In 3 days of hiking, the "not enough food" thing is nonsense.  When im back country hunting, living out of a tent, i survive on as few calories as possible.  The reason being is i dont want to pack a ton of heavy food with me into my camp site, every oz in the pack counts.  So despite the fact that ill be burning a lot of calories, i run off of about 1000 cal per day, with food that has high calorie to weight ratio.  Things like walnuts are great for that.  Am i hungry? Yeah.  Do i lose several lbs in a week?  Yeah.  Does it keep me from being physically able to function?  Not at all.