What causes obesity & overweight?

Several factors can play a role in gaining and retaining excess weight. These include diet, lack of exercise, environmental factors, and genetics. Some of these factors are discussed briefly in the following section. The National Heart, Lung, and Blood Institute offers more information on the causes of overweight and obesity.

Food and Activity

People gain weight when they eat more calories than they burn through activity. This imbalance is the greatest contributor to weight gain.

Environment

The world around us influences our ability to maintain a healthy weight. For example:

• Not having area parks, sidewalks, and affordable gyms makes it hard for people to be physically active.
• Oversized food portions increase Americans’ calorie intake, making even more physical activity necessary to maintain a healthy weight.
• Some people don’t have access to supermarkets that sell affordable healthy foods, such as fresh fruits and vegetables.
• Food advertising encourages people to buy unhealthy foods, such as high-fat snacks and sugary drinks.¹

Genetics

Research shows that genetics plays a role in obesity. Genes can directly cause obesity in such disorders as Prader-Willi syndrome.

Genes also may contribute to a person’s susceptibility to weight gain. Scientists believe that genes may increase a person’s likelihood of having obesity but that outside factors, such as an abundant food supply or little physical activity, also may be required for a person to have excess weight.²

Health Conditions and Medications

Some hormone problems may cause overweight and obesity, such as underactive thyroid, Cushing syndrome and polycystic ovary syndrome.

Certain medicines also may cause weight gain, including some corticosteroids, antidepressants, and seizure medicines.¹

Stress, Emotional Factors, and Poor Sleep

Some people eat more than usual when they are bored, angry, upset, or stressed.

Studies also have found that the less people sleep, the more likely they are to have overweight or obesity. This is partly because hormones that are released during sleep help control appetite and the body’s use of energy.¹