Overweight people who weigh themselves regularly tend to be more successful at losing weight and keeping it off, studies report. But do special bathroom scales that measure body fat offer additional advantages?
Available in countless models, body-fat scales use a technology called bioelectrical impedance to estimate how much body fat you have. When you step on sensors on the scale, an imperceptible electrical current passes up one leg, across the pelvis and then down the other leg. Because it contains much more water, muscle conducts electricity better than fat does, so the greater the resistance, the more body fat you have. The scales use formulas to calculate a body-fat percentage from this resistance information, along with other data that you enter (height, weight, age, gender). Some also include hand electrodes to better estimate overall body fat. And an increasing number come with fancy features such as wireless transmission of data to your computer.
Are they accurate?
One problem with body-fat scales is that they are often inaccurate. Many variables affect the results, including how hydrated you are, when you last ate and exercised, and even whether your feet are highly calloused or dirty, as well as the type and quality of the product itself. Studies have found that different body-fat scales produce widely varying readings and that these often differ from standard methods of fat measurement. (Devices that also have hand electrodes tend to fare somewhat better.) In a study published in Obesity Facts in 2008, scales with only foot electrodes underestimated body fat in people with lots of body fat and overestimated it in leaner people. Even the manuals say the devices may be less accurate for elderly people, highly trained athletes, children and people with osteoporosis, among others. Consumer Reports no longer tests body-fat scales because of their inaccuracies.
Do you really need them?
It’s debatable whether you need to know your body fat in the first place. True, body weight can be deceptive because it doesn’t indicate how much is from fat and how much is from muscle. But there’s no widely accepted standard for ideal body fat; it depends on age, sex, fitness and ethnicity. According to some experts, a “healthy” range is 23 to 33 percent for middle-aged women, 11 to 21 percent for middle-age men and up to 35 percent for older women and 24 percent for older men. Athletes typically have much less body fat.
In addition, more important than total body fat is where the fat is distributed—and body-fat scales don’t tell you this. Excess fat in the abdominal area (as in an “apple-shaped” body) is linked to cardiovascular disease, diabetes and some cancers, while fat in the thighs and hips (a “pear-shaped” body) is not a health problem and may even be protective. In a 2009 study in Obesity, readings from two body-fat scales were only weakly correlated with obesity-related risk factors, such as blood lipids and fasting blood sugar.
On the other hand, some researchers say that body-fat scales can be useful for tracking body fat changes over time and that they can help motivate some people to lose weight (regular scales can do this too, of course). Still, testing your body fat is not something you need to do on a daily or weekly basis.
If you want to monitor your weight, any basic bathroom scale—dial or digital—that gives consistent readings is sufficient. The more fancy extras and the more stylish the look, the higher the price—but not necessarily the better the performance.
One of the best ways to see if you have too much risky body fat is to simply measure your waist. Body mass index, or BMI, which takes into account both your weight and height, is another good measure unless you are very muscular. The NIH has this online BMI calculator.
If you want to know your body fat percentage, however, you may be able to have it measured by a trained professional at a university research facility, hospital or sports medicine clinic. The methods used include hydrodensitometry (underwater weighing, considered most accurate), a Bod Pod (which uses air displacement instead of water) and DEXA scans (a type of X-ray similar to a bone density test). Body fat can also be assessed using skin-fold calipers, but the accuracy of the results depends on the skill of the person doing the test.
Published February 26, 2013