Health Benefits of regular physical activity,Journal of the Louisiana State Medical Society;Grundy, et al.
Body Mass Index (BMI) versus Body Fat
Body Mass Index (BMI)
Body Mass Index (BMI) is an assessment of body weight relative to height. Evidence has shown that an elevated BMI increases risk for such diseases as cancer, diabetes mellitus, hypertension, hypercholesterolemia, and atherosclerosis. Research has proven that the higher your BMI, the greater the risk for a premature death. A healthy BMI is 20.0 – 24.9 kg/m 2. A BMI of at least 27 kg/m 2 indicates obesity and increased health risk. A BMI of 30.0 kg/m 2 indicates grade II obesity, while 40.0 kg/m 2 and greater is morbid obesity.
BMI is the preferred body composition assessment for the obese population because calipers lose their accuracy with large skin folds and variance in fat density. Bio-impedance and near-infrared typically underestimate body fat percentage in this population. Although BMIs don’t account for body fat percentage, excess body fat is already known in the obese.
Important Limitations of BMI:
BMI does not distinguish between fat mass and fat free mass. This is important because ACSM defines obesity as a percent of body fat at which disease increases. People with large amounts of lean tissue may have a high BMI while having their body fat percentage in a healthy range. In addition, a healthy BMI does not necessarily mean that body fat is within a healthy range.
BMI does not give any information on the location of the body fat which is important in determining obesity-related risk for disease.
General Interpretation Guidelines:
Underweight: BMI <18.5
Normal: BMI = 18.5-24.9
Overweight: BMI = 25.0-29.9
*see grades of obesity classification
A BMI score of 20 to 25 is associated with the lowest risk of excessive or deficient adipose tissue. Obesity is divided into three grades:
Grade I = 25.0-29.9
Grade II = 30-40
Grade III = 40+
A BMI of at least 27 indicates obesity and increased health risk. Body mass index increases with age; therefore, age-specific guidelines for interpreting the BMI in the elderly have been recommended.
Body fat, not weight, is a better measure of your health and fitness. Increased body fat is associated with obesity-related health conditions. Because the scale does not distinguish between lean weight and fat weight, a person could be “over-weight” but not be “over-fat”. However, body fat analysis is not appropriate for people who are obviously obese and tests may not provide an accurate result.
Body fat can be measured in many ways. The most common methods are skinfold calipers and Bioelectrical Impedance (BEI). Both of these tests should not be done after exercising; BEI results will be greatly affected if you are not properly hydrated. Each method has a margin of error of approximately 3-5%. For skinfold calipers, the more sites that are assessed, and the greater the skill of the person doing the measurement, the more accurate the result will be.
The gold standard for body fat is underwater weighing; however, most people do not have easy access to an exercise physiology lab to have this test performed. As you age, the acceptable level of body fat increases. The guidelines listed below are not age-adjusted but can give you a general framework to use.
Body Type Female Male
Athlete <17% <10%
Lean 17-22% 10-15%
Normal 22-25% 15-18%
Above Average 25-29% 18-20%
Over-fat 29-35% 20-25%
Obese 35+% 25+%
Sources: ACSM’s Guidelines for Exercise Testing and Prescription, 6th Ed., 2000 and ACSM’s Resource Manual for Guidelines for Exercise Testing and Prescription, 4th Ed., 2001