The main reason that people become fat, or overweight, is taking in more calories than they use. The National Heart, Lung, and Blood Institute states that other causes of being overweight include genetic makeup, family history, health conditions, medications, age, inactive lifestyle, poor diet and lack of sleep.
Maintaining energy balance over time is the key to keeping a healthy weight. According to the National Heart, Lung, and Blood Institute, this means ensuring that the energy the body takes in equals the amount of energy it expends. The energy the body takes in refers to calories in food and drinks consumed, while energy expended includes what the body uses for breathing, digestion and physical activity.