Make sure you are staying properly hydrated during very heavy or sustained cardio. All that water you sweat out has to come from somewhere. If you have access to a good scale, adjust your fluid intake during exercise so that your weight after and before are about equal. (A quart of water is about 2 lbs.) Read the labels, avoid so-called sport drinks that have more sugar than Coca-Cola, but you do need to replenish electrolytes. All those people who are so proud that they lost 4 lbs in an hour on the StairMaster today are fooling themselves; it's mostly dehydration, and the weight will be back in the morning. One of the less serious consequences of dehydration is that your dick may temporarily shrivel up and even stop working until you get the fluid volume back up. Having landed myself in the hospital a few times, I can vouch that there can be more serious consequences as well. (But going from 265 to 180 lbs was sort of worth it; sadly as I've aged and sustained some injuries, quite a bit of it has come back.)