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Posts Tagged ‘hydration’

Asker Jeukendrup on Gatorade and Geb

November 10th, 2011

While I was in New York earlier this week, I had a chance to chat with Asker Jeukendrup, the new “global senior director” of the Gatorade Sports Science Institute. After Amby Burfoot’s wide-ranging interview with him in September, I was curious about what that would mean both for his research and for Gatorade — which, as I wrote about in a Globe article back in 2009, had seemed to be moving away from evidence-based science and toward marketing gimmicks like adding theanine to “improve focus.”

So, after our conversation, I’m now at liberty to reveal… well, nothing. Asker promises that the Gatorade formulas will change, possibly in the short term but certainly in the medium term as new research directions yield results. What those research directions are he wouldn’t reveal, but he emphasized a shift from hydration to the full spectrum of performance nutrition. Two key focuses: energy delivery and recovery. What does this mean? His previous work on “multiple transportable carbs” (which he made famous for PowerBar) showed that the bottleneck for athletes trying to get carbs to their muscles is getting the carbs across the intestinal wall. My totally uninformed guess is that he’s looking at novel ways (different types of carbs? different molecular structures? different ratios?) of getting carbs across that barrier more quickly.

One other interesting nugget. A few months ago, I blogged about a study that included an unverified claim that Haile Gebrselassie lost 10% of his body mass due to dehydration during his world record marathon run. Jeukendrup has worked extensively with Geb — and though that data is confidential, I pressed him a bit about this claim. He confirmed that the 10% figure was reasonable. Geb’s a very small guy who’s capable of working at an inhumanly high work rate for an extended period of time. The result: he generates a huge amount of heat, and thus sweats a lot, likely more than two litres an hour. It’s not practically possible to drink anywhere near this much at world-record marathon pace, so Geb loses a ton of weight.

So the obvious question is: does this rather severe level of fluid loss hurt Geb’s performance? Jeukendrup’s guess: no. The primary mechanism through which dehydration might hurt performance is through reduced blood volume forcing the heart to work harder. But you can compensate for reduced blood volume to some degree: for example, your veins (which return the oxygen-depleted blood to the heart) contract, effectively shortening the circulatory loop. Geb’s example doesn’t prove anything either way — obviously it’s possible that he’d be a 2:02 guy if he drank more. But it’s hard to argue with a world record, unless we have some compelling reason to think that dehydration held him back (and I can’t think of one).

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Dehydration in the lab vs. the real world

November 3rd, 2011

The “right” amount of hydration during exercise is a hot topic these days. For years, lab studies showed that if you forcibly dehydrate someone and then stick them on a treadmill, their performance suffers. But more recently, “real-world” studies (like the one I blogged about here) have shown that in many cases, the fastest finishers in races tend to be the most dehydrated. The difference, according to researchers like Tim Noakes, is that in the real world your brain is able to make pacing decisions that keep your body in a safe state (using the thirst mechanism); on a treadmill at a fixed pace in a lab, your brain is cut out of the loop.

The result is that there are two bodies of research — lab and field — that appear to be answering the same question (how much should we drink?) but that produce completely different answers. So it’s nice to see a study from the “lab” camp that tries to bridge this gap by doing some experiments on an outdoor trail run. Researchers from the University of Connecticut had 14 runners perform two 12K trails runs, one in a hydrated state and the other in a dehydrated state. Here’s what they found:

Pretty straightforward, right? The runners were slower when they were dehydrated. As expected. Case closed. And we should disregard those inconvenient studies that found that faster runners in real races are more dehydrated, according to the researchers:

Although some field studies have found runners to be extremely successful despite considerable body fluid losses, these runners were not compared with a control condition where these same runners remained more optimally hydrated. Therefore, one cannot conclude that performance in these elite runners may have been enhanced if they had maintained or at least attenuated some of their fluid losses while racing.

But hang on a sec. Does this new study really offer valid “field” conditions? Not quite. It may have been conducted outdoors, on nice trails in a local state park, but it nonetheless managed to reproduce all the usual problems of lab studies. First of all, the runners weren’t freely paced: they were instructed to run at a set heart rate, which imposes a rather arbitrary limitation. So they didn’t go slower because they were unable to keep the pace, but because they weren’t allowed to increase their heart rate. More importantly, the dehydrated runners weren’t allowed to drink or eat “high water content foods” for 22 hours before the test run! The result:

So what can we conclude from these results? If you subject volunteers to a punishing dehydration regimen before your experiment even starts, their performance will suffer. This is very important to bear in mind next time you’re stranded in the desert. But as for how much water you should drink during your next run, this study has basically nothing to say.

(I should point out that the researchers did measure a number of other physiological parameters in the study, like gastrointestinal temperature. This is useful data. They also argue that holding heart rate steady is useful because it’s “similar to how a cross country or track coach may advise their athletes to maintain a certain intensity level during a run.” Sure, I guess. But if that’s what they’re measuring, then why are they using the results to make claims about what happens in real-world marathons, where nobody starts after a full day of dehydration?)

