Posts Tagged ‘hydration’

Sports drinks hydrate you but water doesn’t?

November 4th, 2010

There’s an interesting abstract in the November issue of the British Journal of Sports Medicine on how well various drinks hydrate you. We’re talking purely hydration here: how fluid is absorbed and how much blood volume expands, not about whether you get extra energy and so on.

Very simple experiment: have the volunteers drink 500 mL of either water, 3% carb drink, or 6% carb drink (the “standard” sports drinks on the the market are about 6% carb). Use a radioactive stable isotope tracer (deuterium oxide) to follow where the ingested fluid goes, and take blood samples before (two samples) and after (eight samples over the following hour). The results: there was no difference in the carb drinks — both of them increased blood and plasma volume. In contrast, plain water DIDN’T increase blood or plasma volume. The explanation:

This is likely to be due to the sodium and carbohydrate content of these drinks.

Okay, I have to admit I’m a little confused. We know that too much carb (>6% or so) or sodium in a drink will slow the rate at which water empties from the stomach. Now this result is saying that too little will also slow it. This seems plausible, given that osmosis dictates the rate of gastric emptying — though it’s then strange that there was no difference between the two carb drinks. I have a couple of other questions:

1) If they’d kept taking blood samples for longer than an hour, would the blood volume of the water drinkers eventually have increased? Or is there some other route for the water to exit? (I find it hard to believe that they’re going to get diarrhea from drinking pure water.)

2) How did the plasma osmolality of the subjects change? That’s what some researchers believe is the key marker of hydration, as opposed to simply blood volume.

Part of the reason I don’t have the answer to these questions is that I’ve only seen the abstract to this paper. It’s in the “electronic pages” of the current BJSM issue, and I can’t for life of me figure out if there’s a full paper, and if so how I get it. Anyone who knows the answer (to the questions above, or simply to how to get the paper!), please let me know.

UPDATE 11/04:

Okay, some helpful comments below… but I’m still confused. The reason this result jumped out at me, I think, is that I’ve been looking through some of the old literature on hydration for a forthcoming article. So, for instance, I was reading Costill and Saltin’s 1974 article in the Journal of Applied Physiology, “Factors limiting gastric emptying during rest and exercise,” which says right in the abstract “At rest the addition of even small amounts of glucose (> 139 mM) induced a marked reduction in the rate of gastric emptying… These data demonstrate the importance of minimizing the glucose content of solutions ingested in order to obtain an optimal rate of fluid replacement. In combination with high-intensity exercise even small amounts of carbohydrate can block gastric emptying.” There are a whole bunch of studies with similar findings; here’s one from 1988 that found that plain water emptied faster than a variety of glucose concentrations while cycling.

On the other hand, I’ve certainly heard lots about how isotonic solutions are most quickly absorbed. How do I reconcile these two sets of data? Is it in the difference between gastric emptying and plasma volume expansion? Where else does the water go?

Collapsing after a race: dehydration or blood pooling?

September 15th, 2010
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Watch the finish line at big endurance races, and you’ll see plenty of people staggering to a halt and then wobbling, looking dizzy, and perhaps even collapsing if race volunteers don’t get there in time to usher them to the medical tent. What’s going on here? Why are they collapsing after the race, and how should they be treated? A new paper posted online at the British Journal of Sports Medicine takes a look at the two leading theories: (1) they’re dehydrated and overheated, or (2) the blood vessels in their extremities are dilated, so when they stop exerting themselves, the blood pools there and leads to shortage of blood going to the brain.

The right treatment depends on which of those theories you believe. If it’s dehydration, you give IV fluids. If it’s blood pooling, you put the athlete in the “Trendelenburg position” (lying down with legs elevated above heart) and let them drink as much as they want. Researchers from the University of Cape Town conducted a randomized clinical trial at the 2006 and 2007 South African Ironman Triathlons and the 2006 Comrades Marathon, randomly assigning 28 collapsed athletes to the two treatment protocols based on whether they had an even or odd race number.

