The National Heart, Lung and Blood Institute defines body mass index as "a measure of body fat based on height and weight that applies to both adult men and women." The optimum BMI is between 18.5 and 24.9, however this is not an indicator of health.
How Is BMI Used?
BMI is a screening tool that can help you assess your body fat and identify weight problems. Although BMI can be a helpful tool to assess your body fat in relation to your height, the NHLBI recommends using three key measures, BMI, waist circumference and your risk factors for diseases associated with obesity, to give you a more accurate picture of your weight status.
Calculate BMI
To calculate your BMI using pounds and inches, list your weight in pounds divided by your height in inches, multiplied by 2 and then multiplied by 703. The formula is:
BMI = [weight ÷ (height x height)] x 703
Here is a sample calculation from the Centers for Disease Control and Prevention: A woman weighs 150 pounds and stands 5 feet, 5 inches tall, or 65 inches tall. Using the formula, her BMI would be 24.96, or [150 ÷ (65 x 65)] x 703 = 24.96
BMI Categories
A person with a BMI of less than 18.5 is considered underweight. Normal weight is someone with a BMI of 18.5 to 24.9. Someone whose BMI is between 25 and 29.9 is considered overweight; and a person whose BMI is 30 or greater is classified as obese.
A Healthy BMI
A normal BMI is 18.5 to 24.9; however, it is difficult to classify this as a healthy BMI without taking into account your body composition, fat distribution, your diet, physical activity, your other risk factors for disease, family history and other appropriate health criteria.
Limitations of BMI
The Centers for Disease Control and Prevention notes that BMI is just a screening tool to identify weight problems in adults, but it is not a diagnostic tool. Although BMI may be a body fat indicator for some people there are clearly limitations to its use according to a 2002 article by Adrian Bagust and Tom Walley, published in the QJM, a leading medical journal. In their article, "An alternative to body mass index for standardizing body weight for stature," the researchers discuss how BMI has its limitations because it does not assess body fat independent of height, making BMI a poor indicator of obesity in the young and the elderly. Their article also points out that BMI is influenced by body proportions so that it underestimates the weight appropriate for the shortest individuals and overestimates that of the tallest individuals. Furthermore, BMI does not take into account body composition, such as lean body mass or fat mass. This makes the BMI a poor measure of body fat in athletes, who have higher levels of lean body mass.
References
- National Heart Lung and Blood Institute
- Centers for Disease Control and Prevention
- "QJM," medical journal; An alternative to body mass index for standardizing body weight for stature; A. Bagust and T. Walley; 2000
Member Comments