If our diets alone are making us obese, why are vervet monkeys and mice overweight too?

Aeon asks some interesting questions about what’s really making us fatter – and why:

As Richard L Atkinson, Emeritus Professor of Medicine and Nutritional Sciences at the University of Wisconsin and editor of the International Journal of Obesity, put it in 2005: ‘The previous belief of many lay people and health professionals that obesity is simply the result of a lack of willpower and an inability to discipline eating habits is no longer defensible.’

Consider, for example, this troublesome fact, reported in 2010 by the biostatistician David B Allison and his co-authors at the University of Alabama in Birmingham: over the past 20 years or more, as the American people were getting fatter, so were America’s marmosets. 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. Allison, who had been hearing about an unexplained rise in the average weight of lab animals, was nonetheless surprised by the consistency across so many species. ‘Virtually in every population of animals we looked at, that met our criteria, there was the same upward trend,’ he told me.

In a study published in the Social Science and Medicine journal last year, Wells and his co-authors found that, in a sample that spanned 68 nations, for every two obese men there were three obese women. Moreover, the researchers found that higher levels of female obesity correlated with higher levels of gender inequality in each nation. Why, if body weight is a matter of individual decisions about what to eat, should it be affected by differences in wealth or by relations between the sexes?

Sleeplessness and stress, for instance, have been linked to disturbances in the effects of leptin, the hormone that tells the brain that the body has had enough to eat. What other factors might be at work? Viruses, bacteria and industrial chemicals have all entered the sights of obesity research. So have such aspects of modern life as electric light, heat and air conditioning. All of these have been proposed, with some evidence, as direct causes of weight gain: the line of reasoning is not that stress causes you to eat more, but rather that it causes you to gain weight by directly altering the activities of your cells.

According to Frederick vom Saal, professor of biological sciences at the University of Missouri, an organic compound called bisphenol-A (or BPA) that is used in many household plastics has the property of altering fat regulation in lab animals. And a recent study by Leonardo Trasande and colleagues at the New York University School of Medicine with a sample size of 2,838 American children and teens found that, for the majority, those with the highest levels of BPA in their urine were five times more likely to be obese than were those with the lowest levels.

BPA has been used so widely — in everything from children’s sippy cups to the aluminium in fizzy drink cans — that almost all residents of developed nations have traces of it in their pee. This is not to say that BPA is unique. In any developed or developing nation there are many compounds in the food chain that seem, at the very least, to be worth studying as possible ‘obesogens’ helping to tip the body’s metabolism towards obesity.

[via Jen Dornan-Fish]

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