Weight gain occurs when you eat more calories than your body uses up. If the food you eat provides more calories than your body needs, the excess is converted to fat. Initially, fat cells increase in size. When they can no longer expand, they increase in number. If you lose weight, the size of the fat cells decreases, but the number of cells does not.
- Obesity, however, has many causes. The reasons for the imbalance between calorie intake and consumption vary by individual. Your age, gender, genes, psychological makeup, and environmental factors all may contribute.
- Genes: Your genes may play a role in efficiency of metabolism and storage and distribution of body fat.
- Family lifestyle: Obesity tends to run in families. This is caused both by genes and by shared diet and lifestyle habits. If one of your parents is obese, you have a higher risk of being obese.
- Emotions: Some people overeat because of depression, hopelessness, anger, boredom, and many other reasons that have nothing to do with hunger. This doesn't mean that overweight and obese people have more emotional problems than other people. It just means that their feelings influence their eating habits, causing them to overeat.
- Environmental factors: The most important environmental factor is lifestyle. Your eating habits and activity level are partly learned from the people around you. Overeating and sedentary habits (inactivity) are the most important risk factors for obesity.
- Sex: Men have more muscle than women, on average. Because muscle burns more calories than other types of tissue, men use more calories than women, even at rest. Thus, women are more likely than men to gain weight with the same calorie intake.
- Age: People tend to lose muscle and gain fat as they age. Their metabolism also slows somewhat. Both of these lower their calorie requirements.
- Pregnancy: Women tend to weigh an average of 4-6 pounds more after a pregnancy than they did before the pregnancy. This can compound with each pregnancy.
- Certain medical conditions and medications can cause or promote obesity, although these are much less common causes of obesity than overeating and inactivity. Some examples of these are as follows:
- Obesity can be associated with other eating disorders, such as binge eating or bulimia.
- The distribution of your body fat also plays a role in determining your risk of obesity-related health problems. There are at least two different kinds of body fat. Studies conducted in Scandinavia have shown that excess body fat distributed around the waist ("apple"-shaped figure, intra-abdominal fat) carries more risk than fat distributed on the hips and thighs ("pear"-shaped figure, fat under the skin).
Medically Reviewed by a Doctor on 12/22/2014
Gayle M. Galletta, MD, FACEP
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