If it were just kids, we could blame obesity on the cutbacks in phys-ed classes, school vending machines that sell high-calorie junk, and the substitution of videogames for kickball. If it were just adults, we could blame obesity on supersizing, fast-food meals, and pedestrian-unfriendly towns that force everyone into a car. But while 68 percent of American adults qualify as overweight or obese, and 17 percent of children do (compared with 5 percent in 1971), there are other increasingly pudgy populations. Meet some overweight pets, lab animals, and even urban rats.
David Allison, an obesity researcher at the University of Alabama at Birmingham, has long criticized the Big Two explanation for America’s thundering thighs: dwindling physical activity as a result of social changes like fewer sidewalks, and increasing calorie intake as a result of nefarious food-industry marketing. By chance, he came upon a record of marmosets in a Wisconsin research colony: the little primates’ weight had soared over the previous 15 or so years, even though they had not been bred for larger size, switched chow supplier, or undergone any other change that would obviously explain their extra heft. That set Allison looking for weight records of other animals.
With colleagues, he scrutinized the weight histories of 24 populations, from alley rats in Baltimore to lab macaques in California and even control groups of mice in federal toxicology studies. In a paper to be published in Proceedings of the Royal Society B (for biology), they report that in 23 of the 24—eight species, 20,000-plus animals—the percentage of obese individuals has risen since the 1940s (or since the oldest records they found). The odds of that happening by chance are 8 million to 1. And since neither feral rats nor lab chimps nor any of the others have cut back on phys ed or patronized vending machines more, says Allison, we need to look for explanations beyond the Big Two.
No one who understands the ironclad laws of thermodynamics disputes why people gain weight: calories in > calories out. But the reasons for that inequality are almost certainly more complicated than PlayStation replacing tag and Froot Loops replacing oatmeal, or even the fattening school lunches that will be given a whole-grain-and-fruit makeover under a new federal law. For instance, fascinating new research shows that bacteria in our gut affect how many calories we extract from foods; with more calorie-extracting microbes, more of that cheeseburger winds up on your hips, while with more nonmetabolizing bugs, your annoying friend can gorge without breaking a size 6. Even Weight Watchers recognizes that not all calories are equal, and has revamped its points system. By focusing only on obvious explanations, we risk missing what might be more powerful (and interesting) influences on the caloric imbalance—influences that might bring a bigger payoff.
Which brings us to the pudgy pets, lab rodents, and wild animals Allison has chronicled. In macaques living in research colonies, average weight rose about 10 percent per decade. Chimps in labs had a 14-fold increase in obesity, with weight increasing 34 percent per decade. Pet cats: a 38 percent increase in obesity, with weight up 10 percent per decade. Pet dogs: a weight gain of just 3 percent per decade. (Good boy, Spot!) Those alley rats: a 21 percent increase in obesity. Government lab mice: weight gain of 12 percent per decade.
Food marketing, more TV, and less phys ed can no more explain these fatter animals than they can the epidemic of obesity in babies under 6 months. All these creatures live near or with people, however, which raises the intriguing possibility that common factors might explain their obesity as well as ours. Such as? Sleep debt, which increases blood levels of the appetite-stimulating hormone ghrelin and reduces levels of satiety-causing leptin. (Average sleep among U.S. adults has fallen from nine hours per night to seven.) Endocrine-disrupting chemicals such as BPA, which bind to receptors that trigger proliferation of fat cells. More central heating and AC, which means we burn fewer calories to stay warm in winter and don’t get the appetite-killing effects of sweltering in summer. Infection with adenovirus-36, which causes obesity in lab animals and is correlated with it in people.
There are numerous others. Controlled trials have already failed to show that more phys ed reduces kids’ weight (they seem to compensate by being more couch-potatoey at home). It’s time to expand the net of possible suspects in our expanding girths before it’s too late.
Sharon Begley is NEWSWEEK's science editor and author of Train Your Mind, Change Your Brain: How a New Science Reveals Our Extraordinary Potential to Transform Ourselves.