What do "overweight" and "obesity" mean?

Dr. Mehmet Oz, MD
Cardiology (Cardiovascular Disease)
The words "overweight" and "obesity" have specific meanings and are not interchangeable. They are based on your body mass index (BMI), a calculation based on height and weight, which is statistically related to body fat. For adults, a BMI below 18.5 means you are underweight, while a BMI between 18.5 and 24.9 indicates a normal weight. Having a BMI between 25 and 29.9 means you are overweight, while one above 30 signals obesity. To put this in perspective, a woman who is 5 feet 5 inches tall and weighs 144 pounds has a BMI of 24 (normal weight). At 168 pounds, her BMI would be 28, meaning she is overweight. At 180 pounds, her BMI would be above 30, so she would be considered obese.
For adults, overweight and obesity ranges are determined by using weight and height to calculate a number called the "body mass index" (BMI). The body mass index was established by an expert panel convened by the National Institutes of Health in 1998. For most people, the BMI correlates with their amount of body fat.

For example, according to the Centers for Disease Control and Prevention (CDC), an adult who has a BMI between 18.5 to 24.9 is considered to be at a healthy weight. An adult with a BMI of 25 and 29.9 is considered overweight, and an adult who has a BMI of 30 or higher is considered obese. 
Robert S. Kaufmann, MD
Internal Medicine
The terms "overweight" and "obesity" refer to a person's overall body weight and whether it's too high. Overweight is having extra body weight from muscle, bone, fat, and/or water. Obesity is having a high amount of extra body fat.

The most commonly used measure of overweight and obesity is body mass index (BMI). BMI is calculated from your height and weight.

In adults, a BMI of 18.5 to 24.9 is considered normal. A BMI of 25 to 29.9 is considered overweight. A BMI of 30 or above is considered obese. Over two-thirds of American adults are overweight, and almost one-third of these adults are obese.

Overweight is defined differently for children and teens than it is for adults. Children are still growing, and boys and girls mature at different rates. Thus, BMIs for children and teens compare their heights and weights against growth charts that take age and gender into account. This is called BMI-for-age percentile.

Being overweight or obese can raise your risk for heart disease and heart attack. This is mainly because overweight and obesity are linked to other heart disease risk factors, such as high blood cholesterol and triglyceride levels, high blood pressure, and diabetes.

This answer from the National Heart, Lung, and Blood Institute has been reviewed and/or edited by Dr. Robert S. Kaufmann.

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