In the past, many people thought that obesity was simply caused by overeating and under-exercising. Although these are significant contributing factors, doctors recognize that obesity is a complex medical problem that involves genetic, environmental, behavioral, and social factors. All these factors play a role in determining a person’s weight.

Lack of Energy Balance

Energy IN = Energy OUT  (over time) Weight stays the same
Energy IN > Energy OUT  (over time) Weight gain
Energy IN < Energy OUT  (over time) Weight loss

Inactive Lifestyle

People who are inactive are more likely to gain weight because they don’t burn the calories that they take in from food and drinks. Many people aren’t very physically active. One reason for this is that they spend hours in front of TVs and computers doing work, schoolwork, and leisure activities. In fact, more than 2 hours a day of regular TV viewing time, has been linked to overweight and obesity.

Other reasons for not being active include:

  • Relying on cars instead of walking.
  • Fewer physical demands at work or at home because of modern technology.
  • Lack of physical education classes in schools.

Genes and Family History

Overweight and obesity tend to run in families. Your chances of being overweight are greater if one or both of your parents are overweight or obese. Your genes also may affect the amount of fat you store in your body and where on your body you carry the extra fat. Because families also share food and physical activity habits, a link exists between genes and the environment.

Health Conditions

Some hormone problems may cause overweight and obesity, such as underactive thyroid (hypothyroidism), Cushing’s syndrome, and polycystic ovarian syndrome (PCOS).

  • Underactive thyroid is a condition in which the thyroid gland doesn’t make enough thyroid hormone.
  • Cushing’s syndrome is a condition in which the body’s adrenal glands make too much of the hormone cortisol or it can develop if a person takes high doses of steroids, for long periods.
  • PCOS is a condition that affects about 5–10 percent of women of childbearing age. These problems are caused by high levels of hormones called androgens.


Our environment doesn’t support healthy lifestyle habits; in fact, it encourages obesity. Some reasons include:

  •  Lack of sidewalks and safe places for recreation.
  •  Long work schedules.
  •  Oversized food portions.
  •  Lack of access to healthy foods.
  •  Food advertising.


Certain medicines may cause you to gain weight. These medicines include some corticosteroids, antidepressants, and seizure medicines. These medicines can slow the rate at which your body burns calories, increase your appetite, or cause your body to hold on to extra water. All of these factors can lead to weight gain.

Emotional Factors

Some people eat more than usual when they’re bored, angry, or stressed. Over time, overeating will lead to weight gain and may cause overweight or obesity.


Some people gain weight when they stop smoking. One reason is that food often tastes and smells better after quitting smoking. Another reason is because nicotine raises the rate at which your body burns calories, so you burn fewer calories when you stop smoking.


As you get older, you tend to lose muscle, especially if you’re less active. Muscle loss can slow down the rate at which your body burns calories. If you don’t reduce your calorie intake as you get older, you may gain weight.


During pregnancy, women gain weight to support their babies’ growth and development. After giving birth, some women find it hard to lose the weight. This may lead to overweight or obesity, especially after a few pregnancies.

Lack of Sleep

Lack of sleep increases the risk of obesity. People who sleep fewer hours also seem to prefer eating foods that are higher in calories and carbohydrates, which can lead to overeating, weight gain, and obesity. Getting less than seven hours of sleep a night can cause changes in hormones that increase your appetite. You may also crave foods high in calories and carbohydrates, which can contribute to weight gain.