The Significance of BMI

The significance of BMI, the body mass index, is debatable. BMI has been used as a standard, sometimes the standard, measurement for assessing people's body fat since the National Institutes of Health made BMI its definition of obesity in 1985. Proponents of BMI argue that its correlation with risk of obesity-related diseases makes it very significant, but the Centers for Disease Control and Prevention (CDC) and many other critics contend that it has very significant weaknesses.

New Tables For Doctors

Doctors used weight-to-height tables for decades as the standard measurement for obesity for several decades, according to "Essentials for Health and Wellness." There is a correlation between weights higher than the recommended weights and obesity-related diseases, but the tables are more subjective and less accurate because they are based partly on whether you have a small, medium or large frame. The BMI index allowed doctors to tell you your risk of certain diseases by dividing your height in meters squared into your weight in kilograms (W/HH).

Muscle and Fat

Your weight includes muscle and fat. The CDC reports that the BMI-body fatness correlation is "fairly strong," but warns "BMI is not a direct measure of body fatness," and is less significant if you are muscular. You are assessed as overweight if your BMI is between 25 and 29.9 and obese if it's higher, but additional weight might not indicate an increased heart disease risk. Your weight is "healthy" if your BMI is between 18.5 and 24.9, but your weight could be unhealthy if you lack muscle.

Waist Circumference

The American Diabetes Association, the American Society for Nutrition and "other prominent medical groups" are lobbying to have waist circumference replace or supplement BMI as the standard measurement for body fat, reports "Beyond BMI," an article in "Slate" magazine's July 20, 2009, issue. The article reports that waist circumference is more significant than BMI because its measurement of fat and assessment of obesity-related disease risk is more accurate, just as inexpensive and just as easy to measure.

Other Factors

BMI became the measuring standard for fat because of its low cost and ease of use, but it should be just one of several criteria for assessing obesity-related risk because it's not significant by itself, according to "Beyond BMI." Besides waist circumference, underwater weighing and skinfold thickness as measured by calipers should also be used, according to the article. "Essentials" also recommends waist-to-hip ratio because fat distribution is important, and the CDC also recommends dual energy x-ray absorptiometry.

Age and Gender

The significance of BMI is weakened because it doesn't consider age or gender unless you are under 18 years old, according to the University of Pennsylvania's Office of Health Education and the "Beyond BMI." Women and older people have more fat than men and younger people with identical BMIs, they report. In fact, the researcher whose work popularized BMI, Ancel Keys, "warned against using the body mass index for individual diagnoses" because it ignored age and gender, according to "Beyond BMI."