Is Your Body Fat Measurement True?


A little over a week ago, Samantha, one of my nurses, approached me and said that her trainer told her that her body fat percentage was 30%.  This surprised and upset her and she, consequently, suggested that I do a post about the accuracy of these measurements.  Since then, I have read a number of sources about this topic.  Similar to measurements of caloric expenditure, it is difficult to get a really clear idea about the accuracy of measurements of body fat percentage.

To frame this discussion, however, it is important to first review body composition and “normal” ranges of body fat percentages.  The human body is composed of fat and lean mass.  Fat mass is divided into essential fat (the fat that is required for normal physiologic function), which is approximately 3% for men and approximately 12% for women, and non-essential fat, which is anything above those baseline figures.  For athletes, typical total body fat percentages are 6-13% and 14-20%, for men and women, respectively.  For fit men and women, typical percentages are 14-17% and 21-24%, respectively.  For average men and women, typical percentages are 18-24% and 25-31%, respectively.  Finally, obese is considered to be above 25% in men and above 32% in women.  Typical percentages rise slightly with aging.  Separate from specific measurements of body fat, measurement of body mass index (BMI) can be used.  This is calculated by dividing weight in kilograms by height in meters squared.  A BMI of 25 or greater is considered overweight, while a BMI of 35 or greater is considered obese.

There are a number of methods available to estimate body fat percentage.  These include skinfold caliper testing, bioelectrical impedance analysis, dual-emission X-ray absorptiometry (DXA or DEXA scan), hydrostatic (underwater) weighing, whole-body air displacement plethysmography (the Bod Pod), near-infrared interactance, ultrasound, CT, and MRI.  Further discussion of the most commonly-used methods is as follows:

Skinfold caliper testing.  This inexpensive and widely available test involves measuring the loose skin that can be pinched in specific areas of the body.  Typical protocols involve measuring 7 areas of the body.  The measurements are entered into a formula and body fat percentage is then estimated.  This method cannot measure deep belly fat.  It is also limited by the expertise of the person doing the testing.  In one reference, compared to DXA (which is considered the research standard), the skinfold caliper method was off by approximately 7%.

Bioelectrical impedance analysis (BIA).  This is the method that Samantha’s trainer used at the gym.  In her case, she held both hands onto a handheld device.  Similar devices that use the same principles are in scales that estimate percentage of body fat along with measuring weight.  These devices operate under the principle that an electrical current passes more easily through lean tissue than fat.  Therefore, by measuring the current, percentage of body fat can be calculated.  According to several studies, this method can be off, compared to DXA, by approximately 3-9%.  This wide range is explainable by the sensitivity of  BIA to hydration (dehydration leads to an overestimation of body fat %), recent exercise (underestimates body fat % due to decreased electrical impedance), recent meals (underestimates body fat %), the variety of manufacturers and models of these devices, and the different ways these devices are used (for example, using two hands or two feet vs. all four extremities for electrodes).

DXA scan: In this method, the body is scanned by X-rays of two different levels of energy.  Since X-rays of one level of energy are better absorbed by fat than the other, software associated with DXA scan machines can calculate the difference between fat and lean mass and, therefore, can estimate the body fat percentage.  Inaccuracy has been estimated at approximately 1% and this method is, therefore, used as the current gold-standard.

Hydrostatic weighing: In this method, an individual exhales fully and then gets completely submerged in a tank of water to estimate his or her mass per unit volume.  Hydrostatic weighing is based on Archimedes’ principle using measurements of the weight of the body outside the tank of water, the weight of the body when completely immersed in water, and the density of water  Through a series of calculations, body fat percentage can be calculated.  Inaccuracy has been estimated at 0.1-1.2%, hence this method used to be the gold standard for measurement of body fat percentage.  However, this method can be difficult to perform perfectly and can be uncomfortable for the individual who is being measured.

The Bod Pod:  This method is theoretically similar to hydrostatic weighing, but uses air rather than water.  In this case, the individual who is being tested gets into a “pod” that looks a lot like a science fiction space-capsule.  There are more variables to control in this method than in hydrostatic weighing, such as clothing and facial hair.  Therefore, it is unsurprising that inaccuracy has been estimated at 1.5-5.3%.

In understanding the different methodologies of measurement of body fat, it is helpful to also understand the concept of body components and how these are divided by the different methodologies used.  For example, in most testing, such as BIA, hydrostatic weighing, and the Bod Pod, the body is essentially divided into two components: fat mass and lean mass.  In DXA scans, the body is divided into three components: bone, fat, and soft tissue that is not fat.  Finally, there is also four component testing, in which, for example, radiologically marked water (with deuterium) is used in combination with a DXA scan to further divide the body into fat, protein, water, and mineral (bone).  This final division into four components can be helpful to get the most accurate information possible.  This is because hydration can be a confounding factor to the accuracy of DXA scans.  For example, there is is study that demonstrated that a 5% change in hydration can lead to a 2.7% change in results from DXA scans.

The measurement of body fat percentage can be very valuable to health care providers to help measure and guide health and can be further valuable as a motivator.  There are numerous methods to measure body fat and all have advantages and disadvantages.  In general, quick and easy methods, or methods that require a lot of expertise to perform well (skin caliper measurement) are more prone to error.  So, in answer to Samantha, your body fat percentage may be much lower (or higher) than the measurement you received from you trainer.  The most important thing, however, is that you were motivated enough to embark on an exercise program and to get a starting point.

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