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.
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?