In the movie The Martian, why does Mark Watney lose so much weight as to literally start eating himself by Sol 561 when, according to Vincent, he has enough potatoes to last him till Sol 609, or 48 more Sols?
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 his fault intake, but Watney makes it quite clear he's not getting enough.
Even a minimal reduction in intake could cause an accumulative loss of body mass, and 560 Sols is about 80 weeks, so it's not surprising if Watney lost 20-40% (15-30kg) of his body weight, assuming he's in the healthy weight range for his height at the start of the movie.