Obesity is defined as an excess of body fat. Being overweight is different from being obese. Overweight refers to excess body weight compared to normal standards. The excess weight may come from muscle, bone, fat, and/or body water. Obesity refers specifically to having an abnormally high proportion of body fat. Health care practitioners use a number of methods to determine if an individual is overweight or obese. Body mass index, or BMI, is the measurement used to assess whether a person is overweight or obese. BMI is calculated using a mathematical formula that takes into account both a person’s height and weight. A person is considered overweight if they have a BMI of between 25 and 29.9. A BMI of more than 30 is generally considered a sign of moderate to severe obesity.

Obesity is associated with many serious preventable diseases including heart disease, diabetes, high blood pressure, stroke, gallbladder disease, osteoarthritis, and respiratory disorders. The risk of developing these diseases is even higher when weight is concentrated near the waist. According to the National Institutes of Health, 60% of American adults are overweight and 25% are considered obese. For both men and women, the prevalence of obesity increases with age, but this problem is growing in children and adolescents — approximately 25% of American children are overweight or obese, and the numbers are rising.

Signs and Symptoms

Most practitioners use the following BMI ranges as indications that a person is overweight or obese:

  • BMI 25 to 29.9 (overweight)
  • BMI 30 to 39.9 (moderately obese)
  • BMI 40 or above (extremely obese)

A high waist to hip ratio (indicating that fat is centered around the waist — also known as central obesity) increases the risk for developing serious, even life-threatening conditions associated with obesity. Physicians consider a very high waist circumference to be greater than 102 cm for men and greater than 88 cm for women.

Conditions that may accompany obesity include:

  • High cholesterol (including high triglyceride levels)
  • Diabetes
  • High blood pressure
  • Sleep apnea (episodes when a person stops breathing while asleep)
  • Osteoarthritis
  • Gallstones
  • Stress incontinence (inability to control urine; small amounts of urine are released when a person laughs, coughs, or moves abruptly)


While there is no single underlying cause of obesity, the bottom line is that excessive weight reflects an imbalance between energy input and energy output. Both genetic and behavioral factors play a role in the development of excessive weight. For example, an individual’s total number of fat cells (which may predispose an individual to weight gain) is determined genetically, but behavioral factors, such as a high-calorie, high-fat diet and lack of physical activity, must be present in order for weight gain to occur.

Other causes of obesity include:

  • Rare congenital disorders (conditions present at birth), such as Prader-Willi syndrome and Laurence-Moon-Biedl syndrome
  • Hormonal disorders such as Cushing’s Syndrome and polycystic ovary syndrome
  • Insulinoma (tumors of the pancreas)
  • Hypothyroidism (diminished production of hormones from the thyroid gland)
  • Brain injury (or lesion) from trauma or surgery
  • Certain prescription medicines, including steroids, tricyclic antidepressants, anti-epilepsy drugs, or drugs used for high blood pressure