Dieting and concern about body image are an American pastime. At any given moment, some 50 million Americans are on a weight-loss regimen. Although few of those who wish they were thinner suffer from a true psychiatric eating disorder, the number of individuals who have either of the two major forms of eating disorders - anorexia nervosa and bulimia - has risen dramatically in recent decades.

Obesity is sometimes considered a third eating disorder. Technically, it is not viewed as a true...