**Body Mass Index** (noun, “Body Mass IN-dex”)

Body Mass Index is often known by its abbreviation: BMI. It is a mathematical formula. BMI is someone’s weight in kilograms divided by the square of their height in meters. The equation looks like this: BMI = kg/m^{2}.

Muscle weighs more than the same volume of fat. And BMI does not account for how much muscle someone has. So very lean athletes might have a BMI described as “obese,” when in fact they just have a lot of muscle.

Consider an adult who is 1.67 meters (about 5.5 feet) tall and 60 kilograms (132 pounds). They would have a body mass index of 21.3. This seems like a random number. But many doctors and companies use it to determine if someone is underweight, average, overweight or obese. This can pose a problem, however, because someone’s weight to height ratio doesn’t say anything about how healthy they are. It also does not indicate how much body fat they have or diagnose them with a disease.

In addition, the boundaries some health officials draw between what is “normal” and what is “overweight” have changed over time. In fact, a BMI of 27 was considered “normal” until 1998, when the National Institutes of Health changed it to 25. Currently, the Centers for Disease Control and Prevention states that a BMI for adults under 18.5 is “underweight.” Between 18.5 and 24.9 is “normal.” A BMI between 25 and 29.9 is ranked as “overweight,” and a BMI over 30 is described as “obese.”

In children and teens, Body Mass Index is calculated not just by weight and height, but also by age. A BMI is calculated and then compared to a chart of average BMIs for someone’s age group. Kids and teens then get placed into a BMI percentile. A BMI less than the 5th percentile is “underweight.” It means that 95 percent of children that age have a higher BMI. A BMI greater than the 85th percentile is “overweight.” It means that 85 percent of children that age have a lower BMI.

Age makes a big difference for Body Mass Index in kids. A BMI of 18, for example, is average for a 12-year-old girl, but in the overweight category for a girl who is seven and a half.

The Body Mass Index was developed by a man named Adolphe Quetelet. He was a mathematician and astronomer in Belgium in the 1800s. Quetelet was interested in statistics — analyzing groups of numbers to understand their meaning. He developed the BMI as a way to describe how human weight changes as people grow taller. Quetelet used the BMI to describe the “average man.”

After World War II, scientist Ancel Keys became interested in studying body fat in different groups of adult men. He found BMI to be more accurate than other measures, such as measuring the thickness of a fold of skin. But it still wasn’t very accurate. Body fat can vary a lot from person to person. BMI can only explain about half of that variation. So it is only really useful when looking at large groups of people. Both Keys and Quetelet wanted to use BMI to look at groups of people, not one person at a time.

When used to describe individuals, BMI can influence how people are treated. People who are categorized as “overweight” or “obese” face a lot of stigma — disgrace — for their size. Teens can be shamed for their BMI by their peers, parents or even their doctors.

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As teens’ BMI’s increase, their brains become less sensitive to tasting fatty food.