Why BMI and your scale are imperfect

While surfing the web the other day I saw a headline including the phrase “Why BMI is Useless” and I just had to click. The article (from the Daily Mail) went on to describe how a New York firm called Body Labs did full-body scans of different people, all with the same body mass index (BMI). The group then made a graphic that showed (surprise) that each of the 5’9” volunteers had very different amounts of fat weight versus lean muscle weight.

It was a startling graphic to showcase what many health-conscious people already know: BMI is imperfect. It’s a simple formula that uses weight and height alone to classify a person’s weight status (underweight, normal weight, overweight, or obese), and its limited usefulness is a topic that surfaces in the news every few years.

But what doesn’t get said enough is that pretty much all the ways we measure our bodies, from BMI to body fat percentage, when used alone, are imperfect.

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As the Body Labs graphic shows, BMI can’t calculate how much fat (or what kind) you have, nor can it assess your muscle mass. That means a person who has an above average weight for their height, but is very muscular with a low body fat percentage, may score in the overweight range for BMI, because the assumption is that any excess weight is well, excessive. I work with professional athletes, and because of high muscle mass, many do score in the overweight range for BMI, even though they earn their living moving their bodies.

Not only that, a “normal” BMI falls into a range, from 18.5 and 24.9. For a woman who is 5’4” that can mean a weight anywhere between 108 and 145 pounds, and that’s pretty broad. But more importantly, scoring in the “normal” BMI range doesn’t necessarily mean you’re healthy.

Over the years I’ve counseled many people with normal BMIs who were either “skinny fat” (normal weight, but with less muscle mass and a higher than optimal body fat percentage) or had excess belly fat. One recent study, published in the Journal of the American College of Cardiology, found that among people with a normal BMI who carried excess belly fat, the risk of dying was just as great as smoking a pack of cigarettes a day or having very high cholesterol. In other words, weight for height doesn’t provide much information about overall health. In fact, some of the thinnest people I know have the least healthy diets, don’t exercise, smoke, and are incredibly stressed.

And the opposite is also true: having more body fat doesn’t necessarily mean you’re unhealthy. Throughout my career I’ve met many people who had higher than optimal body fat percentages but were incredibly fit, and some Olympic athletes fall into this category. A study published in the European Heart Journal shows that ‘fat but fit’ adults may be at no greater risk of developing or dying from heart disease or cancer than those who are fit and “normal” weight.

So why is BMI so talked about, and why do we put so much pressure on ourselves to slim down? Well, BMI is simply an estimate, and it’s often used when other personal measurements, like body fat percentage or waist measurements can’t be taken. Plus, many people who score in the overweight or obese categories for BMI are statistically more likely to experience health problems (key word: statistically). So in that way BMI is useful to researchers looking at large groups of people.

It’s also true that many people will tell you their health went south when they gained too much weight, and got dramatically better when they lost.

But the bottom line is that everyone’s different. For individuals, weight, BMI, or body fat percentage, should never be used alone; not one of them are reliable indicators of overall health or fitness.

To really assess your health, focus on everything: the quality and consistency of your diet, your energy, mood, strength, endurance, injury track record, sleep patterns, digestive health, immunity, lifestyle (e.g. alcohol intake, smoking, stress), and other clinical measurements (blood pressure, blood sugar, blood nutrient levels, and cholesterol breakdown, etc.), along with your weight.

All of these things matter—not just your BMI or your dress size.

In the pursuit of a lower weight, BMI, or body fat percentage, I’ve seen people wind up worsening their overall wellness, or even upping health risks, and in my book, that trade off just doesn’t make sense. Bottom line: numbers sometimes do lie, and your size alone doesn’t equal your health.

Cynthia Sass, MPH, RD, is Health's contributing nutrition editor. She privately counsels clients in New York, Los Angeles, and long distance. Cynthia is also the sports nutrition consultant to the New York Rangers NHL team and the New York Yankees MLB team, and is board certified as a specialist in sports dietetics.

This article originally appeared on Health.com.

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