Obesity is caused by consuming more calories than are burned (used up) over time. The relationship between how much a person eats and how many calories he or she burns is influenced by hereditary and environmental factors. In some cases, obesity may be caused by specific diseases or head injuries that affect the release of hormones that cause weight gain.

Environmental factors contribute significantly to obesity in people. Lifestyle changes that include eating larger portions of high-calorie food combined with less physical activity have become common in much of the industrialized world.

A well-balanced diet should supply enough calories to maintain a healthhy bodyy weight. However, in the United States and most industrial nations, the amount of food available is usually more than necessary. The wide variety of available foods often leads people to consume more calories than needed. In addition, high-calorie snack foods have replaced more nutritious fruits and vegetables in most American diets. For a complete discussion of well-balanced diets.

Physical inactivity is an important cause of obesity. Much of this inactivity is related to a modern lifestyle. Many people in industrialized countries have sedentary jobs, which involve sitting or standing still. People also engage in social and leisure activities that burn few calories. Such activities include watching television, playing video games, using computers, and traveling short distances by automobile instead of walking or bicycling.