I believe Benjammin got that right. I'm not sure how all the facets of metabolism work, but if you look at stories of people who were in gulags or PoW camps where starving to death was a deliberate strategy employed by the captors... It becomes pretty clear that getting any amount of food in any form is extremely important if you want to last. Every little bit helps, even if it means collecting bread crumbs or gathering insects, at least as long as it doesn't make you sick.