A tablespoon of water helps the exercise go on

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My new Sweat Science columns are being published at www.outsideonline.com/sweatscience. Also check out my new book, THE EXPLORER'S GENE: Why We Seek Big Challenges, New Flavors, and the Blank Spots on the Map, published in March 2025.

- Alex Hutchinson (@sweatscience)

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Neat study from some researchers in Greece, posted last month at Medicine & Science in Sports & Exercise, that suggests that the sensation of cool, wet water flowing down your throat may be more important than the actual hydration the water provides for helping athletic performance.

Hang on just a sec, you say. Doesn’t this sound a bit like the “mouth-rinsing” experiments that have caused such a stir over the past few years, where subjects get a boost from swishing some sports drink in their mouth and then spitting it out? Well, it’s similar… but different. The fluid in the mouth-rinsing experiments contains carbohydrate, and the evidence suggests (very strongly) that we have previously unknown carbohydrate sensors in our mouths. If you’re exercising at an intensity and duration where carbohydrate availability is a potential concern, then when your brain detects incoming carbohydrate, it lets you go faster/harder/farther — even if you then trick it by spitting the carbs out.

The new experiment only used water. They took 10 endurance-trained males, and put them through a two-hour dehydrating protocol (alternating cycling and walking in a heat chamber) that dehydrated them by 1.9% of their body mass. Then then did a ~20-minute cycle to exhaustion under one of three different conditions (each subject tried all three, separated by at least a week): either no water permitted at all, or 25 mL of water to drink every five minutes, or 25 mL of water to rinse their mouth and then spit out every five minutes.

Note that 25 mL is a very small amount of water — it works out to about 1.7 tablespoons. After 20 minutes, they’d have received a total of 100 mL, which weighs 100 grams. In comparison, the pre-test dehydration protocol had taken away about 1,500 grams on average.

The results: they lasted 17.7 minutes in the no-water condition, 18.7 minutes with the water rinse, and 21.9 minutes with water drinking. All the physiological measurements (heart rate, lactate) plus perceived exertion were the same between the groups at the end of the test:

[T]he efficacy of water suggests that probably the sensation of swallowing along with the cool sense in the digestive track can motivate moderate dehydrated subjects and lead to an increase in performance… There is evidence that drinking itself activates the oropharyngeal receptors which in turn, influence significantly fluid balance, thermoregulation and possibly exercise performance.

In other words, in addition to a carbohydrate warning system in the mouth, the brain also seems to gather information about incoming fluid from the throat. This makes sense: it allows us to adjust and moderate behaviour based on incoming fluid and energy rather than waiting for the fluid and energy to be processed and distributed throughout the body. But it leaves us susceptible to being tricked by sneaky scientists.

This isn’t the first time this idea has been investigated. The discussion section of the paper describes a really cool study:

Moreover, in a classic study published by Figaro and Mack, subjects performed three identical dehydration protocols followed by 75 min of rehydration consisting of 1) ad libitum drinking (control), 2) infusion of a similar volume of water directly into the stomach via a nasogastric tube (infusion) and 3) ad libitum drinking with simultaneous extraction of ingested fluid via a nasogastric tube (extraction). The researchers found reflex inhibition of AVP [vasopressin, a hormone that controls the regulation of fluids in the blood] and thirst in control and extraction but not during infusion, suggesting that oropharyngeal reflexes modulate thirst and the secretion of AVP.

Translation: this isn’t just a placebo effect. Drinking water by swallowing it down your throat tells your body that water is coming, and various systems throughout the body respond accordingly. If you’re rehydrated without swallowing, your body doesn’t realize the fluid is coming (at least not immediately), and doesn’t respond. And if you rehydrate by swallowing water but immediately have an equal amount of water sucked out your nose (is this a cool experiment, or what!), then your body still thinks it got rehydrated.

So what’s the message here? Well, it seems that rinsing and spitting doesn’t do everything. But the principle is the same: late in a race, when you don’t want to be downing a full bottle of anything, taking a few swallows of a drink can still trigger your carb response (in the mouth) and your hydration response (in the throat).

Electrolytes and overdrinking: Noakes vs. Gatorade

THANK YOU FOR VISITING SWEATSCIENCE.COM!

My new Sweat Science columns are being published at www.outsideonline.com/sweatscience. Also check out my new book, THE EXPLORER'S GENE: Why We Seek Big Challenges, New Flavors, and the Blank Spots on the Map, published in March 2025.

- Alex Hutchinson (@sweatscience)

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We all know by now that drinking too much during exercise can have fatal consequences, because your sodium levels get dangerously diluted (hence the name of the condition: hyponatremia, meaning too little sodium). So it seems logical to think that drinking beverages containing sodium, rather than plain water, would mitigate this risk. That’s certainly what sports-drink makers want you to think.

