The balance between calorie intake and energy expenditure determines a person’s weight. If a person eats more calories than he or she burns, the person gains weight (the body will store the excess energy as fat). If a person eats fewer calories than he or she burns, he or she will lose weight.

Therefore the most common causes of obesity are overeating and physical inactivity. At present, we know that there are many factors that contribute to obesity, some of which have a genetic component:

 
• Genetics. A person is more likely to develop obesity if one or both parents are obese. Genetics also affect hormones involved in fat regulation. For example, one genetic cause of obesity is leptin deficiency. Leptin is a hormone produced in fat cells, and also in the placenta. Leptin controls weight by signaling the brain to eat less when body fat stores are too high. If, for some reason the body cannot produce enough leptin, or leptin cannot signal the brain to eat less, this control is lost, and obesity occurs. The role of leptin replacement as a treatment for obesity is currently being explored.  
• Overeating. Overeating leads to weight gain, especially if the diet is high in fat. Foods high in fat or sugar (e.g., fast food, fried food and sweets,) have high energy density (foods that have a lot of calories in small amount of food). Epidemiology studies have shown that diets high in fat contribute to weight gain.  
• A diet high in simple carbohydrates. The role of carbohydrates in weight gain is not clear. Carbohydrates increase blood glucose levels, which in turn stimulate insulin release by the pancreas, and insulin promotes the growth of fat tissue and can cause weight gain. Some scientists believe that simple carbohydrates (sugars, fructose, desserts, soft drinks, beer, wine, etc.) contribute to weight gain because they are more rapidly absorbed into the blood stream than complex carbohydrates (pasta, brown rice, grains, vegetables, raw fruits, etc.) and thus cause a more pronounced insulin release after meals than complex carbohydrates. This higher insulin release, some scientists believe, contribute to weight gain.  
• Frequency of eating. The relationship between frequency of eating (how often you eat) and weight is somewhat controversial. There are many reports of overweight people eating less often than people with normal weight. Scientists have observed that people who eat small meals four or five times daily, have lower cholesterol levels and lower and/or more stable blood sugar levels than people who eat less frequently (two or three) large meals daily. One possible explanation is that small frequent meals produce stable insulin levels, whereas large meals cause large spikes of insulin after meals.  
• Slow metabolism. Women have less muscle than men. Muscle burns more calories than other tissue (which includes fat). As a result, women have a slower metabolism than men, and hence, have a tendency to put on more weight than men, and weight loss is more difficult for women. As we age, we tend to lose muscle and our metabolism slows, therefore, we tend to gain weight as we get older particularly if we do not reduce our daily caloric intake.  
• Physical inactivity. Sedentary people burn fewer calories than people who are active. The National Health and Examination Survey (NHANES) showed that physical inactivity was strongly correlated with weight gain in both sexes.  
• Medications. Medications associated with weight gain include certain antidepressants (medications used in treating depression), anti-convulsants (medications used in controlling seizures such as carbamazepine and valproate), diabetes medications (medications used in lowering blood sugar such as insulin, sulfonylureas and thiazolidinediones), certain hormones such as oral contraceptives and most corticosteroids such as Prednisone. Weight gain may also be seen with some high blood pressure medications and antihistamines.  
• Psychological factors. For some people, emotions influence eating habits. Many people eat excessively in response to emotions such as boredom, sadness, stress or anger. While most overweight people have no more psychological disturbances than normal weight people, about 30 percent of the people who seek treatment for serious weight problems have difficulties with binge eating.  
• Diseases such as hypothyroidism, insulin resistance, polycystic ovary syndrome and Cushing’s syndrome are also contributors to obesity.