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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?

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    He has enough food to "last" till Sol 609, that means, enough to survive, not to maintain his current weight...
    – Gustavo Gabriel
    Jul 26, 2019 at 20:29

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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.

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    Additionally, he wasn't sure when he was getting rescued. What if it wasn't within those next 48 sols? He was under heavy rationing to try and eek out his food as long as possible.
    – Gamora
    Jul 31, 2019 at 12:38
  • @Gamora Mark knows exactly when he's going to be rescued... or not. Once the Hermes commits to the Pournelle maneuver, the rescue date is essentially set in stone, and Mark has no options other than to be rescued on that date, or die on (or in space near) Mars.
    – notovny
    May 15, 2022 at 15:07

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