Originally Posted by
MoAlpha
It would have to be active transport and the article you found seems to imply an active process in the upper airway, which can dry out. That makes sense, but it's related to drying, not exercise. Incidentally paper is too old for me to be able to access the full text online, but it is irresistible to note the small sample and the huge multiple comparisons problem with the minute-by-minute measurements. It's from back in the era when I started my scientific career and boy were things easier!
I noticed the small sample also, and even though I did find the full text online I wasn't satisfied with the data, which is why I personally wouldn't critique possible problems with it. Mainly it's *so* easy now to find some study that it's also easy to be a little superficial with them. They show a change in water respiration with effort. Replicated and validated? I don't know. Ignored as sloppy and irrelevant? ditto. No one cares, I don't know either. I'm not digging into it and potentially wasting time if it's an outlier or grad thesis that no one has bothered with. But it's enough to make me think, maybe water vapor does vary with energy production.
My other problem, as we've discussed, is what that signal might be. To answer your question, yes, a meaningful amount of water is produced by carbohydrate metabolism—it can be estimated and be taken into account when computing a patient's or an athlete's water balance—but it is safe to say that it is undetectable in an exercising athlete because of the much, much, greater rate of fluid losses through sweating (which is unquantifiable too) and respiration.
The metabolic signal would be an interesting question, provided there is one. Sweating would be easy to quantify IMO - encase the athlete's body in an impermeable garment and weigh it. Respiration, not so easy. If you go that far, might as well measure CO2 concentrations.