I personally think that what seems in the light of modern affluent societies to be an incomprehensible and nearly irresistable desire for fats, salt, and sugar was at one time highly adaptive. After all, for Paleolithic man, fats for instance were not available for the price of a bag of potato chips at the corner store; fats were a relatively scarce and incredibly valuable source of not only highly concentrated, precious calories, but also essential fatty acids and fat soluble vitamins. Pretty hard to come by for Paleolithic man and woman, and at least in the case of animal fat requiring a considerable expenditure of energy to obtain. Unfortunately for moderns living in affluent idustrialized societies fats are no longer taken at the cost of a hard day's hunt; sugar is no longer found in fruits which ripen only seasonally, and salt is not found only in rare mineral form or in tissue salts in prey animals- these can all be consumed, and what's more in very refined pure forms, without what would have been in nature a complement of accompanying vitamins, fibers, et cetera, by virtually anyone. The sedentary nature of most occupations simply serves to excacerbate the problem. Another case, unfortunately, of homo sapiens
being too smart for its' own good.