Why BMI is a flawed measure of body fat, explained by an eloquent 14-year-old

In a letter that's now gone viral, a 14-year-old girl from Indiana is schooling the world in why body mass index, or BMI, is an imperfect measure of the body.

Tessa Embry says she's a "bigger" girl who's active and loves softball. But her BMI score — her weight divided by her height squared — classifies her as obese. When her teacher asked for her BMI in a recent health quiz, the eighth-grader responded with an eloquent diatribe about why the calculation is fraught with "obvious flaws" and why it's harmful to one's body image:

BMI is an outdated way of defining normal weight, under weight, over weight, and obesity by taking one person's height divided by their weight.

One of the formula's obvious flaws... is that it has absolutely no way of discriminating fat and muscle.

So, let's say there is a fairly athletic woman who maintains a decent diet, she's five feet, six inches, and she weighs 190 pounds, but 80 per cent of her body is muscle.

That doesn't matter when calculating BMI! ... How could someone who stays fit, eats healthy, and has a low metabolism be in danger of heart disease and diabetes? ...

In conclusion, BMI is an outdated way of determining a person's body health, and it's a measurement that should not be used in a school setting where students are already self-conscious and lacking confidence in their unique bodies.

(You can read Tessa's essay in full here.)

Why BMI is flawed

We do have an obesity and weight problem in the US, and there's good reason to be on the lookout for these problems. According to the National Institutes of Health, "The higher your BMI, the higher your risk for certain diseases such as heart disease, high blood pressure, type 2 diabetes, gallstones, breathing problems, and certain cancers." That's why doctors are inclined to use this measure of overweight and obesity with patients.

But Tessa is mostly right about the limitations of BMI. The main flaw: It's an indirect measure of body fat that doesn't take into account important details about age, sex, bone structure, and fat distribution, one study in the Journal of Obesity explained. Again, it's just two numbers: weight divided by height squared.

This can lead to miscalculations for pretty intuitive reasons, especially in older people, the researchers wrote. "A person's percentage of body fat is known to increase with age, whereas muscle mass decreases, but the person's weight and height do not necessarily reflect such changes in body fat and muscle mass. Some elderly persons who are portly but have low muscle mass have normal or even low BMI scores, an underestimation of body fat."

Conversely, muscular and athletic folks — including actors like Tom Cruise — may be placed in the overweight and obese categories.

Why we still use BMI

There are a couple of reasons BMI persists. First, it's not always wrong. As NIH obesity researcher Kevin Hall explained in the Washington Post, "Despite its limitations and notorious counter-examples, BMI … correctly categorizes people as having excess body fat more than 80 percent of the time." (If you're curious, you can find your BMI using this online calculator.)

Second, no good alternative has yet caught on. People have proposed more accurate measures of body fat, like MRI scans or "underwater weighing" — in which patients are submerged in a tank of water to calculate their body volume, density, and body fat. And since researchers know "abdominal obesity" (or belly fat) is especially problematic for health, waist circumference or waist-to-hip ratio has also been proposed.

But MRIs are expensive, and "underwater weighing" is both costly and labor-intensive. Waist measurements can be error-prone, since there's no standard way to do them. So BMI dominates, even though, as Tessa reminds us, it can be flawed and even harmful.

With particular poignancy, the eighth-grader touches on the shame society's focus on fat can bring upon people with bigger bodies. As she writes, "At the beginning of the year, I started having very bad thoughts when my body was brought into a conversation. I would wear four bras to try and cover up my back fat, and I would try to wrap ace bandages around my stomach so I would look skinnier."

Tessa's mother took her to a doctor for a checkup, and the doctor said she was just fine. Tessa reminds us that the problem with BMI is that it only gives us an indirect measure of one thing — body fat — and that sometimes doesn't reflect health. When we talk about BMI, we should see it as something that's more provisional, and not the final answer.