Is Body Fat Testing Necessary?

Is the test really a good measure of health or an outdated annoyance?

Every so often, body fat testing will creep back into the news. (Remember the media field day around Paul Ryan’s alleged 6 percent body fat?) When these headlines come out, it can be tempting to run out and get tested to see how you stack up—but does it really say anything about your health?

Dr. Sriram Machineni, a physician in Massachusetts General Hospital’s Weight Center, says yes and no. So that’s not confusing at all.

Machineni says that body fat testing does say something about general health and remains popular because “everyone wants an objective measure to see how much progress they’re making,” but overall, it isn’t a truly meaningful measure of well being.

“As we find out more about body fat, we realize fat is not the same at all parts of the body. Visceral fat, which is fat around the organs, is the one that causes a lot of the medical problems that people have,” Machineni explains. “A person might have the same amount of total fat, but if they have it all in the visceral part, you have a sicker person. Just knowing the total amount of fat is a better measure than body mass index, but it still doesn’t get us all the way to understanding how sick or how healthy a person is.

Furthermore, Machineni says the tests available to measure body fat actually decrease in accuracy the heavier someone is. The test typically used to measure body fat involves passing a small electric current through the body, and using that current to test the resistance of different tissues to determine what is a fat mass and what is fat-free. But because those calculations are based on assumptions about the body’s physiological makeup, a higher than average weight, or even dehydration, would throw off the test. “It’s accurate in most physiological states,” Machineni says, “but as the weight keeps going higher and higher it starts becoming more and more inaccurate and cannot be used.”

The alternative testing option, using calipers to measure skin fold thickness, has accuracy problems of its own. “Even slight differences in the way it is done can give wildly different results,” Machineni notes. “Unless someone has been doing it consistently and does it at exactly the same place, there is no standardization. They can probably get a few numbers, but they cannot figure out an accurate number of body fat levels.”

Because of those flaws, Machineni says he mostly relies on other methods to chart weight loss progress, especially for patients who are trying to lose weight and are frustrated by a scale that won’t budge. “Because of the inaccuracies and the limitations of the testing that we have right now, what I usually ask [patients] to do is check their waist circumferences,” he says. “The waist circumference is a much better measure of how much fat is around the organs than total body fat testing.”

As for Paul Ryan’s 6 percent number? Machineni says between 10 and 25 percent is a healthy range for men, while women should shoot to be between 20 and 35 percent. In other words, don’t lose sleep trying to get down into single-digit range.