He has limited supplies of water. Of course he hasn't showered in his entire 561-Sol stay - it's possible (and in Mark's case necessary) to maintain hygiene while spending much, much less water than a shower does.
The current (and longstanding) recommended calorie intake for an active male adult is about 2,500 kcal
However, it is possible to survive extended periods on much less - at a cost. A 25% reduction in daily calories can cause about a 0.5kg to 1kg a week reduction in body mass. I can't remember if it's explicitly stated in the movie how much he is short on ...