I draw the line at PARPG "nutrition" but at least having a ballpark numbers is useful.
I will probably do something like:
N = some base kcal consumption per hour awake
N/5 = kcal consumed while resting/asleep
Nx3 = kcal consumed under heavy activity (combat, hiking)
X2 per hour for activity outside in extreme cold
For every 100 kcal you are below your "target" per day, your minimum fatigue goes up by 5 pts/day, and your minimum stress goes up 1pt/day. These points would be recoverable by hitting 110% of target. Recovery is 2x loss, so 10pts/fatiuge, 2 pts. stress.
See
http://wiki.parpg.net/Health_and_Damage for more details about fatigue and stress.