In many areas there is no food to be foraged upon when you need it. MANY calories are spent on foraging.
One thing I stress heavily in my classes is just how hard it is to acquire enough calories by foraging. Vitamins, minerals, antioxidants, etc are easy even in some pretty desolate places but calories can be close to impossible. Burning lots of calories to find calories is a negative-sum game.
You are better off keeping a bag of hard candies in your pocket at all times.
-Blast