How is body fat assessed?

Some of the more common methods used to estimate body fat percentage include skinfold measurements and bioelectrical impedance assessment (BIA). Skin-fold measurements are commonly performed in health clubs by personal trainers. Meanwhile, BIA equipment can be found in clinical settings, health clubs as well as in homes in the form of bathroom scales. Underwater weighing, BodPod and DXA (dual energy x-ray absorptiometry) provide a more accurate and precise estimation of body fat however they require specialized equipment and trained personnel. These assessments are usually performed in medical clinics and university research labs.

 

How does skin-fold assessments work?

Skin-fold measurements technique is based on the premise that the layer of fat found beneath the skin, called subcutaneous fat, is a reliable indicator of total body fat. Skinfolds are pinched and measured with calipers from regions (“sites”) of the body such as the back of the arm (tricep), midback (subscapular), above the hip (suprailiac), abdomen, and thigh. Care must be taken to pinch only the skin and the underlying layer of fat, not the skeletal muscle beneath. The measurements can then be used in an equation to determine body fat percentage. These equations were mostly generated from underwater weighing studies within specific groups of people, such as female college students, male swimmers, women or men ages thirty to fifty, and so forth. Therefore, to be accurate, we need to use the equation most applicable to the person being assessed.

 

The accuracy of skin-fold assessment in estimating body fat percentage depends largely on the person doing the assessment. The average of multiple measures should be applied and equations using multiple skin-fold sites should be applied. A minute or more to allow compressed tissue to recover should separate the multiple measurements at the same site. Also, a pinch should not be held for more than 4 to 5 seconds before taking a measurement. If performed correctly, skin-fold measurements can be accurate and trackable for programs.

 

How does Bioelectrical Impedance Assessment (BIA) assess body fat?

Bioelectrical impedance assessment or BIA is based on electrical conductance. Electrodes, which transmit and receive electricity, are in contact with two limbs and a tiny electric current is passed from one electrode to the other using our body as a conductor. Body fat will act as an insulating material, while lean tissue such as muscle will serve as a conducting material. This is because muscle contains a lot of water and electrolytes while fat tissue contains relatively little water and electrolytes. Therefore, the amount of body fat relative to leaner tissue in the body will determine the speed of conduction of the electric current which in turn is used to estimate body fat.

 

How does underwater weighing and BodPod work?

Underwater weighing and BodPod apply the same general principle of densitometry (density measurement) to estimate body fat. However; to do so the former uses water and the second uses air displacement to estimate body volume. In both situations a person weight and volume is used to determine their density (Density = Mass/Volume), which in turn is used to estimate percent body fat. Underwater weighing has been done at universities for decades and is still considered one of the “gold standards”. Since our body is about 60 percent water, this weight would be negated when we are submerged in a tank of water. After removing as much air from the lungs as possible, the remaining body weight underwater is largely attributed to the relative amounts of body fat and nonfat or lean body mass (LBM). A person with a higher percentage of body fat will be less dense and thus a little more buoyant than a leaner person who weighs the same. Thus the person with the higher body fat level would actually weigh less underwater than a leaner individual of the same body weight.

 

How does DEXA measure body fat?

While dual energy X-ray absorptiometry (DEXA or DXA) was first commonly used to assess bone health, it also provides one of the most accurate means for assessing body fat. In fact one of the advantages of DEXA is that it also estimates regional body fat, such as in the abdomen, arms and legs. Two x-ray beams with differing energy levels are transmitted at the body. Since the absorption of these beams varies with different tissue, this can used to estimate body fat as well as bone mass. DEXA scans are not commonly done for body composition assessment, however if you are have a DEXA scan performed for bone health status be sure to ask for your body composition as well.