Body fat percentage is the amount of fat you have on your body in relation to your lean mass. The only way to know for sure how much fat you have, and where, is to look inside your body, which is both invasive and dangerous. There are several methods for estimating the amount of overall body fat, and, although some are more accurate than others, each has its strengths and weaknesses and there is no one method that is the most accurate.
Dual Energy X-Ray Absorptiometry
Dual energy X-ray absorptiometry, DEXA, is one of the more accurate methods of measuring body fat. Based on the principle that you can measure the density of bones by the amount of photon energy absorbed, this method measures bone mass, lean muscle mass and fat tissue mass. The DEXA scanner uses low-dose X-rays that read the bone and soft tissue mass and requires only one pass to get a reading. This method is very accurate in reading average and fit individuals but is not as accurate in reading extremely obese individuals.
Underwater weighing works on the principle that fat floats, while the denser muscle and bones sink. A technician weights the client on land and again on a special underwater scale. He then estimates the estimated body fat based on those two measurements. This method is considered very accurate for average individuals, but elderly individuals with lower bone density may show artificially high body fat measurements while athletes with higher bone density may show artificially low body fat measurements. This method also does not take into account the buoyancy of the air in the lungs. You need to expel as much air as possible. Some people are not able to expel as much air, and that will also skew the readings. Underwater weighting generally requires at least three readings for the most accurate result.
Bioelectrical impedance, or BIA, works on the principle that muscle and bone conduct electricity better than fat. The subject holds onto a device, or stands on a special scale, with metal contacts that send a weak electrical signal between the two plates. The device measures how long it takes the signal to complete the circuit and estimates body fat percentage. This method is considered accurate because it takes an actual measurement of lean mass to get a more comprehensive estimate of the fat mass. The disadvantage to BIA is that it only measures the lean mass within the circuit, which means it measures only the upper body or the lower body. Individuals who store more fat, or have more lean mass, in one area or another may skew the results. Moisture in the skin also affects the reading, and a reading after a shower or workout session will skew the results.
Skinfold testing works on the principle that the density of the subcutaneous fat, or the fat under the skin, is proportionate to overall body fat. The test uses a special set of calipers to take readings from pinched areas of skin on different sites on the body. The technician takes readings from up to seven sites on the body, and the sites differ between men and women due to the way each gender deposits fat. The technician then calculates the body fat percentage based on an average of all readings. This method is considered accurate because it takes readings from multiple areas of the body. Where this method fails is in technician or operator error. If the technician does not pinch the skin or place the calipers correctly, it will skew the readings. The technician must take multiple readings to ensure the most accurate result.
- "Personal Trainer Manual"; American Council on Exercise; 2008
- “Physiology of Sport and Exercise”; Dr. Jack H. Wilmore, et al.; 2007