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Body mass index vs. body fat percentage: Only one of them actually matters

Both markers are supposed to measure how healthy you are, but BMI is flawed.

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You can measure your BMI with a scale, but it's not accurate as an indicator of health.

Malte Mueller/Getty Images.

Maybe you've been to the doctor's office for a check up, or had your height and weight measured in primary school, or even did a simple Google search -- at some point in your life, you've probably calculated your body mass index, or BMI.

BMI is widely used as a marker of health, but it turns out that it's not all that accurate -- especially for people of color. Instead, we should look at body fat percentage and body fat distribution -- these two numbers give a far better picture of one's overall health.

If you're curious about the history behind BMI or want to know more about how your body fat percentage relates to your health, read on -- we have all your questions answered.

Where did the BMI come from?

To figure out the history behind BMI, we have to look at Flemish statistician Lambert Adolph Jacque Quetelet, who gave us the concept of "social averages." He was searching for the definition of the "average man" and wanted to find a distribution of body fat that would give him a bell-shaped curve. Quetelet discovered in 1835 that a fairly accurate equation for the relationship of body mass to height was given by squaring the ratio of weight to height. And thus, body mass index was born (though it wasn't called this quite yet).

That's a lot of math lingo, but the important part is that Quetelet was not a physician, nor was he studying obesity or health -- he was looking for a way to analyze populations. Quetelet's Index also didn't differentiate between fat and muscle, only taking into account total body mass. So how did BMI start being used as an indicator of an individual's health?

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A typical BMI chart.

IIFYB

In the 1970s, researcher Ancel Keys conducted a study with more than 7,500 participants, trying to find the most effective way to measure body fat. After testing body mass index, water displacement and skin calipers, the first of the three was determined to be the best and most cost-effective way to do so. The problem with Keys' study? The participants were all men, and predominantly white.

Nevertheless, in 1985 the National Institute of Health changed its definition of obesity to include BMI, saying that it is "a simple measurement highly correlated with other estimates of fatness." And thus, BMI became an easy way to measure one's risk of obesity-related illnesses.

Why BMI is not accurate

But just because BMI is simple and cost-effective to measure, this doesn't mean that it's accurate for everyone. This is partially because muscle weighs more than fat, so if your body fat percentage is low, but you weigh more than what's average for your height, your BMI could say that you are obese when you aren't.

In one study with thousands of participants, African American women with the same BMIs as white women were found to have better health markers in areas like blood pressure and cholesterol, suggesting that BMI can define black people as "unhealthy" when in reality they aren't.

Other studies echo similar ideas -- a researcher at the University of Tennessee concluded that, "Compared to Caucasians, African Americans of the same age, gender, waist circumference, weight and height may have lower total and abdominal fat mass." Thus, BMI will overestimate their body fat, and isn't accurate.

BMI also fails to accurately assess health in Asian people, but in the opposite direction of African Americans. One study that tracked the health of more than 78,000 women in the US found that Asian Americans were at a higher risk of developing obesity-related illnesses at lower BMIs than their white counterparts.

BMI vs. body fat percentage

Now that we've determined that BMI isn't a super accurate way to measure a person's health and risk of disease, we can turn to one measurement that is far more effective: body fat percentage.

Body fat percentage has been shown to be an accurate indicator of the risk of osteoporosis with aging, high blood pressure and other cardiometabolic risks, diabetes and increased mortality in general -- all when people's BMIs or weight would indicate they are otherwise healthy.

Researchers don't know the full story behind BMI, body fat percentage and obesity, but the reason why body fat percentage is more accurate is probably because it actually takes into account a person's adipose tissue, instead of lumping body fat and lean muscle together.

How to measure body fat

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The Naked Labs Body Scanner can measure your body fat, just by standing in front of it.

Tyler Lizenby/CNET

There's a bunch of different ways you can measure your body fat percentage, and for the best readings you'll want to get it done at a doctor's or dietician's office. They'll have machines like an underwater weighing station or the ability to do a DEXA scan, which are far more accurate than anything you can do at home.

If you don't have the resources to make the trip to a health care provider, however, you can get a fairly good estimate at home. The American Council of Exercise has a calculator where you can plug in some skinfold measurements for a rough idea of what your body fat percentage is.

Body fat percentage still doesn't tell the whole story --  where the fat is stored on your body is important, too. Some people carry fat around their midsection, while others have an "hourglass" shape, where you carry fat in your chest and hips. Body fat distribution is determined in part by environmental factors, like alcohol intake and cigarette use, but it also has a strong genetic component

To get an idea of your body fat distribution, measure the ratio between the circumference of your waist and the circumference of your hips. The more fat you carry around your waist (being apple-shaped as opposed to pear-shaped), the higher your risk is for heart disease. Ratios over 0.85 for women and 0.9 for men put people at substantially increased risk for obesity-related illnesses.

If your ratio or body fat percentage is higher than you'd like, the good news is that it's not permanent. A 2011 study suggested that a diet low in processed foods may lower your waist-to-hip ratio. If you're concerned about your body fat percentage, talk to your health care provider -- they'll have ideas on how you can manage your weight better in the long run.

The information contained in this article is for educational and informational purposes only and is not intended as health or medical advice. Always consult a physician or other qualified health provider regarding any questions you may have about a medical condition or health objectives.