Phamilton

I’ve commuted on several different types of bikes with several different types and widths of tires and I always get to work at about the same time.

MoAlpha

Doesn't work that way. The respiratory epithelium, where gas exchange takes place, is saturated all the time or you're dead. So, only purely physical factors, such as humidity, respiratory rate, and airflow velocity, determine the amount of vapor expired.

If you want some other reasons why you can't do it, glucose metabolism releases water, but there's also a lot of water bound in the crystalline structure of glycogen in a non-stoichiometric way, which could only be accounted for approximately. also, the amount of water diverted to urine depends minutely on hydration and sodium repletion status.

MoAlpha

You come home in a water surfeit, accumulated over your ride? Most people drink a lot and come home dehydrated.

znomit

This is true, however I find if I use slow fat tyres I'll often take the scenic river path home, which takes longer.

wphamilton

Some good points. I still think that water concentration in breath increases with exertion rate even after accounting for minute ventilation and external factors, but I realize from your remarks you would have to measure the volume of air flow. Which makes it a show-stopper for the kitchen table. Thanks for the answer.

MoAlpha

I'm pretty certain that's not true or possible, but, for the sake of discussion, what mechanism do you have in mind for it?

wphamilton

Active transport maybe? That water has to come from somewhere after all, and it's not necessarily just passive osmosis. These guys imply that that it happens https://www.ncbi.nlm.nih.gov/pubmed/3823651

My initial idea was just weigh the body (& perspiration) before and after exercise, the difference in weight could only be from whatever is respired. CO2 component would also vary, originating in the body, perhaps an immeasurable difference in weight (?) It would be calculable for a certain energy production, given an assumption about the energy pathways utilized.

My original questions were: 1) how much H2O is produced in the first place, enough to measure? and 2) *could* it be related to water in respiration? You say 2 is impossible, and I don't really have a problem with that even if I don't fully accept your reasoning. But the showstopper is all the *other* water expelled, which you can't begin to account for without knowing at least the per-minute respiration volume. So that would all be back to a real lab.

bruce19

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!

The gas exchange surface in the alveoli, is huge by comparison to the upper airway, like 70 m sq., and that's where most of your transpired water comes from. That surface is one of the body's holiest and evolutionarily conserved places, where everything is optimized and tightly regulated for gas exchange. Drying there is clinically unknown and when it gets too wet, as in pneumonia or pulmonary edema, very bad things happen, so while it's impossible to rule out, I would be very surprised if it responded to metabolic signals by changing the rate of water transpiration.

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.

Well that's too much and I'm sure I haven't convinced you on the primary question, but it's fun to think about this stuff.

wphamilton

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.

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.

Drew Eckhardt 

It has been quantified using Chung's (@rchung) virtual elevation model which outputs both CdA and Crr.

Carbonfiberboy 

Depends on the length of the ride. After a long ride, I'll pee normally after, but will then continue to pee more than normal over the next 24 hours or so. It's not just me. A long ride is over 200 miles. One really doesn't want to get dehydrated on a long ride - steady-state is best. Could be glycogen, could be accumulated sodium or some other factor. On the usual short training rides what you say is true. My experience may not apply to everyone, but the phenomena is known.