We’re constantly being barraged with public service announcements from public figures that we all eat too much, exercise too little, and as a result society has a (sorry) growing obesity problem. However, as David Berreby points out, it’s not as simple as that:
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.
It isn’t hard to imagine that people who are eating more themselves are giving more to their spoiled pets, or leaving sweeter, fattier garbage for street cats and rodents. But such results don’t explain why the weight gain is also occurring in species that human beings don’t pamper, such as animals in labs, whose diets are strictly controlled. 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. Obviously, if animals are getting heavier along with us, it can’t just be that they’re eating more Snickers bars and driving to work most days. On the contrary, the trend suggests some widely shared cause, beyond the control of individuals, which is contributing to obesity across many species.
Such a global hidden factor (or factors) might help to explain why most people gain weight gradually, over decades, in seeming contradiction of Bloomberg’s thermodynamics. This slow increase in fat stores would suggest that they are eating only a tiny bit more each month than they use in fuel. But if that were so, as Jonathan C K Wells, professor of child nutrition at University College London, has pointed out, it would be easy to lose weight. One recent model estimated that eating a mere 30 calories a day more than you use is enough to lead to serious weight gain. Given what each person consumes in a day (1,500 to 2,000 calories in poorer nations; 2,500 to 4,000 in wealthy ones), 30 calories is a trivial amount: by my calculations, that’s just two or three peanut M&Ms. If eliminating that little from the daily diet were enough to prevent weight gain, then people should have no trouble losing a few pounds. Instead, as we know, they find it extremely hard.
Many other aspects of the worldwide weight gain are also difficult to square with the ‘it’s-just-thermodynamics’ model. In rich nations, obesity is more prevalent in people with less money, education and status. Even in some poor countries, according to a survey published last year in the International Journal of Obesity, increases in weight over time have been concentrated among the least well-off. And the extra weight is unevenly distributed among the sexes, too. 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?