What does matter—whether you’re seeking improved health, a better physique, or both—is what you’re made of: how much of you is muscle, bone, skin, and other lean tissue, and how much is fat? And more importantly, how are those numbers changing over time?
To find that crucial information out, you need a body composition test—a tool that indirectly estimates body fat and lean tissue (the only direct way to measure these things is dissection, which we don’t recommend).
Right now, there are numerous such tools available. Here’s our breakdown of the pros and cons of each one:
You’ve seen this one: A tech grabs a pair of calipers, pinches some flesh at several sites on your body, feeds the resulting data—along with your age, height, weight, and gender—into a formula which then ballparks how much fat you’re carrying. Crude, but effective.
Pros: For convenience and price, skinfold testing is near the top. It takes five minutes or less, it’s available at most gyms (there are dirt-cheap home models available as well), it’s easy, and it’s fast. If you’re looking for a simple way to track how your body fat and lean mass are trending over time, it’s a reasonable bet.
Cons: Having a stranger pinch the very spots on your body you are most self-conscious about can be unpleasant. And unlike other methods, the technician who does the measuring matters, big time. Giving accurate skinfold tests is an art, and if the trainer giving the test isn’t skilled, your results will suffer. Hydration levels also skew your results.
Accuracy: Exercise scientist Vivian H. Heyward estimates that, under ideal conditions, skinfold testing is accurate to within 3.5 percentage points—meaning if an experienced, competent tech measured your body fat at 10 percent, it might actually be as high as 13.5 percent or as low as 6.5 percent. That’s a wide margin—the difference between “reasonably fit” and “shredded.”
The Verdict: For fast and cheap, you can’t beat it. And if your tech is experienced and skilled, it’s a decent indicator of how you’re trending over time. Just don’t take those numbers to the bank.
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Bio-Electrical Impedance
This is the next level of techiness. Depending on the device being used, you’ll grasp a couple of metal handles, stand on some metal plates, or both. The machine sends a mild, painless electrical current from one arm or leg to the other. Since fat and muscle slow the current at different rates, the machine can then estimate how much of each you carry.
Pros: Convenience. You’re fully clothed, and no stranger is pinching your pudgy spots. A home model you can use forever is available for less than 100 bucks.
Cons: Since water is a conductor of electricity, bioelectrical impedance is profoundly affected by hydration. The area measured is limited, so if you carry lots of fat in your legs, a handheld device will underestimate your fat levels and a stand-up model will overestimate them.
Accuracy: Bioelectrical impedance is about as accurate as skinfold measurement, according to a 2006 study.
The Verdict: A good—if less than fully accurate—frequent-use option. If you’re going to go this route, test yourself at the same time of day, before any activity—ideally just after waking, before any food or drink—to ensure consistency.
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DEXA
Used in medical circles primarily as a tool to measure bone density, DEXA scanning uses two simultaneous X-rays, each of a different energy level, to detect and measure bone, muscle, and fat mass. The radiation exposure is very low—about 20 percent of the exposure in the average dental X-ray—and all the non-invasive scan requires that you lie comfortably on your back for just a few minutes.
Pros: Of the higher-tech, more expensive body fat testing methods out there, DEXA scanning is considered your best bet. Increasingly, DEXA is available commercially through companies like BodySpec at a cost of around $50 per scan. Compared to hydrostatic weighing, it’s more widely available, and more convenient: no need to get wet or simulate drowning to get a reading.
Though it’s on the pricey side, a scan gets you much more than a single number. A scan gives you left-to-right side comparisons on lean mass and bodyfat, so you can detect asymmetries, as well as fat distribution, bone density, and an estimate of your resting metabolic rate. There’s a special section for visceral fat, too—an essential number for those concerned about cardiovascular and metabolic disease.
Cons: 50 bucks isn’t cheap, and you do have to schedule an appointment—which, including the consultation, lasts an hour or so—and make a visit to a testing facility. In major cities, they’re pretty accessible, less so in more remote areas.
Accuracy: DEXA is very accurate at measuring bone and lean soft tissue (muscles, skin, organs), estimating fat mass by subtracting your lean body mass from your overall mass. Unlike skinfold and bioelectrical impedance, it detects fat head to toe, not just in the areas being tested.
The Verdict: At present, this is the Gold Standard. It’s expensive, but if you want the real goods, this is the way to go.
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Hydrostatic Weighing
For a long time, hydrostatic weighing—the dunk tank—was considered the most accurate way of measuring body fat. You get into a large tub, blow out all your air, and hold for a few seconds while the machine registers your underwater weight. Because muscle, organ tissue, and bone weigh more than fat, a technician can compare your underwater weight to your weight outside the tank and run some simple numbers for an estimate of your body fat percentage.
Pros: Unlike skinfold testing or bioelectrical impedance, underwater weighing measures the fat all over your body, head to toe. As long as you blow all your air out, human error—say, from pinching too much, or not enough—is largely eliminated. Unlike DEXA, there’s no radiation exposure, so the dunk tank is considered safe.
Cons: Reread the part about submerging yourself in water and blowing out all your air. It’s quite a to-do, and, of course, you’re soaking wet afterwards. Your results are skewed by unusually lower by unusually high bone density—something many lifters should take note of. At about $50 a pop, it’s among the most expensive options.
The Verdict: Worth a try if there aren’t better options available. But if you’re going to drop 50 bucks on a bodyfat test, there are more accurate and hassle-free ways to do it.
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Bod Pod
If you’ve ever wondered what a chicken feels like before it hatches, now’s your chance to find out: the Bod Pod is an egg-shaped chamber you sit in while the machine measures your body density, weight, and lung capacity, and estimates percentages fat and lean tissue.
Pros: Fast (each scan takes less than eight minutes), convenient (you can do the test in gym clothes), and informative (you’ll emerge with a printout of your basic data).
Cons: At 45 bucks a pop, it’s on the expensive side, but it’s not as accurate, and you don’t get the breadth of information you get with a DEXA scan.
Accuracy: Two studies—one from 2000, one from 2008, found that, relative to the dunk tank, the Bod Pod tended to underestimate body density and overestimate body fat levels. Another study suggested that the Bod Pod is especially unreliable at tracking changes in individual’s body composition over time. Bodybuilders—who are likely to want to use technology like this for that very purpose—take note.
The Verdict: A reasonable option for cross checking if you’re so curious that you want a second opinion, and worth it if other high-tech methods aren’t available. But if the main reason you’re getting tested is to track your progress over time, the Bod Pod isn’t a great option.
