Obesity is a condition in which a person has an unhealthy amount and/or distribution of body fat.

To measure obesity, researchers commonly use a scale known as the body mass index (BMI). BMI is calculated by dividing a person’s weight (in kilograms) by their height (in meters) squared (commonly expressed as kg/m2). BMI provides a more accurate measure of obesity than weight alone, and for most people it is a fairly good (although indirect) indicator of body fatness.

Other measurements that reflect the distribution of body fat—that is, whether more fat is carried around the hips or the abdomen—are increasingly being used along with BMI as indicators of obesity and disease risks. These measurements include waist circumference and the waist-to-hip ratio (the waist circumference divided by the hip circumference).

The standard weight categories based on BMI for adults age 20 years or older are

BMI in kg/m2 Weight Category Below 18.5 Underweight 18.5 to 24.9 Normal 25.0 to 29.9 Overweight 30.0 to 39.9 Obese 40.0 or higher Severely obese

The National Heart Lung and Blood Institute has a BMI calculator at http://www.nhlbi.nih.gov/health/educational/lose_wt/BMI/bmicalc.htm.

For children and adolescents (younger than 20 years of age), overweight and obesity are based on the Centers for Disease Control and Prevention’s (CDC’s) BMI-for-age growth charts, which are available at http://www.cdc.gov/growthcharts/clinical_charts.htm:

BMI Weight Category BMI-for-age at or above sex-specific 85th percentile, but less than 95th percentile Overweight BMI-for-age at or above sex-specific 95th percentile Obese

The CDC has a BMI percentile calculator for children and teens at http://nccd.cdc.gov/dnpabmi/Calculator.aspx.

Compared with people of normal weight, those who are overweight or obese are at greater risk for many diseases, including diabetes, high blood pressure, cardiovascular disease, stroke, and many cancers. Extreme or severe obesity is also associated with an increased death rate; heart disease, cancer, and diabetes are responsible for most of the excess deaths (1, 2).