How quickly is water absorbed after you drink it?

October 19th, 2011

I’ve always been curious about this. Sometimes, after drinking a big glass of water, it seems like I pee it all out literally just a few minutes later. Is this just in my head, or is ingested fluid really processed that quickly? A new study by researchers at the University of Montreal, published online in the European Journal of Applied Physiology, takes a very detailed look at the kinetics of water absorption and offers some answers.

The study gave 36 volunteers 300 mL of ordinary bottled water, “labelled” with deuterium (an isotope of hydrogen than contains a proton and a neutron instead of just a proton) to allow the researchers to track how much of that specific gulp of water was found at different places in the body. They found that the water started showing up in the bloodstream within five minutes; half of the water was absorbed in 11-13 minutes; and it was completely absorbed in 75-120 minutes.

Here’s what the data looks like:

On the left, it shows how quickly the water was absorbed in the first hour, measured in the blood. On the right, it shows the gradual decay of deuterium levels over the subsequent 10 days, measured from urine samples. This, of course, shows that when I pee after drinking a glass of water, I’m not peeing out the same glass of water! Within ~10 minutes, fluid levels in my blood will have risen sufficiently to trigger processes that tell me to pee — but, according to this data, it takes about 50 days for complete turnover of all the water in your body.

The other wrinkle in this data is that the subjects showed two distinct absorption patterns (shown on the bottom and top), with about half in each group. In the top group, the water is very rapidly absorbed into the blood (possibly because these people get water out of the stomach and into the small intestine very quickly) before running into a slight bottleneck as the water is then distributed throughout the body to all the extremities. The second group, on the other hand, doesn’t hit this bottleneck: the flow of water out of the stomach and into the small intestine is slow enough that extra water doesn’t have a chance to build up in the blood before being distributed throughout the body.

So what does this all mean? I don’t have any particular practical applications in mind — I just thought it was kind of cool.

Extreme heat, dehydration and sodium balance

July 28th, 2011

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!

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Pre-drinking to hyperhydrate, and other heat-related research

July 25th, 2011

There’s a great (and timely) article called “Myths About Running in Heat” in the current issue of Running Times (linked to from Amby Burfoot’s latest blog entry), in which Phil Latter takes a look at six common myths relating to topics like thirst and acclimatization. It’s all good stuff, and worth a read.

The one that I hadn’t really thought about before was the idea of “hyperhydration” before running in the heat. In general, if you try to load up on fluid in the days or hours before a run, you’ll just pee it out. But Latter suggests two options. First:

Hyperhydrating, or drinking more fluid than is necessary to maintain fluid balance within the body, is effective right before an event because blood flow is severely reduced to the kidneys during exercise, thus limiting fluid excretion. “The trick then is to be able to absorb quickly and then tolerate the bloating feeling for a couple of minutes into the exercise period,” says [University of Sherbrooke exercise physiologist Eric] Goulet. “As the exercise progresses the intestine will slowly absorb the fluid, which will then be used for physiological regulation.”

Second is the idea of drinking “lightly salted water in the several hours preceding hot weather exercise” — a technique with a long anecdotal history that Goulet is currently testing in the lab:

The biggest trick, Goulet concedes, is making the substance palatable. For his trials, Goulet had the salt water ( just over ¼ teaspoon of table salt per cup) blended with Crystal Light and served at roughly 35 degrees, but adds, “You have to find what works best for you.”

One other interesting point is the idea that drinking fluids helps you deal with heat better — a claim that most people (including exercise physiologists) accept absolutely uncritically. Not everyone agrees, though:

After performing a thorough meta-analysis, Loyola University’s Jonathan Dugas, a well-known blogger on the Science of Sport website, explains why. “I’m not saying there’s no effect of fluid on body temperature, but you have to really qualify it,” he says. “The effect is really small. Maybe a half degree in temperature, maybe less.” [...]

His research suggests hydration levels have almost no effect on one’s likelihood of suffering from even the most extreme of all heat-related issues, heat stroke. [...]

“On a very hot day,” Dugas says, “no amount of drinking is going to change the fact that you’re going to go slower. You can drink up to 100 percent of your body mass, and it won’t keep you from running slower.

For practical purposes, of course, this doesn’t mean that water isn’t a special concern on hot days. The heat will make you sweat more, so you’ll need to drink more that match thirst. But it challenges the widespread assumption that when someone at a race collapses from heat stroke, one of the causes was that they didn’t drink enough water:

Current research suggests that some combination of genetic predisposition, infection, muscle damage, sleep deprivation and high levels of exertion may lead to heat stroke… water intake (or the lack thereof) isn’t mentioned.

 

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A tablespoon of water helps the exercise go on

July 7th, 2011

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

March 13th, 2011

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

February 2nd, 2011

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

November 30th, 2010

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?

November 17th, 2010

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.