There are two reasons behind this. One is to avoid muscle and bone turning into brittle mush due to lack of gravity. The other is a wacky phenomenon called orthostatic intolerance. On Earth, it's the reason you sometimes feel dizzy when standing up too fast. Off Earth, it graduates to being the reason your slacker ass would freaking hate space. Your body is normally able to deal with orthostatic intolerance by raising your heart rate and blood pressure until you get a grip ... but in space, your heart is a goddamn p***y. Realizing it doesn't need to do the whole "fighting against gravity" crap, it happily gets smaller and weaker, and your blood pressure drops. When orthostatic intolerance hits in a situation like that, you'll pass out. A lot.
Some of you may adapt better than others.
Your only remedy? Constantly hitting the gym. You have to exercise a good 2.5 hours a day, every day, as long as you're out there. It's not the cool kind of exercise, either; no, that s**t is all cardio, all the time. There will be no idle ogling of asses while "sipping from your water bottle" -- you'll spend those hours strapped into one of the three exercise contraptions available (a gym cycle, a treadmill, and a space Chuck Norris Total Gym called ARED), forever grinding in a tiny room that is filled with cables and presumably smells like jock straps.
Imagine an RV full of dirty gym clothes and the nearest fresh breeze is 200 miles away.