Tag: hydration

Posted on

Sports drinks hydrate you but water doesn’t?

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)

***

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?

Posted on

Collapsing after a race: dehydration or blood pooling?

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)

***

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).

Posted on

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

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)

***

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.

Posted on

How salty is your sweat? A home test kit

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 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?

Posted on

Artificial sweeteners can’t fool your subconscious brain

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)

***

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!]