Calories and metabolism, food and activity, fat and fuel: these are the key terms in Balestrino's explanation of why some people are fat and some are skinny. No one could argue with the conclusion: ""If he has low metabolism a fat person may never get much thinner. . . Most fat people can be thinner. They must eat less."" Understandably at this level Balestrino ignores recent research such as the discouraging evidence of fat cell counts being fixed in early childhood, but his fuzziness on the relationship between metabolism rate and activity and between consuming and burning calories can be frustrating. About all he does is introduce some of the variables traditionally considered in connection with obesity, but if he isn't as clear and certain about their interaction as we would like, perhaps it's because no one is.