"Animal experiments show that when rodents first consume a sugary food the brain releases dopamine, a chemical also involved in drug addiction. It signals pleasure and helps drive motivation. The more scrumptious the food, the more dopamine is produced. The same reaction is found in humans. But over time a glut of sugary foods seems to change the brain’s circuitry. Obese people become conditioned to getting excited by the sight of yummy food. Just a glance at a fried Oreo can trigger higher activity in the frontal cortex (linked to reward and motivation) of a fat person than it would in those of normal weight. At the same time fat people seem to have fewer dopamine receptors. This is a dangerous combination—they get more worked up by the prospect of junk food, but also get less pleasure from eating it. This may drive compulsive overeating. And as an individual gets fatter, levels of leptin, the fullness hormone, rise so much that the brain seems to stop responding to it. When he starts to lose weight, leptin levels drop and the brain signals that he is starving, even if he still has plenty of fat to spare."
It is what I simply detailed in a presentation named:
Is obesity a reward?
"Yet not everyone is obese. Studies of siblings and twins show that the propensity for getting fat is largely determined by genes. In the 1980s Rudolph Leibel of Columbia University and his colleagues were among the first to identify some genes that influence the way the brain regulates food intake. Since then genetic analyses have revealed many more genes involved in this process. Genetics does not guarantee that someone will get fat, says Philip James of the International Association for the Study of Obesity, nor does it explain the dramatic rise in the number of fat people in recent decades. But genes do help explain why, given similar conditions, some people are fatter than others."
In the debate of nature vs nurture keep this in mind...
"“The evolutionary process has led to a gene pool that is designed to defend body weight against falling below a minimum,” Dr Leibel explains, “but not to defend against gain.”"
"All this biology, however, cannot explain one notable feature of the obesity epidemic. The rich and well-educated have mostly managed to stay slim, or at least have got less fat than the less well off. In America obesity rates in children with college-educated parents are less than half the rates of children whose parents lack a high-school degree. Marion Devaux and her colleagues at the OECD, a rich-country think-tank, found an inverse relationship between education levels and the likelihood of getting fat in Australia, Canada and England. The picture is slightly different in poor and middle-income countries, where the more affluent urbanites are usually the first to put on weight. As the economy develops, however, the poor become obese in disproportionately large numbers."
Sorry but I disagree you did not mention epigenetics. It is probable that the poor are more epigenetically oriented toward food consumption and storage by their own history it was the core of the thrifty gene hypothesis.