According to the American Medical Association, 30 percent of adult Americans are overweight. Many public health officials believe that these individuals suffer adverse health effects due to their excess weight. For example, the U.S. Department of Health and Human Services reports that obesity is associated with "an increased risk for diabetes, heart disease, stroke, hypertension, gallstones, and certain types of cancer."

Losing weight, however, has proven to be extremely difficult for large numbers of people. Diets—including the major weight- loss programs offered by Weight Watchers International, Jenny Craig, and Nutri/System—often help people shed pounds. Nevertheless, most of these dieters regain the lost pounds (many gain more than they initially lost), and some fall into a pattern of "yoyo dieting"—repeatedly losing weight only to gain it back. Researchers have suggested that one reason it is so...