On my recent 16 miler in the hot and humid Chicago summer, I lost between 5-6 pounds of water weight despite guzzling like a horse while out on the lakefront course. This is not hyperbole. The weight loss is typical for me on long summer runs and one of the water fountains is actually an old horse trough from when these were bridle paths not running paths.
I typically gain most of it back within a day or two with my overall weight slowly drifting downward as I hit peak mileage before a race. But I'm increasingly sensitive to this because I am not interested in a repeat of kidney stones due to dehydration and weight yooyooing of 6-8 pounds within a 36 hour period can't be that good for you. The good news is that this calms down dramatically as the heat and humidity fade.
Any thought about how common this is and what I can do about it?