Let's talk about finding calories in the woods for a bit. Most vitamins and minerals are easy to come by from edible wild plants but calories are a lot more difficult. There's a basic rule of thumb which states that while at rest your body will consume calories equal to ten times your body weight. If you are working hard this can jump up to twenty times your weight (find your calorie requirements here).
I'm 6'5" and 200 pounds. To meet my energy needs to make it through a hard day backpacking I'll need 20 x 200 = 4,000 calories. Food-wise how much is this?
Snickers Bar...........136 calories per oz
Peanut butter..........168 calories per oz
Whole wheat flour......97 calories per oz
Baked potato (plain)...26 calories per oz
A regular Snickers Bar is 2oz, so I'd need to eat FOURTEEN of them to fuel myself. I kind of like the sound of that, though the resulting dentistry bills would suck. What if I eat something healthier like potatoes? A large, plain baked potato weighs about 10oz, so I'd need to eat FIFTEEN of them.
I don't want to carry fifteen potatoes per day when I'm out in the woods.
So, what about wild edibles?
Acorns................112 calories per oz
Pecans................197 calories per oz
Apple..................15 calories per oz
Cattail tuber..........19 calories per oz
Blackberries...........12 calories per oz
Nuts are definitely the main source of non-animal-based calories in the wild. I would need to eat 35oz (2.2 lbs) of acorn nut meat, which means collecting somewhat more than that weight of acorns as I need to shell, crush, then extract the tannin from them before eating. Pecans do much better, I'd only need to eat 20oz of shelled pecan nut meat.
But nuts are only available for a short time. What about berries or some sort of tuber? Unfortunately, these have approximately 1/5 the calories of nuts. One of most common sources of calories in the woods are greenbrier roots. They have slightly fewer calories than potatoes so I'm going to have to dig up around ten pounds of them which is time consuming and hard. Plus they don't taste very good. Cattail tubers are easier to dig but give even less calories than greenbrier. Berries give still fewer calories, I'd need to find twenty pounds of them to meet my energy needs.
Most of us have some built-in fat reserves to get us through short periods of famine. However it does not take long for irritability, confusion, and weakness to set in when the body is deprived of its necessary calories. For more information on the effects of starvation I recommend reading They Starved So That Others Be Better Fed: Remembering Ancel Keys and the Minnesota Experiment