In the 1970s, scientists wanted to develop new rat models of obesity. This was harder than it sounded; rats ate only as much as they needed and never got fat. Various groups tried to design various new forms of rat chow with extra fat, extra sugar, et cetera, with only moderate success – sometimes they could get the rats to eat a little too much and gradually become sort of obese, but it was a hard process. Then, almost by accident, someone tried feeding the rats human snack food, and they ballooned up to be as fat as, well, humans. The book:Variety is another factor; one reason the !Kung are thin is that most of their calories come from meat and mongongo nuts. The more variety available to you, the more you eat.
Palatable human food is the most effective way to cause a normal rat to spontaneously overeat and become obese, and its fattening effect cannot be attributed solely to its fat or sugar content.
So what does cause this fattening effect? I think the book’s answer is “no single factor, but that doesn’t matter, because capitalism is an optimization process that designs foods to be as rewarding as possible, so however many different factors there are, every single one of them will be present in your bag of Doritos”. But to be more scientific about it, the specific things involved are some combination of sweet/salty/umami tastes, certain ratios of fat and sugar, and reinforced preferences for certain flavors.
Modern food isn’t just unusually rewarding, it’s also unusually bad at making us full. The brain has some pretty sophisticated mechanisms to determine when we’ve eaten enough; these usually involve estimating food’s calorie load from its mass and fiber level. But modern food is calorically dense – it contains many more calories than predicted per unit mass – and fiber-poor. This fools the brain into thinking that we’re eating less than we really are, and shuts down the system that would normally make us feel full once we’ve had enough. Simultaneously, the extremely high level of food reward tricks the brain into thinking that this food is especially nutritionally valuable and that it should relax its normal constraints.
So the basic reason we have gotten fatter since 1980 is that our food tastes better and is more varied. Plus, the main reason some people are fat and others are thin is genetics. So the best ways to stay thin are 1) be born with the right genes and 2) eat the most boring diet you can think of.
Have a nice day.