Troops who are insufficiently supplied lose 1 hit point per day and suffer a cumulative -1 to attack throws and damage rolls. Furthermore, they lose the ability to heal wounds normally, though magic will still work. If troops eat enough food for a day, they regain the ability to heal, recover 1 hit point lost to hunger, and reduce by 1 any penalties to attack throws and damage rolls. Thus, troops that receive rations every other day (or half rations daily) can function almost indefinitely.
However, even if an army physically survives a lack of supply, it may not survive psychologically. Each week a unit is partially or completely unsupplied counts as a calamity and the unit must make a loyalty roll.
If an army has enough supplies to feed some of its units, but not all, the army's leader must choose which units to supply. The supplied units do not suffer any penalties, nor do they need to make weekly morale checks. However, the unsupplied units suffer an additional -1 penalty to their loyalty rolls, as it is evident that they are being left to starve while others feed.So that's a fair bit harsher than the rules from the core book - no two-day grace period (though that would be consistent with having no water, rather than just no food), and accumulating to-hit and damage penalties. The morale rolls seem more reasonable than punitive, though depending on circumstances it may still make sense for a starving unit to stick with its PC leaders exactly long enough to get home out of the wilderness. They do fill a hole I predicted in effects of starvation on morale, though, which is nice.