Body mass index (BMI) and body fat percentage are two different methods for classifying one's body type. BMI doesn't take into account the weight difference for a person with a large fat mass in comparison to a person with a large muscle mass. Thus, a person of an athletic build may be misclassified using BMI. Body fat percentage, however, gives a more accurate picture of body composition, but is generally more time-consuming and/or expensive to determine.
BMI
Body mass index (BMI) is a standardized measure that renders body mass in relation to height. It allows for the classification of a person as underweight, normal, overweight, or one of the varying degrees of obese.
The BMI is calculated by dividing the weight in kilograms by the square of the height in meters (the units are kg/m2). BMI classifications are as follows:
A BMI less than 18.5 is classified as underweight; 18.5 to 25 is considered normal; greater than 25 and up to 30 is classified as overweight; greater than 30 and up to 35 is obese; greater than 35 and up to 40 is classified as very obese; and greater than 40 is extremely obese.
The BMI does not differentiate between increases in muscle mass and increases in fat mass. So people with a muscular physique may not be properly classified.
Body Fat Percentage
A clinical professional determines a patient's body fat percentage by one of the following methods: skin fold assessment by using a bioelectric impedance device, by dual energy X-ray absorptiometry (DEXA) scan, using hydrostatic weighing, or by body plethysmography.
According to the National Institutes of Health, the recommended amount of body fat for men is between 13 percent and 17 percent. A man is considered obese when his body fat percentage is 25 percent or higher. The recommend body fat percentage for women is between 20 and 21. A body fat percentage greater than 30 is considered obese for a woman.
BMI and Body Fat
BMI closely correlates with the amount of excess body fat. Thus, the body fat percentage can be estimated using the following equations, established and published in the British Journal of Nutrition in 1991 by nutrition researchers Paul Deurenberg, Jan Weststrate and Jaap Seidell:
Females % fat = 1.2(BMI) + 0.23(age(y)) -- 5.4
Males % fat = 1.2(BMI) + 0.23(age(y)) -- 16.2
Fat Distribution
Though BMI and body fat percentage are useful ways to classify people based on height, weight and body composition, studies show that disease risk and mortality are more closely linked to the location of body fat--as well as to the amount. Patients with greater abdominal or central obesity are at increased risk for disease and mortality. According to Drs. Rena Wing and Mary Lou Klem, contributing authors of "Psychiatric Secrets," abdominal fat can be estimated using waist circumference measures. Waist circumferences of greater than 102cm or 40 inches for men and 88cm or 35 inches for women are correlated with increased risk of morbidity and mortality.
As Predictors of Health
As stated by Drs. Victoria Catenacci, James Hill and Holly Wyatt in their article, "The Obesity Epidemic," increased BMI and body fat percentage are correlated with increased morbidity and mortality, particularly associated with the following diseases: cardiovascular disease, especially coronary artery disease, Type 2 diabetes mellitus, various forms of cancer, obstructive sleep apnea, fatty liver disease, gallbladder disease and osteoarthritis.
References
- "DeGowin's Diagnostic Examination, 9e, Chapter 4;" Richard F. LeBlond MD, Donald D. Brown MD, Richard L. DeGowin MD; 2009
- National Institutes of Health, Medline Plus: Weight Management
- "British Journal of Nutrition"; Body mass index as a measure of body fatness: age- and sex- specific prediction formulas; Paul Deurenberg PhD, Jan A. Weststrate PhD, Jaap C. Seidell PhD; 1991
- "Psychiatric Secrets, 2nd ed., Ch. 79"; Rena R. Wing, PhD, Mary Lou Klem, PhD; 2001
- "Clinics in Chest Medicine"; Victoria A. Catenacci, MD, James O. Hill, PhD, Holly R. Wyatt, MD; 2009.
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