training run, then do so. If you find carrying a water bottle or fuel belt cumbersome, in most situations you’ll probably be fine without it.


Although there are studies that show that if you subject yourself to purposefully increased heat exposure you will perform better under hot conditions (as well as cooler conditions), there’s no sense in torturing yourself prematurely leading into the summer months.

But you can test your body by running in increasingly tougher conditions as the summer begins and temperatures suddenly jump. It’s very tempting to wait until the sun goes down or to run very early in the morning, or even bathed in A/C on the treadmill at the gym. But these avoidance tactics will delay the inevitable.

As the summer heat begins to set in, ease into your typical training routine. If you are a mid-day runner, back off a touch during the first few warm days. Always run on feel first, responding to the perceived effort, especially on easy days. The acclimatization period can last a couple of weeks, and this is a good time to do a few easy runs without a watch, so that you practice listening to your body more than the paces on the Garmin.

Brain training and the central governor theory in hot weather running

There’s been much talk in the last couple of years about the “central governor theory” — the idea that the body is not so much governed by actual physical limitations, but that the brain has a ‘governor’ that constantly monitors how things are going and prematurely shut down a physical effort if it’s getting to a point where it could become too demanding. Needless to say, efforts like, say, a marathon are designed to challenge the central governor.

If we do indeed have a central governor, it could be posited that, just like with training meant to get us through the last 10 to 12K of a marathon, we can train our brains to not prematurely pull the plug on a tough run on a hot day. In reality, so long as we don’t enter into severe dehydration or allow our core body temperature to exceed 40 C, we will probably be OK.

One proposed method is to take in a mouthful of fluid here and there throughout a long run or during a difficult stretch of running (preferably sport drink, as it has the sugars and electrolyte composition that our brain is looking for), but instead of swallowing it, spit it out. A study attempting to prove that fluid intake was less vital than we’d previously thought utilized this technique and the results do indeed reveal that, at least for a period of time, it’s more important for our bodies that there’s the promise of fluid intake than the actual intake itself. This trick can be used quite effectively during hot weather races, when often it’s difficult to stomach fluid. It’s also a fantastic tool during a marathon.

A recent study also suggests that cooling the neck area may also improve performance in the heat. Functioning in much the same way as with taking in and then spitting out fluids, the sensation on the neck sends signals to the brain that override the central governor.

Also, and this sounds sort of silly and obvious, but it’s worth really thinking about what you are setting out to do before you go for a run, especially on a hot day. Recent studies have also revealed that performance is connected to what’s called “anticipatory calculations,” which can be seen as a sort of primer for what the central governor is looking for during the run. So, if you leave your house and you’ve reviewed your workout in detail, your brain is sending signals to your body in advance and preparing you for that level of effort.

Air quality issues

One final consideration when you leave your door during a sweltering, humid summer day is air quality. In many major cities air quality has become a very real issue, especially for those of us who are opting to suck back large volumes of it while stressing our bodies for significant periods of time. It’s worth checking Environment Canada’s air quality health index for your area on windless, humid days. If air quality does effect your performance, high index days are of course to be avoided. Environment Canada provides a detailed breakdown as to what each level of the index means.

A few additional tips:

Always carry a water bottle with you throughout your non-running day. Proper hydration has very little to do with fluid intake during a run, but instead mostly to do with how properly hydrated you are throughout the day leading into the run. Ironically, perhaps the only time one should not be carrying a water bottle around is during a run.

Don’t overdo it with the drinking, however. In truth, the body can only absorb a small amount of fluid at a time. Stick to a regimen of a few sips or mouthfuls of fluid every 15-20 minutes and no more. By the end of the day you’ll have consumed more than enough fluid to be perfectly hydrated — enough so that your urine is either clear or a light lemony colour at worst. If it’s yellow or has an odour you are not hydrated enough.

If you run first thing in the morning, pay more attention to hydrating before bed than in the morning. It’s much less effective to force down fluids (that won’t absorbed anyhow) just before heading out the door than to drink smaller amounts throughout the evening. This practice is especially crucial leading into an early morning race and is worth practicing on morning long runs.

If you religiously carry fluids with you, on a moderate day where you are not running long try leaving the bottle at home and see how it goes. Chances are you’ll be fine.

Less is not always more in terms of clothing. When it’s extremely sunny, a hat and a white t-shirt is better than no shirt at all and nothing covering your head and face.

Always start off easy and then gradually increase your effort on hot days. Often, you’ll find that you can execute a workout at the prescribed level, but it’s critical to ease into hard efforts during the summer months. The only thing worse than not sticking to the numbers is failing a workout! Also, perceived effort on a hot day should always trump what your watch is saying.