But some evidence suggests otherwise — and that has led to an explosive scientific debate. The latest blow comes from a paper in the British Journal of Sports Medicine by Tim Noakes, in which he takes data from an earlier Gatorade-funded study [full text freely available] and reanalyzes it to reach essentially the opposite conclusion. In typical Noakes fashion, he pulls no punches about what he believes is causing the confusion: the title of his paper is “Changes in body mass alone explain almost all of the variance in the serum sodium concentrations during prolonged exercise. Has commercial influence impeded scientific endeavour?

It is instructive to review the industrial connections of those who wrote the 2007 ACSM Position Stand [Noakes writes]. Of the six authors, four – Drs. Maughan, Burke, Eichner and Stachenfeld – have direct and longstanding involvement with Gatorade and the Gatorade Sports Science Institute (GSSI), but only three (Drs Maughan, Eichner and Stachenfeld) deemed it necessary to disclose in the Position Stand the existence of that relationship. The two remaining authors – Drs. Sawka and Montain – are employed by the United States Army Research Institute for Environmental Medicine (USARIEM)… It is perhaps surprising that given the large number of experts available to it, the ACSM should consistently choose the authors of its influential Position Stands from such a narrow selection of group thinkers.

Leaving aside the politics, what about the evidence? Noakes argues that the sodium content of your drinks makes little difference; what matters, rather, is how much you drink. If you drink too much, your sodium levels will drop, perhaps dangerously. If you don’t, they won’t. As evidence, he replots the data from Lindsay Baker (formerly of Penn State, now working for Gatorade)’s 2008 paper:

What’s clear here is that how much you drink (which determines the change in body mass) is the dominant factor in where you sodium levels end up. If you drink enough to gain weight, your sodium levels are going to drop no matter how much sodium you ingest. There are some small differences between the drinks (which is why the three dots in each cluster aren’t right on top of each other), but they seem pretty minor. Even Baker, in her original paper, acknowledges this:

[Sodium] consumption attenuated the decline in [serum sodium levels] from pre- to postexperiment during the 0% and +2%?BM trials, but the differences among beverages Na+0, Na+18, and Na+30 were not statistically significant

Which makes it a little odd that when she summarizes her conclusions, she neglects to mention that the results weren’t statistically significant:

[C]ompared with [sodium]-free beverages, consumption of beverages with [sodium] attenuates the decline in [serum sodium levels] from pre- to postexercise…

So, Baker concludes in this study funded by the Gatorade Sports Science Institute, endurance athletes should consume sports drinks containing electrolytes.

Personally, I don’t think the case is as clear-cut as Noakes suggests. After all, there are some differences between the drinks, as you can see in the graph above. But I also think he’s right to question why a study that failed to find statistically significant differences between drinks with different sodium levels would conclude that athletes should consume drinks with sodium. Maybe it’s true, maybe it isn’t — but the conclusions should follow from the data.

[Thanks to Joe Baker at York for pointing out the paper to me!]

Hydration: faster marathoners lose more weight

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My new Sweat Science columns are being published at www.outsideonline.com/sweatscience. Also check out my new book, THE EXPLORER'S GENE: Why We Seek Big Challenges, New Flavors, and the Blank Spots on the Map, published in March 2025.

- Alex Hutchinson (@sweatscience)

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A nice, simple little study was posted online at the British Journal of Sports Medicine a few weeks ago that adds some data to the big debate about hydration that I wrote about in November.

The details: 643 runners at the Mont Saint-Michel Marathon were weighed before and after the race. Sure enough, the fastest finishers lost the most weight, with those under 3:00 averaging 3.1% weight loss, compared to 2.5% for 3:00-4:00 and 1.8% for greater than 4:00. If you break it down the opposite way, those who gained weight averaged 3:58; those who lost 0-2% averaged 3:54; those who lost 2-4% average 3:47; and those who lost more than 4% average 3:40. Here’s the distribution of weight loss:

And here’s the graph of finish time versus weight change:

So what does the second graph tell us? “There was a significant linear relationship (p<0.0000001) between the degree of BW loss and race finishing time so that lesser degrees of BW loss were associated with longer finishing times. However, the predictive value of this relationship was small.” In other words, don’t head out and try to lose weight in your next marathon!

Seriously, as one just-published response has already noted, there are limits to what you can infer from a field study like this. It’s correlation, not causality, and I’m sure we can all think of many other reasons that back-of-the-packers are drinking more than the race leaders. So take it for what it is: a data point, and one that suggests that the old chestnut about greater-than-2% dehydration impairing performance needs to be rethought.

Drinking during exercise: maybe you don’t need as much as you thought

THANK YOU FOR VISITING SWEATSCIENCE.COM!

