Comment by InitialLastName

At some point your body will also fail to exert energy; there are also metabolic safeguards ahead of the point where your body can no longer manage the energy to keep basic functions going where the lower-priority functions (immune system, cognitive function, motion muscles) start to degrade.

You don't need to be at the limit of human energy intake to see what happens to people when their energy expenditure greatly exceeds the energy available from food over a sustained time period; there is quite a lot of medical literature on the effects of such... er... malnutrition.