A combination of poor diet and lack of exercise is the main cause of obesity. This typically involves a high-calorie, highly processed diet combined with a sedentary lifestyle.

Portion sizes have ballooned in restaurants, homes and schools. Many people favor sugars, starches and saturated fats over vegetables, fruits, high-fiber whole grains, lean protein and monounsaturated fats. In addition, fewer than one-third of Americans meet basic levels of physical activity, often defined as at least 30 minutes a day most days of the week.

However, the development of obesity is complex, and there are many reasons, including environmental and inherited factors, that can lead to a person becoming obese. Some of the most common are:

• Alcohol. Consuming too much alcohol can lead to obesity because alcohol is high in calories without offering nutritional benefit.
  
  
• Medications. Drugs that can cause weight gain include insulin, some antidiabetic agents (sulfonylureas, thiazolidinediones, meglitinides) corticosteroids and some medications used for psychiatric conditions.

• Eating disorders or other psychological issues. An estimated 10 percent of mildly obese people who attempt to lose weight through weight loss programs or on their own have a binge eating disorder, according to the National Institute of Diabetes and Digestive and Kidney Diseases. Among those who are severely obese, this disorder is even more common. Also, some people eat, even when they are not hungry, as a way of dealing with stress, anger, sadness or other emotions.
  
  
• Genetic predisposition. If one or both parents are obese, the child’s chances of becoming obese are increased. Though there is no escaping some familial traits, it is possible to maintain control when it comes to weight.
  
  
• Race and ethnicity. Rates of obesity and diabetes are higher among minority populations, especially Native Americans, black Americans and Hispanics. For example, black Americans and Puerto Ricans have diabetes rates that are 1.8 times higher than non-Hispanic whites, according to the National Diabetes Education Program (NDEP). Rates among Mexican Americans are 1.7 times higher than in white Americans and 2.2 times higher in American Indians and Alaska Natives than in whites. Factors that may contribute to these discrepancies include genetics as well as culture, diet and economic status.
  
  
• Medical conditions. An overproduction of hormones by the adrenal glands (Cushing's syndrome), low thyroid function (hypothyroidism), polycystic ovarian syndrome, low metabolic rate or other medical problems can all lead to obesity. However, these conditions account for less than 2 percent of all cases of obesity.
  
  
• Physiological influences. Metabolic rates vary by the individual. This means that two people with the same height, weight and body type can require a different number of calories to maintain a normal body weight.