The results: no statistically significant difference between the protocols: 52 minutes before discharge for the IV group, 58 minutes for the non-IV group, with a standard deviation of 18 to 23 minutes. So this doesn’t really settle the debate about what’s causing it — but, applying the “simple is better” principle, it suggests that you don’t really need an IV if you’re dizzy at the end of a race (but don’t have, say, a temperature above 40C).

Sports drinks don’t help for one-hour exercise (unless you’ve been fasting)

April 29th, 2010

If you’re running a marathon or cycling for several hours, you need to ingest some carbohydrates during the session to maintain your performance. If you’re sprinting for 100 metres, on the other hand, you can leave the buffet belt at home. Where things get murky is the middle ground, for sessions lasting about an hour: several decades of research have produced lots of conflicting results. A study in the April issue of the Journal of Sports Sciences, by researchers from Loughborough University in Britain, offers some new insight.

The basics: 10 trained runners did two trials in which they ran as far as they could in one hour (they used a neat treadmill that automatically speeds up if you move towards the front of the belt, and slows down if you lag towards the rear, so it was a freely paced trial). In both cases, they ate a high-carbohydrate meal three hours before the run. They drank either a sports drink containing 6.4% carbohydrates or a placebo before and during the run. The results: no difference whatsoever in performance, blood glucose, lactate, respiration, carbohydrate burning, perceived exertion, or anything else they measured.

What’s interesting is that the same group published a similar paper last year, where the only difference is that the runners were fasted before the trial instead of having a meal three hours prior. In that case, the sports drink group significantly outperformed the placebo group.

So it seems pretty clear: you only need supplemental carbs for a one-hour exercise bout if you haven’t topped up your carb supplies beforehand. There are two ways your body stores glycogen: in your muscles (which is then used exclusively by your muscles), and in your liver (which feeds glucose into your bloodstream to fuel your heart and brain and keep blood sugar levels stable). When you sleep overnight, your muscle glycogen stays relatively stable, but your liver glycogen drops by more than 50 percent (because your brain and heart are still running all night). So the researchers believe that, if you don’t have a pre-exercise meal, the sports drink is needed to make up for your depleted liver glycogen stores.

Practically speaking, this means you don’t need to worry about carbs during short exercise bouts in the afternoon or evening, since you’ll have had a meal or two. In the morning, though, you need to make sure your liver glycogen is restocked, even for a short one-hour run. You can do that by getting up early enough to eat beforehand — or, apparently, by consuming some carbs immediately before and during your session.

One other interesting note from the paper. They discuss the performance boost that comes from “rinsing and spitting” with a sports drink, which some researchers have suggested could explain why sports drinks sometimes help with short exercise sessions even when muscle glycogen stores are full. Interestingly, a couple of recent papers suggest that the same conditions also apply in this case: you get benefit from rinse-and-spit if you’ve fasted beforehand, but no benefit if you had a pre-exercise meal. That suggests that your brain is monitoring levels of carbohydrate throughout your body, and only responds positively to the carb stimulus if your body actually needs it:

Although speculative, the idea of central monitoring of whole-body carbohydrate status, which in turn influences the self-selection of exercise intensity, is worthy of further investigation.


How salty is your sweat? A home test kit

March 17th, 2010

I just noticed, a little belatedly, that an article I wrote for the March issue of Canadian Running magazine is now available online. It describes my experiences with a home sweat analysis kit from Medion Corporation, and compares the results to a laboratory sweat test I did with Lawrence Spriet of the University of Guelph and the Gatorade Sports Science Institute.

In the name of science, I dabbed some shaving cream on the back of my leg and scraped clear a fist-sized area of bare skin. I was about to undertake a new home sweat test to find out exactly how much salt my sweat contains, and I needed to make sure the absorbent patches would stay glued to my skin once the fluid started to flow…

It was an interesting experience, and the home test kit was pretty neat (though a little pricey at $250). It measured a lower sodium concentration than I got in the lab test, something that I think may be due to the fact that I was dehydrated before the lab test (I cycled for 40 minutes in hot sun just to get to the lab).