My new Sweat Science columns are being published at www.outsideonline.com/sweatscience. Also check out my new book, THE EXPLORER'S GENE: Why We Seek Big Challenges, New Flavors, and the Blank Spots on the Map, published in March 2025.

- Alex Hutchinson (@sweatscience)

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My column in today’s Globe takes on a pretty controversial topic: how much water can you afford to lose as sweat before your performance suffers? I gave a little preview of one of the studies in a blog entry a few weeks ago, but this article is a much more detailed look at some recent research suggesting that the amount of weight you lose during exercise doesn’t necessarily correspond to the amount of water you lose:

Here’s a riddle posed recently by South African scientists: A group of soldiers undertook a gruelling 14.6-kilometre march during which they lost an average of 1.3 kilograms. But sophisticated measurements with isotope tracers showed the total amount in water in their bodies actually increased by 0.53 kilograms. Where did this “extra” water appear from?

Groups such as the American College of Sports Medicine have long advocated weighing yourself before and after exercise to determine how much fluid you lost. A litre of water weighs exactly one kilogram – so by this calculation, if you’re a kilogram lighter, that means you sweated out one litre more fluid than you replaced by drinking. Lose more than about 2 per cent of your starting weight, the ACSM warns, and your performance will suffer due to dehydration.

But the South African study, published in the British Journal of Sports Medicine by researchers at the University of Pretoria, adds fuel to a simmering debate about whether weight loss during exercise corresponds to water loss. They argue that some of the weight loss is from the energy stores you burn, and that your body has “hidden” stores of water that are released during exercise – which may mean we need to rethink how we approach hydration. [READ THE REST OF THE ARTICLE…]

The print version of the article is accompanied by a fantastic graphic by Trish McAlaster that breaks down the various ways your body gains and loses water during a marathon. So far it’s not available online (I’m hoping it will be posted later), but it you have a copy of the paper around, check it out.

Dehydration and change in body mass: not linked after all?

THANK YOU FOR VISITING SWEATSCIENCE.COM!

My new Sweat Science columns are being published at www.outsideonline.com/sweatscience. Also check out my new book, THE EXPLORER'S GENE: Why We Seek Big Challenges, New Flavors, and the Blank Spots on the Map, published in March 2025.

- Alex Hutchinson (@sweatscience)

***

I (and everyone I know) have always taken this for granted: if you weigh yourself before and after a workout, the difference tells you how much fluid you lost to sweat (after correcting for any water that you drank during the workout). If you lose more than about 2% of your bodyweight, dehydration will impair your performance. That’s what the ACSM guidelines on hydration say:

If proper controls are made, BW [bodyweight] changes can provide a sensitive estimate of acute TBW [total body water] changes to access hydration changes during exercise.

But it turns out there’s actually a hot debate currently raging in the literature about this. The latest salvo just appeared online in the British Journal of Sports Medicine, from researchers in South Africa (Pretoria, not Cape Town, though Tim Noakes is indeed listed as a co-author). They studied 18 soldiers doing a 14.6-kilometre march while drinking “ad libitum” (however much they wanted), and took careful measurements of a whole series of physiological parameters. One of those parameters is “total body water” — the sum total of all water stored in the body, typically totalling about 60% of body mass — which they  measured using radioactive tracers. When we talk about hydration and dehydration, that’s what we’re really talking about: is there sufficient TBW to ensure that all the tissues and cellular processes in the body are working optimally.

The surprise: the subjects lost 1.98% of their body mass on average, but their total body water stayed roughly the same (actually, it increased by 0.53% on average). They drank 0.85 litres per hour, but sweated out 1.289 litres per hour. In other words, they were losing fluid — so how did their total body water stay the same or increase?

Some of the possible explanations are explored in this 2007 paper by Ron Maughan. One is “metabolic water”: when your body converts fat or carbohydrate into ATP, it release some water as part of the sequence of chemical reactions (one estimate is that it releases 0.13 g per calorie burned). A more significant possibility, especially for endurance athletes, is that every gram of glycogen you store ties up an estimated 3-4 g of water. A marathoner who carbo-loads and packs in 450 grams of glycogen, for example, could in theory have 1.35 kg of “hidden” water that will gradually be released into the body as carbohydrate stores are released during exercise.

So what this study claims is that these soldiers were sent out on a march and told to drink however much they wanted; they lost 2% of their body mass, but weren’t dehydrated. Their  interpretation: the body’s thirst mechanism is built to maintain the osmolality (the concentration of “stuff,” essentially) in the blood and tissues, which was indeed preserved in this experiment.

Is the debate over? Far, far from it. For one thing, a laboratory experiment at Penn State published last year found exactly the opposite — that the amount of weight you lose during exercises correlates perfectly with the loss in total body water. How to reconcile these diverging views? I’m not sure, but I’m digging into the literature and doing some interviews for an upcoming article.