Body mass index (BMI) is an assessment tool used to determine if a person's weight is healthy with respect to his height. Both the National Heart Lung and Blood Institute (NHLBI) and the Centers for Disease Control and Prevention (CDC) use BMI, along with waist circumference and certain risk factors, to assess overweight and obesity. The formula for calculating BMI is the same for persons of all ages, but the criteria used to interpret the results varies for different age groups.
Features
The formula for computing BMI, as presented in the National Heart Lung and Blood Institute's Clinical Guidelines on the Identification, Evaluation, And Treatment of Overweight and Obesity in Adults, produces a score based on comparison of a person's height and weight. To calculate BMI, multiply weight in pounds by 703, and then divide by the height in inches squared. The resulting number is the BMI.
Intrepretation of BMI for Adults
A BMI of 30 to 40 is obese in adults 20 years of age and older, according to the National Institutes of Health, and a score over 40 is extremely obese. A normal BMI for adults falls in the range of 18.5 to 24.9. Below 18.5 is underweight and 25 to 29.9 is overweight.
Intertpretation of BMI for Children and Teens
In children and teens, the amount of body fat varies with age. The amount of body fat also differs between boys and girls. For these reasons, interpretation of BMI for children and teens takes into consideration both sex and age, and the statistic for this age group is often called BMI-for-age. Doctors calculate the BMI, and then plot it on a growth chart for persons of the same age and sex to determine a percentile ranking. Most professionals use percentiles, rather than BMI, to assess overweight and obesity in children and teens. According to the CDC, an obese person is in the 95th percentile or greater. Persons in the 5th to less than 85th percentile have a healthy weight, those in the 85th to less than 95th percentile are overweight, and those less than the 5th percentile are underweight.
Significance
Obesity is a contributing factor to several potentially serious health conditions. Obese people are at greater risk of developing high blood cholesterol, high blood pressure, stroke, type 2 diabetes and certain cancers, according to the NHLBI, and the American Heart Association says that "obesity is now recognized as a major, independent risk factor for heart disease."
Considerations
Although BMI is a reliable predictor of total body fat, it does have some limitations. According to the NHLBI, a BMI score may underestimate body fat in older people and others who have experienced a decrease in muscle mass. BMI may overestimate the amount of body fat in persons who have a muscular build, such as athletes.
References
- National Heart Lung and Blood Institute: Aim for a Healthy Weight
- National Institutes of Health: Body Mass Indes
- Centers for Disease Control and Prevention: About BMI for Adults
- Centers for Disease Control and Prevention: About BMI for Children and Teens
- National Heart Lung and Blood Institute: Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults: The Evidence Report
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