Overall, I’m not sure what to make of this information. I’ve just been reading The Runner’s Body, the book by the Science of Sport bloggers, and they argue that the theory linking electrolyte loss to muscle cramps is mistaken (a topic I’m looking forward to digging into a little more deeply). Personally, I’ve generally focused on (relatively) shorter distances, so my training runs don’t tend to be multi-hour affairs — which means I’ve never really worried about electrolytes. But for marathoners and triathletes out there, is knowing how salty your sweat is a piece of information that would help you plan your hydration strategy?

Artificial sweeteners can’t fool your subconscious brain

January 5th, 2010

Here’s a mystery: Why is obesity still such a problem in the age of the magic zero-calorie sweetener? New Scientist has a great article on the latest brain-scanning research, which offers some hints about how these sweeteners may fool us on a conscious level, but don’t manage to trick our unconscious minds. These new studies suggest that “zero-calorie” options may really just lead to “deferred calories” that make us consume more than a full-sugar version would have.

For many years, there have been hints that people who drank sugar-free sodas ended up gaining more weight than those who didn’t. (Travis Saunders described some of this evidence at Obesity Panacea last year.) Guido Frank at the University of Colorado is one of the researchers whose studies help explain this. He fed drinks containing either sucrose (sugar) or sucralose (artificial sweetener) to subjects, who were unable to tell the difference between the two. However:

Sucrose produced stronger activation in the “reward” areas of the brain that light up in response to pleasurable activities such as eating and drinking. Sucralose didn’t activate these areas as strongly… Frank suggests that sucralose activates brain areas that register pleasant taste, but not strongly enough to cause satiation. “That might drive you to eat something sweet or something calorific later on,” he says.

This is still a developing area of research, but it seems highly likely that there’s no (calorie-)free lunch. You can’t have sweetness without (eventually) paying a caloric cost.

The obvious question, then, is whether you’re better off drinking diet soda or full-sugar soda. I’ll join with Travis Saunders in suggesting that you keep consumption of either to a minimum (though, as with most “bad” foods, it should be fine in moderation). But if I’m choosing between the two, now that I know that the overall caloric hit will be about the same for regular and diet soda, I’d rather drink the real thing.

[Thanks to Selam for the tip!]

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NYT: hiking the Larapinta Trail in the Australian outback

November 28th, 2009

Another slightly off-topic post… An article describing my recent hike along the Larapinta Trail, a fantastic route through the empty desert west of Alice Springs in Australia’s “Red Centre,” will run in this Sunday’s New York Times travel section. Actually, it’s not that off-topic — for this hike, we had to pay very close attention to factors like hydration (a frequent Sweat Science topic), since there was literally not a drop of water available other than occasional rainwater tanks. Along some sections of the trail, the distance between tanks was a nine- or ten-hour hike at a pretty fast clip. We met some hikers who couldn’t make it that far in a single day, and were thus forced to carry enough water for two full days with them (and that includes cooking water).

Anyway, it was a great hike — a chance to see some unique and inaccessible landscape, and a real test of endurance. The story is online here.


Slushies: the new weapon for exercising in heat

September 25th, 2009
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Reading up on Australian sports research for an upcoming magazine story, I came across this little nugget about dealing with competition in hot conditions. The Aussies have been leaders in research on “pre-cooling” to lower body temperature before starting extended exercise in the heat. They introduced ice vests at the 1996 Olympics (which have since become widely used commercial products), and in 2004 brought big bathtubs full of ice-water to the Athens Olympic venues, actually immersing their endurance athletes shortly before their competitions.

I can’t imagine the pre-race ice bath becoming a really widespread phenomenon, for many reasons including logistical ones. But for the Beijing Olympics in 2008, the Aussies unveiled a new idea: slushies! As Louise Burke, head of sports nutrition at the Australian Institute of Sport, explained at a conference in Switzerland last fall, ingesting crushed-ice drinks cools the athletes internally — not just with the coolness of the ice, but with the energy of the phase transition as it melts. In tests, the Aussies found their athletes lowered their body temperature by about one degree Celsius by drinking the slushies (which were filled with “a mix of carbohydrates, electrolytes, and ice with other secret ingredients“). So the Australian Olympic team brought seven slushie machines to Beijing.

