Home Scales Can Accurately Measure Body Fat

Q:  To help me stay on track with my weight-loss program, I decided to weigh myself weekly and record my progress. I recently bought a new scale and it came with a feature to measure body fat. The scale instructions give the normal values for body-fat percentages, but I don't know if I should pay more attention to the body fat or to the weight itself.

Can a scale really measure body fat anyway? I thought you had to use skin-fold calipers.

A:  Obesity leads to high blood pressure, diabetes, coronary heart disease, arthritis, heartburn and sleep trouble, just to name a few. The clinical research that concluded that obesity led to these problems is based on the fact that obesity is defined as too much weight for your height.

Put another way, obesity is a body mass index of greater than 30. Many body mass calculators are available online.

Of course, it's not as simple as a BMI alone, because both weight-lifters and sedentary folks will be heavier than exercisers, and it's the extra fat on the body (and extra food that is not burned up with exercise) that contributes to the serious problems. But for most Americans, the body weight tells the story, revealing the risk for problems such as heart disease.

If you're doing a lot of weight training while reducing your food intake in your weight-loss program, your weight may not initially drop because as your fat converts to muscle, you become more dense. In this case, you may wish to record your body-fat percentage along with your weight to get a better picture of your progress.

I must add that recent research has focused on the location of excess fat, not just how much. Apple-shaped bodies (where fat is deposited primarily around the middle) suffer more complications of obesity than pear-shaped bodies (where fat is deposited primarily in the buttocks and lower extremities). Here the best measure is the waist circumference or the waist-to-hip ratio. Normal waist circumference is less than 35 inches in women and 40 inches in men; normal waist-to-hip ratio is less than 0.9 in women and less than 1.0 in men.

Your scale is measuring body-fat percentage with a technique called "bioelectrical impedance." A weak electric current is sent from the scale, up one foot, through your body and back to the scale down the other foot. The speed at which the current travels is dependent on the relative amount of the more conductive lean tissue compared with the less conductive fat. Similarly, hand-held devices send current from one thumb through the body to the other thumb.

This method is about 95% to 97% accurate, a bit better than the skin-fold calipers.

Julie L. Mitchell, MD, MS, is an Assistant Professor of Medicine at the Medical College of Wisconsin. She practices at the Froedtert & The Medical College of Wisconsin General Internal Medicine Clinic - East. Her column appears in the Milwaukee Journal-Sentinel.