It is commonly theorized that some youths become fatter than others simply because they eat too much and exercise too little; i.e., they have an excessive positive energy balance. This theory suggests that obesity prevention efforts should emphasize dietary strategies designed to reduce energy intake, with moderate physical activity (PA) playing a supporting role. However, such interventions have typically been unsuccessful, perhaps because pediatric research that has measured body fatness, rather than weight, has found that the simple energy balance theory is faulty; it is critical to also consider whether the ingested energy is deposited in fat or lean tissue in order to provide a more complete picture of the etiology of pediatric obesity. In some way that is still poorly understood; vigorous PA and mechanical stimulation of the body stimulate stem cells to differentiate into bone and muscle rather than fat, with the result that ingested nutrients tend to be partitioned into lean tissue rather than fat. Thus, active youths tend to ingest more energy than their sedentary peers without increasing the percentage of the body mass that is comprised of fat tissue. Having a high level of both energy expenditure and intake is in accord with the biologic drives of youths because it encourages them to ingest sufficient amounts of the nutrients needed for healthy growth. Thus, public health interventions are likely to be more effective if they devote more attention to increasing PA and less attention to dietary strategies designed to reduce energy intake.