Other interesting points from Burke’s talk in Switzerland: The ice baths actually lowered body temperature more than the slushies, but this turned out to be a negative. In cycling time-trial tests, the athletes felt so good after the ice baths that their internal pace regulation was messed up, so they started too fast and paid for it late in the race. The slushie-fed athletes, on the other hand, started a little slower but ultimately performed better.

And the other reason the Aussies introduced slushies in 2008? They wanted to have something new, Burke says, to elicit a placebo response in their athletes.


Homemade sports drinks

September 14th, 2009

A few months ago, I promised to look into how to make your own sports drink at home. It has taken me a while to follow up, but I thought I’d pass the following along. From the book “Nancy Clark’s Sports Nutrition Guidebook,” as cited in a New York Times blog entry:

1/4 cup sugar
1/4 teaspoon salt
1/4 cup orange juice
1/4 cup hot water
2 tablespoons lemon juice
3 1/2 cups cold water

In a quart pitcher, dissolve the sugar and salt in the hot water. Add the remaining ingredients and the cold water. The drink contains about 50 calories and 110 mg of sodium per 8 ounces, approximately the same as for most sports drinks.

Drinking too much during marathons (hyponatraemia): an update

September 9th, 2009
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Just noticed a University of London preprint that has been accepted for future publication in the British Journal of Sports Medicine that looks at incidence of hyponatraemia in the 2006 London marathon. This topic has received a fair amount of attention in the past few years (justifiably, since at least five people have died recently in the U.S. and Britain, according to the paper), but there are a couple of new wrinkles in this paper.

First of all, this wasn’t your typical hot marathon where people are pouring fluids down their throat with abandon — the 2006 London race was held in “wet, rainy conditions with air temperature 9-12 [degrees] C.” Still, 11 of the 88 runners studied developed “asymptomatic hyponatraemia,” as diagnosed by low sodium levels. They didn’t have any negative effects — or any symptoms at all, actually — but they were on the border, supporting the contention (the authors claim) that hyponatraemia is underdiagnosed.

As expected, the hyponatraemia sufferers drank more (every mile, most commonly, compared to every second mile for the non-sufferers), and they put on weight during the marathon on average, while everyone else lost weight. But there were some anomalies: four of the hyponatraemics actually lost weight, but still somehow ended up overhydrated. It’s not clear how this happened, though the researchers speculate about “inappropriate antidiuretic hormone (ADH) release during exercise causing altered renal function and secondary fluid retention.”

So what do we take from this? Well, it’s hard to get too worried about an asymptomatic condition that doesn’t cause any problems (though of course if they persist into the symptomatic regime, they risk serious problems). On the other hand, these results tell us that quite a few people are still chugging water well beyond their needs. So maybe it’s worth bearing in mind the words of Tim Noakes, the respected South African sports scientist who has been stirring up dissent about our current obsession with proper hydration: “If you are thirsty, drink; if not, do not,” he wrote in 2007. “All the rest is detail.”

The bare facts about heat stroke

July 30th, 2009

In case summer ever decides to start, here’s an article on heat stroke from Gretchen Reynolds of the New York Times. The best line, relating to how heavy clothing can increase risk:

ā€œIā€™m all in favor of naked practice sessions,ā€ [University of Connecticut researcher Douglas] Casa says. Unfortunately, sunburn also is thought to have an impact on your abilitity to dissipate heat.

Other than that, nothing particularly surprising in the article — unfortunately, there’s no quick fix or miracle cure for heat stroke. It’s a matter of caution, acclimating to hot weather (especially if, say, an unusually cold and wet summer has limited your exposure to hot days), and looking out for warning signs like dizziness and confusion. Another interesting point: while hydration is important, it’s perfectly possible to get heat stroke even if you’re fully hydrated.