There are better ways to measure body fat than BMI
We’re using an outdated, inaccurate system.
This post has been updated. It was originally published on January 10, 2018.
Every January, fat’s in the crosshairs of health columnists, fitness magazines, and desperate Americans. This year, PopSci looks at the macronutrient beyond its most negative associations. What’s fat good for? How do we get it to go where we want it to? Where does it wander when it’s lost? This, my friends, is Fat Month.
Like a nice mutton, your body gets more fatty as it gets older. This is just the unfortunate reality of aging: lean muscle slowly degrades, some of it turning into fat (and some people stop thinking you look tasty).
This is not a huge problem for sheep, who lack the mental capacity to understand their body composition might affect their health, but it is a problem for human medicine. The primary way we judge a person’s obesity level is a metric called body mass index, or BMI. It’s calculated by dividing a person’s weight by their height squared, which means it’s really only an index of how a person’s height compares to their weight.
You may notice that the word “fat” did not appear in any part of that explanation. That’s because BMI is not actually a good indicator of how much body fat a person has, even though it’s the primary indicator we use to determine body fat percentage. Athletes who carry lots of lean muscle can end up with an “overweight” BMI because muscle weighs more than fat, and we know that small amounts of belly fat can put your health at risk even when your BMI says you’re within healthy range. BMI may generally correlate with how much body fat a person has, but because it only takes height and weight into account, it’s easy for this metric to miss the mark.
One study found that 8 percent of men and 7 percent of women are incorrectly told they’re obese despite having normal body fat content, while 41 percent of men and 32 percent of women are told they’re not obese even though they are. That error rate is … not great. Especially when obesity can lead to dire health problems like diabetes, heart disease, and cancer.
BMI gets a pass, though, because it has something that none of the other, more accurate, tests can offer: it’s cheap and easy. Every other method requires some kind of technical equipment and training to administer. To calculate your BMI, someone just needs to weigh and measure you. But despite its ease-of-use, there are other options doctors are exploring.
Skinfold thickness
Probably the least appealing of all the methods, skinfold tests involve pinching your body fat to determine how much of it sits on top of your muscle. Just imagine a nurse taking metal pincers and squeezing the fat on your belly—that’s almost exactly what it’s like. This is a useful way of figuring out where your biggest fat deposits are, but not as easy to implement as you might think.
Measuring fat deposits actually requires significant training to do properly and then retraining to make sure healthcare workers continue to do it right. And you have to maintain the pincers themselves, constantly cleaning and calibrating them.
Also, let’s be honest, no one wants this to happen to them. Most people already hate going to their annual check up. They might stop going altogether if someone starts trying to pinch their fat with metal calipers.
Grade: B
More accurate than BMI, but also much less pleasant and not as cheap.
DEXA scan
This is basically the most “Jetsons”-esque way we have of measuring body composition. You just lie down, fully clothed (minus metal jewelry), and an X-ray machine scans you. It’s not so much an estimate of your fat as it is a scientific breakdown of exactly how much fat and muscle you have.
DEXA stands for “Dual-Energy X-ray Absorptiometry,” because it uses the absorption properties of your body to figure out which bits are fat and which are muscles. This is also how x-rays work in general. Your bones are much more dense than everything else in your body, so when someone shines x-rays at you, your bones deflect the rays more. Fat and muscle also reflect radiation differently from each other (and from bone), so we can also use x-rays to determine where you have fat versus muscle.
Unfortunately, the scans cost several hundred dollars. And though they’re fast, you do have to set up an appointment and go to the scanning facility. This is not an easy way to get everyone’s body fat percentage, at least right now.
Grade: A-
Incredibly accurate, but too expensive and inconvenient to be used widely.
Air-displacement plethysmography
Sometimes (upsettingly) called “the BOD POD,” this method uses air displacement (http://ybefit.byu.edu/portals/88/documents/how%20does%20the%20bod%20pod%20work.pdf/) to figure out the size of your body. It’s kind of like a dunk tank, where you can see how much water an object displaces to determine its volume, except it’s you inside a small chamber with a little window you can peek out of. There’s a closely-related body-fat determination method, incidentally, called hydrostatic weighing, where you literally get into a dunk tank.
Unlike going underwater, you don’t have to try exhaling all of your air because the system takes into account how much you’re breathing in and out. You just sit there breathing normally and the system uses basic chemistry principles to figure out your total volume.
Both air- and water-displacement can calculate your body’s density based on its volume and weight, and from that determine your body fat percentage. It’s accurate and automated, but like DEXA scans it requires special equipment.
Grade: A-
Accurate, easy, automated, but again quite expensive and impractical for use in a standard doctor’s office.
Waist circumference and waist-to-hip ratio
This is not technically a way of measuring overall body fat percentage, since it only looks at your waist measurement, but it is an effective metric for assessing health problems related to obesity. Fat accumulated around the midsection is more dangerous than arm fat or leg fat. No one is quite sure exactly why this is, but it seems to be related to the way the fat cells in your belly drain into your liver and pancreas. The drainage affects your metabolic system more because the liver and pancreas control your insulin and blood sugar levels.
Measuring your waist circumference, especially when you compare it to how wide your hips are, gives doctors quite an accurate sense of how at-risk you are for various obesity-related problems in a way that BMI just doesn’t. It’s also really easy.
Unlike DEXA scans and displacement methods, waist circumference only requires a measuring tape. That’s easy for doctor’s offices to keep around, and easier to train people to use than a pair of calipers.
Grade: A
Though it doesn’t give an overall body fat percentage, gives an accurate sense of health risks and is easy and cheap to implement.
