You’re probably familiar with the concept of the Body Mass Index (BMI). You may have been taught that BMI is a good indicator of overall health and physical fitness. What may surprise you is that BMI is not only a poor measure of health, but it was actually never intended as a measure of individual health in the first place.

Actually, BMI was initially designed as a mathematical principle for population studies—not for calculating the health of individual people. Furthermore, it is only useful for describing certain types of population. If you are looking to get some general measures of industrial populations of men, between ages 20 and 65, BMI is not without its uses—but that is really where its utility ends.

Consider this: Ancel Keys, the American scientist who gave BMI its name and championed its use, fully admitted that it was not an accurate way to assess the health of women, children, the elderly or those who are not white.

It comes as little surprise, then, that BMI simply is not a reliable indicator of an individual’s muscle mass—and if you don’t believe it, consider one more fact: According to the BMI model, NBA superstar LeBron James is technically overweight, though of course no one would call him unhealthy!

The bottom line: While BMI has gained a reputation as a serious way of gauging body compositions, many scientists—including eating disorder specialists—say that it is both an inappropriate and a dangerous standard for measuring individual health. It leads to some erroneous expectations about body type that can lead incredibly healthy and fit people to worry about being either underweight or overweight.

Do not use BMI to determine your fitness, or to make key health decisions. If you are worried about something, consult a doctor—and get some information that is truly useful.

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