Originally Posted by
Carbonfiberboy
IME blood volume and tissue H2O storage increases, and then you pee it out over the next day or two if there's been enough riding and effort time to accumulate a serious amount of H2O. And of course the sweat and respiration losses, as you say. That's not precise of course, just IME. There's also the water released by glycogen breakdown, which I think accounts for a lot of that pee.
This is all about measuring the glycogen breakdown as I see it. I figured there'd be a blood volume increase and some would make its way to the bladder, amounts which you couldn't really measure very easily. But if say 10% of it exits in respiration, and you could weigh the difference, then those other accumulations wouldn't change the before/after weight and you could multiply by 10. Or whatever the formula might be. We breathe out water vapor for sure, but I don't know if there is a reliable curve relating water produced and water respired.