A body composition test is done to determine how much of the patient’s body is fat. Knowing that can help the subject plan a diet and/or exercise regime to lose weight, build muscle or both.

There are many ways to measure body composition, and each method has strengths and weaknesses. Some methods are more accurate than others. The more common methods include the following:

1) DEXA Scan
The DEXA scan is a type of X-ray that can distinguish fat from muscle and both from bone. The patient simply lies on a table while the machine takes its measurements. It is highly accurate, but it can also be expensive.

2) Whole Body Plethysmography
Also known as the “bod pod,” this method is also quite accurate. The patient steps into an egg-shaped pod. The technology in the pod measures the air the subject has displaced and the density of the subject’s body. Since mass is calculated by multiplying volume by density, the bod pod can determine the subject’s weight. As the densities of bone, muscle and fat are all known, further measurements and calculations enable the subject to learn what percentage of their body is fat.

3) Hydrostatic Weighing
In hydrostatic or underwater weighing, the subject sits on a scale in a large tank filled with water. The subject exhales all of the air from their lungs and goes completely underwater. They remain submerged and very still for 10 seconds to give the scale a chance to stabilize.

Just as the Bod Pod measures the volume of air displaced by the subject, the scale measures the volume of water displaced. The technician can use the information provided by the scale to calculate the percentage of body fat.

4) Skinfold Calipers
Skinfold calipers are a low-tech way of measuring body composition. The exercise physiologist uses the calipers to measure the thickness of two layers of skin plus any subcutaneous fat. He will usually take measurements from four to six places on the subject’s body. Common locations include the abdomen, triceps, waist, calf and shoulder blade. The subject’s percentage of body fat is then calculated.

While the caliper method has the advantage of being easy to do, it can produce variable results. It is also less reliable than some of the other methods, especially if the subject has localized fat.

5) Bio-Impedance
Bio-impedance measures the body’s ability to conduct electricity. Lean tissues like muscle and bone all conduct electricity well, while fat acts like the insulation on a wire.

The subject either holds a gadget or sits on a device. When they press a button, a small amount of electricity is sent through part of their body. The fatter they are, the more current will be blocked. A large drop in current therefore indicates a high percentage of body fat.

Since bio-impedance devices only measure body fat on given parts of the body, they are less accurate than other methods for testing body composition. The results can also be skewed by such variables as hydration or when the subject last ate.

On BMI
Body mass index (BMI) is not a method for determining body composition. It simply measures a ratio of weight to height, while body composition tests determine the ratio of fat to lean weight (non-fat). It is not as reliable a method for calculating ideal body weight because it tends to penalize big-boned or muscular people, who often have a high weight to height ratio. Consider a football player who is 6’6″ and weighs 275 pounds. He has a BMI of 31.8, which is considered “obese.” A body composition test, however, would show that he has a healthy amount of body fat and is in very good shape.

Call or email Cenegenics Boston today to learn more or schedule your own DEXA Scan.

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