The dominant metabolic theory of obesity is that it develops from excessive positive energy balance. However, preventive interventions based on this theory have often been ineffective. This article proffers a developmental theory, which is based on recent epidemiologic research and a new line of research dealing with differentiation of immature stem cells into mature tissues. This theory posits that the mechanical stimulation provided by vigorous physical activity (PA) causes immature stem cells to differentiate into lean tissue cells rather than into fat cells. If this new theory is confirmed by future research, it implies that interventions that involve adequate doses of vigorous PA, along with appropriate diet composition, may be especially effective in promoting development of healthy bodies. (Contains 2 figures.)