One of the most comprehensive data sets available about Americans—how tall they are, when they last visited a dentist, what sort of cereal they eat for breakfast, whether they have to pee during the night, and, if so, how often—comes from a series of studies conducted by the federal Centers for Disease Control and Prevention. Participants are chosen at random, interviewed at length, and subjected to a battery of tests in special trailers that the C.D.C. hauls around the country. The studies, known as the National Health and Nutrition Examination Surveys, began during the Eisenhower Administration and have been carried out periodically ever since.
In the early nineteen-nineties, a researcher at the C.D.C. named Katherine Flegal was reviewing the results of the survey then under way when she came across figures that seemed incredible. According to the first National Health study, which was done in the early nineteen-sixties, 24.3 per cent of American adults were overweight—roughly defined as having a body-mass index greater than twenty-seven. (The metrics are slightly different for men and women; by the study’s definition, a woman who is five feet tall would count as overweight if she was more than a hundred and forty pounds, and a man who is six feet tall if he weighed more than two hundred and four pounds.) By the time of the second survey, conducted in the early nineteen-seventies, the proportion of overweight adults had increased by three-quarters of a per cent, to twenty-five per cent, and, by the third survey, in the late seventies, it had edged up to 25.4 per cent. The results that Flegal found so surprising came from the fourth survey. During the nineteen-eighties, the American gut, instead of expanding very gradually, had ballooned: 33.3 per cent of adults now qualified as overweight. Flegal began asking around at professional meetings. Had other researchers noticed a change in Americans’ waistlines? They had not. This left her feeling even more perplexed. She knew that errors could have sneaked into the data in a variety of ways, so she and her colleagues checked and rechecked the figures. There was no problem that they could identify. Finally, in 1994, they published their findings in the Journal of the American Medical Association. In just ten years, they showed, Americans had collectively gained more than a billion pounds. “If this was about tuberculosis, it would be called an epidemic,” another researcher wrote in an editorial accompanying the report.