by DAVID BERREBY
As the American people got fatter, so did marmosets, vervet monkeys and mice. The problem may be bigger than any of us
Moral panic about the depravity of the heavy has seeped into many aspects of life, confusing even the erudite. Earlier this month, for example, the American evolutionary psychologist Geoffrey Miller expressed the zeitgeist in this tweet: ‘Dear obese PhD applicants: if you don’t have the willpower to stop eating carbs, you won’t have the willpower to do a dissertation. #truth.’ Businesses are moving to profit on the supposed weaknesses of their customers. Meanwhile, governments no longer presume that their citizens know what they are doing when they take up a menu or a shopping cart. Yesterday’s fringe notions are becoming today’s rules for living — such as New York City’s recent attempt to ban large-size cups for sugary soft drinks, or Denmark’s short-lived tax surcharge on foods that contain more than 2.3 per cent saturated fat, or Samoa Air’s 2013 ticket policy, in which a passenger’s fare is based on his weight because: ‘You are the master of your air ‘fair’, you decide how much (or how little) your ticket will cost.’
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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.
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To make sense of all this, the purely thermodynamic model must appeal to complicated indirect effects. The story might go like this: being poor is stressful, and stress makes you eat, and the cheapest food available is the stuff with a lot of ‘empty calories’, therefore poorer people are fatter than the better-off. These wheels-within-wheels are required because the mantra of the thermodynamic model is that ‘a calorie is a calorie is a calorie’: who you are and what you eat are irrelevant to whether you will add fat to your frame. The badness of a ‘bad’ food such as a Cheeto is that it makes calorie intake easier than it would be with broccoli or an apple.
Yet a number of researchers have come to believe, as Wells himself wrote earlier this year in the European Journal of Clinical Nutrition, that ‘all calories are not equal’. The problem with diets that are heavy in meat, fat or sugar is not solely that they pack a lot of calories into food; it is that they alter the biochemistry of fat storage and fat expenditure, tilting the body’s system in favour of fat storage. Wells notes, for example, that sugar, trans-fats and alcohol have all been linked to changes in ‘insulin signalling’, which affects how the body processes carbohydrates. This might sound like a merely technical distinction. In fact, it’s a paradigm shift: if the problem isn’t the number of calories but rather biochemical influences on the body’s fat-making and fat-storage processes, then sheer quantity of food or drink are not the all-controlling determinants of weight gain. If candy’s chemistry tilts you toward fat, then the fact that you eat it at all may be as important as the amount of it you consume.
More importantly, ‘things that alter the body’s fat metabolism’ is a much wider category than food. 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. If some or all of these factors are indeed contributing to the worldwide fattening trend, then the thermodynamic model is wrong.
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