EVERYONE AGREES THAT obesity results when more calories are taken in than are used up. A debate rages, however, between people who cite environmental factors as the cause of obesity and those who claim heredity is to blame. Moreover, no one understands completely why some people can eat a lot and never gain an ounce while others eat much less and get fat. Proponents of the environmental side of the argument suggest that, except for very rare genetic diseases, people get fat because they overeat. Those on the heredity side of the debate argue that people with a genetic tendency to obesity will weigh more than the accepted norm no matter how much they exercise and how carefully they watch their calorie intake.

What is fat"

Although scientists have not yet come to an...