Why BMI is a bust when it comes to measuring your health

According to a new study from UCLA, 54 million Americans are perfectly healthy — but mislabeled as obese. In my view, the problem stems from our modern cultural bias against “excess” body weight. Even medical researchers possess a strong bias against carrying a little extra meat on the bones…despite the evidence.

In fact, as I reported yesterday, a massive new study linked increased body weight with significantly lower dementia risks. That study followed two million men and women over two decades. And it appeared in one of the most prestigious medical journals of our time. But the researchers couldn’t bring themselves to trust their own findings! And they warned us not to jump to conclusions.

Another part of the problem stems from our reliance on a flawed measure of body composition…the body mass index (BMI). As I have warned before, researchers should not use BMI alone to estimate “excess” body weight or “excess” body fat.

BMI is a flawed, sloppy measure of health

A 19th century French statistician named Alphonse Quetelet first introduced this concept. He called it (in French) the ponderal index.

When I first started working on the association between body size and health outcomes, we still used the terms Quetelet and ponderal index. But these French words may have been too much for our modern medical scientists. So it became known as the “body mass index.” Specifically, the BMI is your weight divided by (controlled for) your height, to the square of height.

In my own research, I found that the concept of dividing weight by height was generally valid in terms of determining excess body weight. But it’s not perfect. And there are a few major problems with it.

First, the BMI does not distinguish between extra body fat and extra lean body mass (muscle mass).

Second, my research shows it’s also necessary to adjust the exponential factor for the height term when you divide.

The “standard” BMI simply uses the square of height — that is, height to the power of two, or mathematically, weight/height2 . However, just using the exponent 2.0 is not always correct for men and women at different weights among different populations.

I found the correct exponent is usually between 1.5 and 2.0. So by simply using 2.0 for height, you “overcorrect” for height. There are other problems as well, which we published in a lively debate among leading nutritional scientists in the American Journal of Clinical Nutrition during the mid-1980s.

We have much better methods to measure health

There is simply no substitute for measuring skin folds and other anthropometric dimensions in order to figure out what is “excess” body fat, versus lean body mass, vs. stature, etc.

I incorporated these more accurate measures into our multi-million dollar studies on diet, nutrition, body size, and health while I was at the National Institutes of Health (NIH). But when I attempted to hire a younger graduate who was an expert in these techniques, they ridiculed the idea. Instead, they hired someone who had “laboratory” skills — as if those skills would help to measure the body correctly.

Unfortunately, when I left NIH, my methodology left with me. And today, we are left with using a sloppy, inaccurate way of assessing “extra” body weight.

So, it’s no big surprise then — after decades of using the flawed body mass index — that the new study from UCLA found 54 million Americans are incorrectly labeled as overweight or obese. They’re actually perfectly healthy.

These findings saddle perfectly healthy men and women with higher insurance rates and possible discrimination regarding employment. They also contribute to social “body shaming” of perfectly healthy men and women based on a flawed measure of excess weight and a deeply flawed index of health.

Of course, I often report all the studies showing that “excess” body weight, as measured by a higher BMI, does not cause health problems. Furthermore, many studies (including the one from yesterday) show carrying a little meat on your bones has health benefits, such as reducing your risk of developing dementia.

What we should be focusing on are the increased risks of people who are under-weight. They run a far higher risk of developing dementia and a host of other diseases.

Watch out Weight Watchers! (As if they didn’t already have enough problems with Oprah and their stock price lately.)




“Misclassification of cardiometabolic health when using body mass index categories in NHANES 2005–2012,” International Journal of Obesity, 2016