This excellent talk from Stephan Guyenet at AHS `14 describes why individuals end up with runaway obesity when eating a poor diet of highly refined, low fiber, fattening foods. He shows how a diet, and obesity itself, can disrupt our leptin feedback system responsible for regulating appetite and keeping us lean.
The talk shows how poor gut health (from diets lacking fermentable fiber) causes inflammation in the hypothalamus that results in leptin resistance. He also shows that refined and processed meals themselves immediately cause inflammation, compared to whole, unprocessed foods. Not surprisingly, exercise reduces inflammation. Finally, he shows how obesity itself inhibits the effectiveness of leptin. (It makes sense for the body to protect its fat stores. This allows people to fatten up before scarce times.)
The bottom line is that we restore leptin sensitivity once we eat whole, high fiber foods, exercise, and lose weight. These same steps also restore insulin sensitivity.
Our feedback systems naturally keep us healthy in a real food environment. You can’t really blame evolution for not handling non-stop Cheetos and Ding-Dongs all those millions of years ago.