According to a recent report from the World Health Organization, it can be estimated that worldwide about 250 million adults — 7 percent of the population — are obese, defined as having a body-mass index (the weight in kilograms divided by the square of the height in meters) of 30.0 or more, and that at least 500 million adults are overweight, defined as having a body-mass index of 25.0 to 29.9. More than 50 percent of adults in the United States, Canada, and some other countries have a body-mass index of at least 25.0. During the past 150 years, the . . .