How Body Composition Is Measured in Children

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If you’re monitoring your child’s weight or trying to help him lose weight, you may wonder what tools are at your disposal besides measures of body mass index (BMI). Since BMI doesn’t calculate body fat directly, nor does it measure lean body mass—it simply reflects a child’s weight relative to his height—you may want to consider other techniques. After all, it’s increasingly recognized that the amount of body fat and lean mass a child has can affect his current health and future health risks. But the techniques that are available for adults aren’t always appropriate for children. Here’s a look at the options.

Low-Tech Measures

Waist circumference: To figure out if your child is slimming down, especially as he grows in height, it can be helpful to bring out a flexible tape measure. You can use it to measure the circumference of your child’s waist (the narrowest part of the midsection between the lowest rib and the top of the hipbone). In a study involving 201 kids between the ages of 7 and 17, researchers at the University of Cincinnati found that the strongest correlate of fat distribution was waist circumference, thus making it the best simple technique for measuring body fat distribution.

Skinfold thickness: Using a set of calipers to measure the thickness of the skin at various points on the body, this technique can assess relative “fatness” as well as isolated fat deposits in kids and adults alike. While this technique can be used to assess changes in a given child, useful reference data to compare a child to his peers still needs to be developed. Another limitation: It can’t be used to assess overall fat-free (or lean) mass.

Bioelectric impedance analysis (BIA): This technique uses electrodes placed on the wrist and ankle to run a small electric current through the body to determine how much resistance there is to its passing through. The more body fat there is, the harder it is for the current to flow through the body. In the past, the accuracy of BIA devices has been poor—they can be skewed by water retention alone—but the technology is improving.

High-Tech Methods

Dual energy X-ray absorptiometry (DXA): While DXA scans, which rely on X-rays, were developed to measure bone mineral density in adults at risk for osteoporosis, they can also be used to calculate body fat and lean mass in children as young as 4 years old. This could be useful for gauging whether weight loss is accompanied by changes in lean mass as well as fat mass. A 2013 study in the U.K. found that DXA was the most accurate method, compared to the other techniques described here, for assessing fat mass and fat-free mass in children.  

Plethysmography: With this technique, a child sits inside an egg-shaped cocoon for about 5 minutes while air is gently blown around him to measure fat and fat-free mass. In a study comparing different methods for assessing changes in children’s body composition, researchers at the National Institute of Child Health and Human Development at the National Institutes of Health found air displacement plethysmography performed better in measuring changes in children’s body fat than did BIA or skinfold thickness but it wasn’t as good as DXA.

Magnetic resonance imaging (MRI): A technique that uses a magnetic field and radio waves to produce images of body tissues and organs in the body, MRI is better than other techniques at estimating regional body composition, especially fat in the belly. The downside is: It’s very expensive and it’s not suitable for young children who will struggle with the measurement protocol (they have to stay very still while they’re inside the tube-shaped machine).

Your best bet is to discuss the various options with your child’s pediatrician. This way, you’ll be able to find out which one makes the most sense for his condition and your concerns—and which ones are available in a clinical setting near you.

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