How Do They Work?
Body fat measurement by way of skinfolds involves using a device, called a caliper, to pinch the skin and underlying fat tissue at a number of pre-determined body points and separate that skin and fat tissue from the muscle underneath. The thickness of the skinfolds at those pre-determined locations (usually 3-7 sites across the body) are combined and plugged into a formula along with age to determine body density. Then, in similar fashion to the volumetric methods, the formula-derived density is plugged into yet another formula along with total body mass (like the Siri Equation) to estimate body fat.
While the logic behind calipers seems relatively simple, and the equations are derived based on sound logic, like all the methods we’ve discussed, there are a number of possible sources of error.
Where is the error?
More so than the other techniques we’ve discussed, this technique’s accuracy is highly dependent on the skill of the administrator. Although the concept itself is rather simple, it takes a fair amount of experience and skill to separate skin and subcutaneous fat from the underlying muscle. Tester error is, in fact, the largest source of variability in skinfold measurements. A skilled tester makes a world of difference! Even amongst the most practiced professionals, however, other factors such as sweat or body lotion may skew results.
Another major source of error with this technique comes from the equations used to predict body density from the skinfold measurements. There are a number of different equations that have been derived to transform caliper measurements to body density. The most common equation, the Jackson-Pollock Equation (no relation to the painter!), was developed using hydrostatic weighing (dunk tank) as the ground truth. If you recall from the volumetric methods segment, dunk tanks can have individual error rates exceeding 5%. Thus, an equation derived using an error-prone method as a ‘ground truth’ will further compound that error.
In fact, even when performed perfectly, calipers can have error rates at the individual level as high as 10-15%. This may be in part due to assumptions made about the nature and distribution of fat. As calipers only directly assess subcutaneous fat at a few sites, the location and magnitude about other regions’ subcutaneous fat volume and visceral fat volume (fat around the organs) are based on educated conjecture. The byproduct of such speculation? Calipers tend to overestimate fat mass in very lean or muscular people and tend to underestimate body fat in overweight and obese individuals.
Like all two-compartment body composition methods, skinfold assessments are insensitive to variations in the density of fat-free mass. That is, for individuals with different-than-average bone density or water retention, body fat percentage will be systematically inaccurate.
Is there a workaround?
Many authors, researchers, and body composition experts recommend against using the two equations (e.g. Jackson-Pollock to convert skinfolds to density and Siri to convert density to BF%) to calculate body fat. Instead, they suggest that the raw caliper values will do a better job of indicating body composition change than their composite score calculated with the two equations. So, if instead of using skinfold thickness in an equation, you just start tracking the skinfold thickness itself, you may have a more reliable metric for tracking change!
The “sells” for skinfold thickness as a metric for measuring and tracking body fat are its low cost and ease of use. Available for just a few dollars online (although be wary! The cheaper the caliper, the higher the error), calipers allow you to track progress in the comfort of your own home. That said, the error can be very high – certainly higher than volumetric methods and DXA.
Are calipers totally useless? No. Especially not if you use the raw skinfold thicknesses as the primary measure you are tracking. However, errors in computing body fat from skinfold testing are nearly inevitable, so take any caliper-derived body fat with an extra-large grain of salt!
Stay tuned for the next part of the series, where we will cover bioimpedance-based body fat measurement.
This article is the fourth entry in Naked's series on the various methods of body fat measurement. These articles were written by Naked's in-house research scientist, Dr. Sam Winter.