Sports drinks hydrate you but water doesn’t?

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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?

11 Replies to “Sports drinks hydrate you but water doesn’t?”

  1. Its not saying that too little CHO will slow gastric emptying, its saying that no CHO or Na will slow it. The negligible differences between 3% and 6% in emptying rate are reasonable since it is higher concentrations (>8%) that really begin to slow emptying. Gastric emptying is not controlled by osmotic rates alone, glucose speeds absorption of water compared to water alone because action of glucose transport into the blood also transports water and Na. However, if plasma volume not increase in water intake why did blood deuterium increase. If deuterium is absorbed regardless of water absorption why was it used as a marker of water absorption?

  2. This has been extensively studied by the medical community already in their search for a treatment for acute dehydration caused by cholera. It was shown that the ratio of sugar to salt is very important as well as the total amount of both in the solution. Oral rehydration therapy has been used since the 1970’s by the World Health Organization and has greatly reduced the number of deaths due to cholera. If you google Oral Rehydration Therapy or Oral Rehydration Solution you will find a lot of information about it.
    Oral rehydration solution is absorbed by the body quicker than plain water or sweetened water. The research also shows that too much sugar in the solution will reduce the blood volume because the sugar draws water into the stomach and intestines. This is a very good reason to avoid the high sugar sports drinks.

  3. @reid beloni
    Thanks for the info, Reid. I was also confused about the deuterium results (another reason I’d like to see the whole paper, if such a thing exists). The only explanation I can think of is that water passes back and forth between the gut and bloodstream in all cases, and the tonicity of the solution just determines how much stays in the blood. But I’m obviously a bit confused!

  4. @Ron
    Ron, thanks for the comment. Very timely — my med-student wife just handed me her Human Physiology text and told me to read a section called “Oral Rehydration Therapy: Sipping a Simple Solution Saves Lives” that explains precisely the mechanisms you’re talking about!

  5. I happen to know both of the researchers since they were the profs for my post grad! I’ll see if I can get my hands on the full text.

    Noa

  6. Alex- I will try and access and send the article later…

    couple of comments:
    1) difference from 3 to 6 % is very small and with this method probably cannot detect such small differences. We have data (just submitted to MSSE for review) showing most athletes in the field with use of gels, water and sports drink actually choose 10-12% solution. A shame this study did not include higher CHO% as well.

    2) results will really depend on CHO blend as well. All the original data on gastric emptying etc. used 100% glucose. There are now several studies showing that glu:fru blends emptying quicker than glu alone (search Jeukendrup AE + Moseley).

  7. Awesome — thanks, Trent! I guess in real-world situations, endurance athletes are worried about carbohydrate status as well as hydration, so they’re willing (or forced) to make trade-offs in balancing the two?

    I still can’t reconcile why Costill & Saltin (1974) found that water emptied from the gut faster than any glucose-containing solution, while this new study found that plain water didn’t increase plasma volume at all (while glucose solutions did)…

  8. Deuterium is not radioactive. The authors probably measured the deuterium concentration using IR spectroscopy. Tritium is the radioactive isotope of hydrogen.

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