Body composition methods: validity and reliability

In a previous blog we explored how different techniques measure body composition. Here we are going to look at how valid and reliable these measures actually are. It is worth noting that the main limitation of all body composition assessments is that they are based on assumptions. The only truly accurate way to assess body composition is cadaver analysis (i.e. dissection). In this article we will focus on the 3 most used methods to measure body composition: Dual X-ray absorptiometry (DXA), skinfolds and bio electrical impedance measurements (BIA). Air displacement measurements (Bodpod is only mentioned in the infographic).

Methodological issues of measuring body composition

The measurement of body composition can be influenced by many factors including prior physical activity, the technician, fasted/fed state, hydration status etc. It is therefore very important measurements are standardised as much as possible. This will improve the accuracy of measures, as well as the reliability if you are taking repeated measures over time. This depends upon the method used, but some standardisation techniques that are easy to employ are:

  • Perform in the morning, before the athlete has eaten anything or performed any exercise

  • If you are not performing the test in the morning, then make sure you always take the measurements at the same time of day

  • Ensure the athlete is well hydrated

  • Use the same machine/technician when you are taking repeated measurements over time (3-9% variability can be attributed to differences between investigators)

How valid and reliable are the different measures?

Whilst there has been significant progress in the techniques used to estimate body composition, a gold standard method does not exist. No techniques have an accuracy better than 1% (1) and this means that, at best, we can estimate body fat to within 700 grams in a 70kg person and in reality, it is usually closer to 1000 grams. This means that a percentage body fat of 15% could really be anything between 9.5kg (20.9lbs) and 11.5kg (25.4lbs) for a 70kg (164lbs) athlete, and body fat would thus be between 13.6% and 16.4%: obviously a wide range. For a 50kg person, this range would be even greater: 13.0% and 17.0%. This is hardly ever taken into account when interpreting the data and we are thus assuming and working with a pseudo accuracy. This is even worse if the measurements are variable from day to day (poor reproducibility or reliability). Whilst measuring more compartments may provide more accurate estimates, they require multiple body composition techniques to be used to measure each of the compartments. This increases expense and time, making it unpractical to be used in a sport setting.


No techniques have an accuracy better than 1%

DEXA, skinfolds and BIA are the most widely used techniques in a practical (i.e. athlete) context. And so, we will now compare those methods in further detail…

DEXA

Whilst DEXA is considered the laboratory reference method, it still has errors within the measurement. Caution should be taken when repeating DEXA measurements, and how often these are made, because it will not pick up on small changes in body composition. The estimated error for prediction of body fat % is between 2-3% (1). An IOC consensus regarding body composition methods concludes for DEXA that whilst it is relatively precise for whole-body estimates of body composition, it is less reliable in producing accurate fat estimates of lean athletes (1). The assessment of total and regional fat free mass (FFM) is generally acceptable if total scanned mass equates to scale mass.

Skinfolds

Over 100 equations have been created to predict body fat %