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Body Mass Index: Body Fat By the Numbers

Body mass index, or BMI, is one of the most pervasive tools used by health professionals to assess wellness. Read on to learn more about what BMI says about you.

It’s a simple enough equation: w(kg) / h(m)². Or, in more literary terms, a person’s weight in kilograms divided by the square of his or her height in meters. In either case, the formula spits out a number that fits into one of four categories: underweight, normal, overweight, and obese. To experienced doctors and counselors, these categories can be used as predictors for health risks such as high blood pressure and type 2 diabetes, giving patients and clients a clearer picture of how their lifestyle affects their overall health.

An Imperfect Measurement

For all its value, however, BMI provides an incomplete picture to assessing body-fat content and health. It doesn’t take into account such factors as types of fat, fat distribution, or the proportion of fat versus muscle—athletes, for instance, will often fall in the overweight range owing to the large amount of muscle and shoulder pads on their frames. Better predictors for mortality might include measuring total body fat, body-fat percentage, waist circumference, and waist-to-hip ratio, as where the body stores its fat is as important as how much it contains in all.

Bonus Points

• Body mass index derives from an equation put forth more than a century ago by a Belgian mathematician and astronomer named Adolphe Quetelet, who sought to apply the same practices of probability and measurement that allowed him to predict the position of the stars to finding the average mass of the human body.
• Adolphe’s equation didn’t gain widespread traction until 1972, when researcher Ancel Keys conducted a study of 7,400 men in five countries that concluded that Quetelet’s equation was the best height-weight formula for determining body-fat percentage.