Indeed, some children may not be getting enough calories. This definition does not distinguish between fat and muscle tissue or body shape although the distribution of fat affects risk. Fundamentally, an imbalance exists in the energy equation between what a person takes in and what he or she expends. Biological and behavioral factors affect this balance. Recent studies of biological factors show that multiple genes affect the body's mechanism for controlling its weight and that specific genetic defects may vary from one person to another.
More research is needed to understand the role of genetic defects.
The proportion of 9th through 12th grade students engaged in daily school physical education has declined, as has the proportion of school physical education time that students spend being physically active. Nearly two-thirds of 9th through 12th graders engage in 20 minutes of vigorous physical activity three or more times per week, while 35 percent watch television 3 or more hours every school day. Unfortunately, a sedentary lifestyle is both a cause and a consequence of overweight. Americans consume too many calories, with too many of those calories from fat, and not enough fiber. Genetic, cultural, and environmental factors influence Americans' diets.