Body composition is something that is measured regularly in sporting contexts. In this article, we will take a look at some of the different methods used to measure body composition. In future articles, we will explore how valid, reliable and practical these measures actually are.
What is body composition?
Our bodies are made up of bone, muscle, tissue and water. Body composition measures what proportion of the body is made up of each of these components. At the most basic level, body composition is measured using a ‘two compartment model’, meaning that it assesses:
Fat mass (FM): as it says, fat stores within the body.
Fat free mass (FFM): all body components (excluding fat), including internal organs, skeletal muscle, bone and body water.
Body composition can be measured up to a maximum of five compartments – atomic, molecular, cellular, tissue and whole body (1). In most cases, the more compartments that are measured, the less error there is in body composition estimates. However, measuring more compartments requires multiple methods of body composition assessment to be used. It is more common the a basic two-compartment model is used to assess body composition in athletes. Although these methods re more practical, estimates of FFM can vary largely between methods.
The more compartments that are measured, the less error there is in body composition estimates
Why measure body composition?
There are a variety of reasons why you may have your body composition assessed as an athlete. This includes:
To determine the effectiveness of an intervention.
To track body composition goals
To assess injury risk. For example, low bone mineral density is linked to increase bone stress fracture risk.
To aid in setting body composition goals.
To assess health risk (could be due to being underweight or overweight).
For the first two reasons you will need a method that provides reproducible results (i.e. is reliable). Even if it is not very accurate it is still possible to use this method to track changes. The other reasons require accurate absolute numbers and therefore the method must be reliable but also accurate.
Methods of body composition measurement
There a range of techniques that can be used to measure body composition which vary in their accuracy, reliability, cost etc. Commonly used methods only provide an estimate of body composition because they are based on assumptions regarding the compartments measured. This is because the only truly accurate way to measure body composition is by dissection! Below is a brief overview of the common methods used…
Dual energy x-ray absorptiometry (DXA)
Two low energy x-rays are passed through the body which are absorbed differently by bone and tissues. A scan takes ~10-20 minutes, exposing subjects to very minimal doses of radiation (equivalent to <2 days of natural background radiation or roughly a 2 hour plane flight). DXA can measure regional body composition, sub-dividing the body into different components (i.e. arms, legs and trunk), as well as bone density. DXA relies on certain assumptions, and when these are violated, errors in measurements can occur. It is therefore extremely important that the ‘Best Practice DXA Protocol’ proposed by Nana et al. is followed as strictly as possible (see reference 2 for details).
Bioelectrical impedance analysis (BIA)
A small alternating electrical current is passed through the body, and the impedance (resistance) to this is measured. Muscle tissue contains a high water content which allows the electrical current to pass through quickly, however the electrical current experiences resistance when passing through fat tissue. The resistance allows total body water (TBW) to be measured, which is then converted to FFM using the assumption that 73% of FFM is water. Single frequency BIA scales are typically used allowing only TBW to be measured, however if multiple frequency scales are used, this can be further differentiated into extracellular water and intracellular water.
ISAK Skinfold measurements
ISAK stands for the International Society for the Advancement of Kinanthropometry who train practitioners to perform skinfold measurements in a standardised way. The skinfold technique measures a double fold of skin, which reflects the subcutaneous fat thickness at various sites across the body. Skinfold thickness is measured in mm, and various population-specific equations have been created to attempt to convert these measures into body fat percentage. However, it is important to note that this is only an estimate of body fat %, based off another estimation measure and these calculations do not take into account differences between populations and/or individuals. Skinfolds are best used as a monitoring tool over time, with the same person taking the measurements each time. The thickness of a skinfold also depends on hydration status. So although this method is relatively easy there are also quite a few limitations.
This is only an estimate of body fat %, based off another estimation measure
Air displacement plethysmography (Bodpod)
Air displacement plethysmography measures body composition through a person sitting within an enclosed chamber (i.e. Bodpod) whereby body volume is indirectly measured through measuring the volume of air the body displaces within the chamber. In other words, the amount of air that you displace when stepping in the chamber is equivalent to your body volume. Volume, in addition to body weight, can then be used to calculate body density, which then allows FM and FFM to be estimated.
Hydrodensitometry (underwater weighing)
This technique involves being fully submerged in a tank of water and expelling all air in the lungs whilst underwater weight is measured. Both bone and muscle have a greater density than water, whereas fat mass has a lower density than water. Therefore, someone with a larger amount of FFM will weigh more in water. Body density is calculated using underwater weight, body weight outside of the water, density of the water and residual volume of the lungs. The residual volume in the lungs is measured by inhaling helium and measuring the dilution. Estimations of FM and FFM can then be made. This technique is perhaps the most direct and accurate technique to measure body fat, but there are few places that have this facility and it is not a very practical method.
In conclusion...
There are a number of techniques that can be used to measure body composition. The technique we should use depends on the goal of the measurement. For example, if we want to know more about bone density, we should use DXA. If we need an accurate measure of body fat, we cannot use skinfold measurements and we should use underwater weighing or DXA. On the other hand, if we need a practical way to track changes over time, we should consider skinfolds. The different techniques vary in their accuracy and their reliability (how reproducible the results are if you do several measurements). This will be discussed in the next blog.
References
Wang ZM, Pierson RN Jr, Heymsfield SB. The five-level model: a new approach to organizing body-composition research. Am J Clin Nutr. 1992;56(1):19-28. doi:10.1093/ajcn/56.1.19
Nana A, Slater GJ, Stewart AD, Burke LM. Methodology review: using dual-energy X-ray absorptiometry (DXA) for the assessment of body composition in athletes and active people. Int J Sport Nutr Exerc Metab. 2015;25(2):198-215. doi:10.1123/ijsnem.2013-0228
