Body Mass Index  — BMI — is something we have discussed before. It’s a simple way of relating a person’s weight to their height, which makes it easier to generalize about populations of people. However, BMI has long been controversial. Many people, most of whom have a BMI over 25, criticize the measurement as being inaccurate, too generic, and meaningless as a measurement of obesity. It turns out that those critics are right, just not how they thought.

Researchers set out to see just how accurate BMI is by measuring both the BMI and body fat percentages of over 500 women. The results? Yes, it turns out that BMI is not accurate enough.

Surprisingly, BMI underestimates the number of obese women in the United States!

In fact, they found that “Current BMI cutoff values recommended by the NIH failed to identify nearly half of reproductive-aged women who met the criteria for obesity by percent body fat.” [emphasis added]. The results were worst for white and Hispanic women. Researchers were even able to put together a mathematical formula “that for a given BMI, white and Hispanic women will have 2.9% higher percent body fat than black women….”

You may ask why the study only included women of reproductive age. The study was published in the journal Obstetrics and Gynecology. Obesity can cause problems during childbirth — to say nothing of raising a woman’s risk of breast cancer, endometrial cancer, and other disorders specific to women. Obesity is a critical factor that obstetricians need to consider in their patients. The researchers admit that the age range they used is a limitation of the study, but point out similar results in post-menopausal American women.

So while BMI calculations are no substitute for body fat measurement, you shouldn’t ignore it either.