The diet theory of obesity has some problems. For instance, the weight of *lab animals with controlled diets* is also increasing:
>As were laboratory macaques, chimpanzees, vervet monkeys and mice, as well as domestic dogs, domestic cats, and domestic and feral rats from both rural and urban areas. In fact, the researchers examined records on those eight species and found that average weight for every one had increased. The marmosets gained an average of nine per cent per decade. Lab mice gained about 11 per cent per decade. Chimps, for some reason, are doing especially badly: their average body weight had risen 35 per cent per decade.
>In fact, lab animals’ lives are so precisely watched and measured that the researchers can rule out accidental human influence: records show those creatures gained weight over decades without any significant change in their diet or activities.
http://aeon.co/magazine/health/david-berreby-obesity-era/
Fat people forced to lose weight very quickly gain it back. After they lose the weight, they have the metabolisms and behaviors of starving people. Despite having a healthy weight.
Conversely, people who artificially increase their calories to gain weight, lose the gained weight almost effortlessly afterwards.
Identical twins raised in different environments have almost identical BMIs. "Weight is more strongly inherited than nearly any other condition, including mental illness, breast cancer or heart disease." Another study I found showed the BMI of adopted children was much more correlated with their biological parents than their adopted family.
http://www.nytimes.com/2007/05/08/health/08fat.html?pagewanted=all