Extreme heat, dehydration and sodium balance


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- Alex Hutchinson (@sweatscience)


Another interesting hydration study [UPDATED WITH LINK TO STUDY] from Tim Noakes and his collaborators, studying South African Special Forces soldiers marching in hot conditions — following up the one I blogged about last year. The basics: 18 soldiers did a competitive 25 km march (taking about four hours), carrying 26 kg packs and wearing full battle dress, in temperatures averaging 40.2 C and reaching a high of 44.3 C (112 F). They were allowed to drink only water. The main point: they did it, despite

environmental conditions that approached those considered to be unsafe for practice and competition by the American College of Sports Medicine. Furthermore, all soldiers completed the study successfully and none presented with either the signs or symptoms of ‘‘heat illness’’.

But it’s the details that are most interesting. They were allowed to drink as much as they wanted, and the amount they chose to drink led them to lose 3.8% of their body mass on average — too much, according to conventional thinking. But they showed no sign of trouble, and there was no link between the amount of weight each soldier lost and his finishing time. But (as their previous study showed), weight loss didn’t correspond exactly to water loss: for every 1 kg of mass lost, their total body water stores only declined by 200 g (for details of how this is possible, read the earlier blog entry).

More importantly, the sodium concentration in their blood didn’t change significantly (and neither did their overall plasma osmolality), even though weren’t taking in anything but water. They lost some salt to sweat, but they also lost some fluid, so the concentration stayed relatively constant.

At this sweat sodium concentration, average total sweat sodium losses during the march could have been >240 mmol. Yet despite such large losses that were not replaced during exercise, participants maintained their serum sodium concentration. This confirms the now well-established finding that serum sodium concentration can be maintained during exercise without the need for acute sodium replacement during exercise.

I’m sure plenty of people will disagree with that last sentence! Noakes’s argument is that, if you allow people to drink as much as they want and choose their own pace, they’ll automatically self-regulate in order to preserve homeostasis — and the crucial parameter that your body monitors is not weight or water content, it’s serum osmolality. So it’s no coincidence that the soldiers allowed themselves to get dehydrated to precisely the degree that matched the salt they lost in their sweat — that’s just the way the body works.

P.S. Random aside on dehydration: the introduction of this paper cites another study claiming that Haile Gebrselassie lost 10% of his body mass while setting the current marathon world record. Now that’s impressive!

11 Replies to “Extreme heat, dehydration and sodium balance”

  1. Very interesting. After the Holyoke Marathon in, I believe, 1963, in extreme heat (it became known as the “Holyoke Massacre”), several runners who finished well were talking and found that they had in common a salt-free diet. Among them was the legendary Ted Corbitt. Now then, why would a salty diet contribute to bailing in hot-weather marathons? “Worthy of further study…” doesn’t begin to describe it.

  2. Very interesting indeed. As an ultra runner who supplements with electrolytes every hour in races in the heat, I find this hard to swallow and couldn’t imagine going without.

  3. I believe that in the Hammer Nutrition literature on electrolytes and endurance performance (http://www.hammernutrition.com/knowledge/essential-knowledge/), the statement is made that with a normal diet the body stores enough sodium for up to about 4 hours of running/cycling. If true, it might explain why Ted Corbitt & Co. were able to do well in extreme heat despite their “salt-free” diets (I bet they got sufficient sodium from celery, meat, etc.). I do know that at Western States, the doctors warm people strongly in the medical briefing the day before the race, of the dangers of low sodium, complete with horror stories of runners being air-evac’ed out of the canyons at great expense, in critical condition.

  4. @Derrick: Yes, I’d be hesitant to extrapolate these findings directly to ultra races (or other very long events like Ironman triathlons). @George’s point that the body has enough sodium to last about four hours is interesting — I’d certainly love to see if any studies have confirmed that.

    A key point: the subjects in this study were clearly losing sodium through sweat. They were able to (subconsciously) compensate for this by allowing themselves to become dehydrated so that the concentration of sodium in their blood stayed constant. But this can’t go on indefinitely. They were 4% dehydrated after four hours — but if it was a 12-hour march, that would present a different challenge. Of course, it’s not really realistic to discuss doing 12 hours of intense exercise in extreme heat taking nothing but water — but the point is, you can’t extrapolate these findings indefinitely, because the athletes are going into temporary water debt.

    The other factor is pacing. This study was self-paced, and the subjects were permitted to take breaks whenever they wanted. If you look at the individual core temperature data (they each swallowed special pills to continuously monitor their core temperature), it’s a sawtooth pattern: temperature rises until it approaches a critical point, then they back off to keep it in a safe region. This is the essence of self-paced exercise, as opposed to lab studies on treadmills with fixed speed, where the core temperature would have risen to a critical point, the subjects would have fallen off the back of the treadmill, and the conclusion would have been “Being dehydrated by more than 2% causes heat stroke.”

    However, I’m not sure what happens when you translate this to an ultra race, where your motivation to place as highly as possible might override your body’s desire to slow down. In this case, can taking extra salt allow you to maintain a harder effort before being cued to slow down by hydration limits? I don’t know, but this study certainly doesn’t rule that out.

  5. Question about Noakes study. Were his soldiers black or white? Blacks (having evolved in a hot humid climate) are thought to have salt preserving mechanisms in the kidney that was developed as a survival benefit in equitorial conditions and a low sodium diet (hunter-gatherers).

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