Yeah, it certainly sucks. But you can be healthy in medical school if you make it one of your priorities. I have 3x as much to study in my 2nd year but I've been exercising much more this year simply because I have an accountability partner who pushes me when I don't feel like it. I have classmates who bike everyday and alot of our female classmates do the BATS (butts, abs, and thighs) aeorbic workout at our school gym. Looking from the outside in, it can seem impossible, but if you put exercise on your everyday todo list, then you can do it.
Eating healthy is a much harder task, especially since most of the free lunches tend to be pizza. I can honestly say that I haven't quite won the battle yet with eating healthy, but I have classmates who have. Overall, in any stressful environment you have to adapt in order to make sure that it doesn't kill you.