Quite simply, obesity is caused by eating too much and/or moving around too little. Your weight is determined by the relationship between the amount of energy you take in and the amount of energy you expend. Energy comes into your body via food and drink; the unit of energy is the calorie. Energy is used by the body in all its processes, especially moving the muscles. If you burn off all the calories you take in, your weight stays the same. If you burn off more than you consume you will lose weight. When you eat more than you burn, the excess calories are stored as fat and you both gain weight and increase the proportion of your body weight that is made up by fat. Over time, this will lead you to become overweight and then obese.

There are genetic and hormonal influences on weight as well. Some people may be more or less likely to put on excess weight than average. Ultimately, these factors all influence the balance of calorie intake and expenditure. Evidence suggests that there are only a very small number of cases of extreme obesity that are caused by genetic mutations in the system the body uses to regulate its fat levels.

Some other situations that may lead to obesity include:

• Quitting smoking : People tend to gain weight after they quit smoking because their metabolic rate (how fast the body burns calories) declines and they tend to eat a little more. However, smoking is a much greater health hazard than the extra weight you may put on by quitting.
• Pregnancy: Weight gain is necessary in pregnancy, but women sometimes have difficulty losing the excess weight after they give birth.
• Taking medications : Some medications may lead to weight gain as a side effect. Examples include some blood pressure medicines and corticosteroids.
• Specific medical problems : Having certain medical problems such as hypothyroidism or excess corticosteroid production (Cushing’s syndrome) can cause significant weight gain. Also, problems that reduce mobility such as chronic back pain or arthritis can reduce physical activity and lead to weight gain.
• Aging: The process of normal aging involves changes which lead to decreased lean weight and muscle mass, thus increasing the percentage of fat in the body.