Obesity is defined simply as an excess of body fat. A person is generally considered obese or overweight when body weight exceeds "desirable" weight for height and gender by 20 percent or more and when the excess weight is fat rather than water, muscle or bone. The "desirable" weight is based on the 1959 Metropolitan Life weight tables.

Obesity is now recognized as a major risk factor for coronary heart disease, which can lead to heart attack. Some of the reasons for this higher risk are known, but others are not. For example, obesity:

• raises blood cholesterol and triglyceride levels.
  
• lowers HDL (the "good" cholesterol linked with lower risk).
  
• raises blood pressure.
  
• can induce diabetes.

But even when there are no adverse effects on the known risk factors, obesity alone imparts an increase in risk. Obesity also has many other harmful effects beyond those on the heart and blood vessel system. It is a major cause of gallstones and it can worsen degenerative joint disease.

Obesity is caused mainly by taking in too many calories and not getting enough exercise. When caloric intake is excessive, some of the excess often is saturated fat, which further raises the blood cholesterol. Intake of dietary cholesterol also may be high. Thus, obesity contributes to higher coronary risk in a variety of ways.

"Ideal" or "desirable" weight tables for height and build have been published by the insurance industry for many years. These tables are based on the industry's analyses of weights and death rates among policyholders. The new tables were revised upward by roughly 10 percent in 1983 because even policyholders who are obese are living longer than they used to.

The revision raised many concerns, since the relationship of weight to health and disease is complex. The AHA does not use the revised table as a basis for determining proper weight. Instead, it follows the table published by the insurance industry in 1959.

What is the body mass index (B.M.I.)?

The body mass index (BMI, weight in kg ÷ height in m) is a method used to measure a person's percentage of body fat. In studies by the National Center for Health Statistics, overweight is defined as a body mass index of 25.0–29.9 (based on U.S. Dietary Guidelines for Americans). Obesity is defined as a BMI of 30.0 or greater (based on criteria of the World Health Organization).

Click here to calculate your body mass index.