Within all human hierarchies those lower in the hierarchy have poorer health than those higher up. In high income countries poor people are fatter than rich people, and, although the gradient did initially go the other way in low income countries, it’s now true in them as well. But why? Monkeys can help answer this question, said Carla Moore, a researcher from Emory University at the World Congress of Prevention of Diabetes and its Complications in Madrid earlier this week.

One possibility that poor people are fatter is because they live in more obesogenic environments, where fattening foods are cheaper and more available. Another possible explanation is that poorer people are more stressed than richer people and that the stress induces overeating. This is the notion of “comfort foods,” which we all recognise. A systematic review conducted by Moore has shown that poorer people have not only higher body weights, but also higher levels of stress and unhealthier diets. This is especially true for women.

But there are methodological difficulties. The systematic review included 14 studies, but 13 were cross sectional, making it difficult to draw inferences about causation. The tools for measuring diet were qualitative and imprecise, and there is variation in how social position is identified—by income, education, occupation, and others means.

These methodological problems can be avoided in studies of monkeys. There are also other advantages, said Moore: the genetic code of monkeys is 93% the same as humans; the monkeys exhibit complex behaviour like humans and have similar neuroanatomy and neurophysiology (unlike rodents). A great advantage that monkeys have over humans is that they don’t smoke, drink alcohol, or use technology—all confounders of the relationship between position in the hierarchy and body weight.

Moore has thus conducted studies in colonies of Rhesus monkeys, which have a rigid hierarchy. Every monkey has its position, which can be easily determined by researchers. Monkeys lower in the hierarchy give way to those higher up and have less control over their lives and less access to resources.

The studies are conducted in colonies that include only females, and when the monkeys are eating the standard laboratory diet, which is low fat and low sugar, then the monkeys that are lower in the hierarchy weigh less than those higher in the hierarchy. It’s possible to measure the activity of the monkeys by attaching accelerometers to their necks, and there seems to be no important difference between the higher and lower status monkeys. Differences must be something to do with food intake.

Moore then went on to study the effects of stress and diet on the colony of monkeys where stress levels were increased by giving the monkeys less space and not allowing them to go outside. For this experiment automated feeders were used that allowed detailed measurement of the food intake of each monkey. When the monkeys were fed a high fat, high sugar, low fibre diet the lower status monkeys ate much more than the higher status monkeys. Interestingly when the monkeys were returned to the normal diet the low status monkeys still ate more than the high status monkeys, which hadn’t been the case before the high fat, high sugar, low fibre diet had been introduced.

This was a study with only 10 monkeys over two weeks, which was not long enough to see changes in body weight, and Moore now plans an 18 week study with 48 monkeys that as well as studying chronic stress will also look at the effect of short term stress.

The results so far do support the theory that poorer women who are stressed (and poverty is stress) and in obesogenic environment will eat more and may have their physiologies reset to continue to eat more. I have a feeling that women living in the East End of Glasgow would not be surprised by these findings.

Richard Smith was the editor of the BMJ until 2004 and is director of the United Health Group’s chronic disease initiative.

Competing interest: RS was speaking in the same session as Carla Moore only on climate change and the prevention of diabetes. His expenses to travel to Madrid were paid by his employer, the UnitedHealth Group.