Comment by pazimzadeh

One theory is that the most important nutrient that we really need a certain amount of every day is protein, and thus the body wants us to keep eating until it thinks we've got enough protein. (And for the vegans, I'm not saying meat - even most green plants and mushrooms are about 1/3 protein by dry weight). In nature almost every food has some amount of protein. If you get meat, you don't need to eat that much for your body to have all the protein it needs. If you are eating cake, it will take an awful lot of cake to have an adequate amount of protein.

In evolutionary past, if one had access to fresh fruit it might make sense to eat a lot of it right away since it won't keep, and the sugar in the fruit is easy for the body to store as fat and use later. In nature it's very rare to find a diet with very high fat and low protein but suppose you live by a macadamia tree, you may need to eat a lot of calories worth of macadamias to get enough protein. I have a feeling though that excess fat can go right through you in some cases like that - because there have been times where I was binging on peanut butter, like easily 16-24oz in a day often, like 2-3000 calories extra on top of my normal diet, and I didn't gain weight, I think a lot of it went through me undigested.

These are just hypotheses I'm not claiming they are necessarily the reason, and definitely are not the only mechanism involved as it's extremely complex. But they make sense as a simple place to start.

I thought this was because the protein can (partially) neutralize stomach acid. Proteins contain an amino group which can be protonated; carbohydrates and fats do not.