Obesity is defined as an excessively high amount of body fat (or adipose tissue) in relation to lean body mass.

Everyone needs a certain amount of body fat for stored energy, heat insulation, shock absorption, and other functions. As a rule, women have more body fat than men. Most health care providers agree that men with more than 25 percent body fat and women with more than 30 percent body fat are obese.

Measuring the exact amount of a person's body fat is not easy. The most accurate measures are to weigh a person underwater or to use an X-ray test called Dual Energy X-ray Absorptiometry (DEXA).

There are simpler methods to estimate body fat. One is to measure the thickness of the layer of fat just under the skin in several parts of the body. Another involves sending a harmless amount of electricity through a person's body. Both methods are used at health clubs and commercial weight loss programs.

Because measuring a person's body fat is difficult, health care providers often rely on other means to diagnose obesity. Weight-for-height tables, which have been used for decades, usually have a range of acceptable weights for a person of a given height. One problem with these tables is that they do not distinguish between excess fat and muscle. A very muscular person may appear obese, according to the tables, when he or she is not.

In recent years, body mass index (BMI) has become the medical standard used to measure overweight and obesity.

BMI uses a mathematical formula based on a person's height and weight. BMI equals weight in kilograms divided by height in meters squared (BMI = kg/m2).

A BMI of 25 to 29.9 indicates a person is overweight. A person with a BMI of 30 or higher is considered obese.

Like the weight-to-height table, BMI does not show the difference between excess fat and muscle. BMI, however, is closely associated with measures of body fat. It also predicts the development of health problems related to excess weight. For these reasons, BMI is widely used by health care providers.

All information provided in this site is offered for educational purposes only, and it is not intended nor implied to be a substitute for professional medical advice. Always consult your own physician or healthcare provider with any questions you may have regarding a medical condition.