How is Body Composition measured?
Body composition can be assessed in a number of different ways, with methods ranging in technicality, cost and accuracy. The only fully accurate measurement of body composition is cadaver whole-body dissection analysis, previously undertaken as part of the ‘Brussels Cadaver Analysis Study’ . In light of this, there are no in vivo techniques for body composition analysis that will provide complete accuracy, and so body composition assessment is an estimate, often made on assumptions regarding the proportions and properties of FM, FFM, water, protein and other minerals.
Methods which are likely more accessible to the general public include skinfold calipers and bioelectrical impedance analysis, whereas research laboratories and elite sports teams may have access to hydrostatic weighing, DEXA and Whole Body Plethysmography (BodPod). Each method has its pros and cons, and it is likely that there is no one technique that is optimal for all situations – this will be discussed in the following sections.
Dual-Energy X-Ray Absorptiometry (DXA)
DXA is based on a three-compartment model that measures bone mineral content, FM and FFM. A full body DXA involves the individual lying on an open scanner for approximately 8 minutes, whilst the ‘arm’ of the machine moves over the length of the body, and scanning using two x-ray beams. The more energy that is absorbed, the denser the tissue. Two energies are used to allow estimates of soft tissue absorption, separate to bone .
DXA is considered the gold standard measurement tool for the diagnosis of osteopenia and osteoporosis . It is fast, non-invasive, and only exposes individuals to a small amount of radiation. The DXA also has the added benefit of providing segmental body composition analysis, which may be of particular interest when looking at bone mineral density in some athletic populations.
Although it is often considered one of the most accurate methods of body composition analysis, it is not without limitations. In athletic populations, longitudinal data from repeated measurements of body composition may be affected by muscle glycogen levels, hydration status and changes in muscle metabolites such as creatine . This may lead to a misrepresentation of FFM, and these factors should be considered when interpreting the results of DXA estimates of body composition.
Skinfold calipers measure a double fold of skin and subcutaneous adipose tissue and apply a constant pressure to the site. Skinfold measurements make the assumption that adipose tissue compresses in a predictable manner, that the thickness of skin is negligible, and the double layer compression is representative of an uncompressed single layer of adipose tissue. Measurements give results in millimetres, which can be then converted to a body fat percentage, with dozens of equations available for varying populations.
A potential limitation of skinfold measurements is that they are dependent on the competency and accuracy of the person taking the measurements (i.e. intra-rater reliability). To minimise technical error of measurement, measurement sites and techniques have previously been defined . Practitioners can become accredited in anthropometric measurement through the International Society for the Advancement of Kinanthropometry (ISAK).
To qualify, practitioners must assess various skinfold sites, girths and breadths, and report values within 10% of a Level 4 anthropometrist, and within 7.5% of their own values when repeating measurements. Skinfold measurements taken just one centimetre away from the defined ISAK sites have previously been shown to produce significant differences in measurement values at each site, indicating how important it is to mark and measure skinfolds correctly for accurate data . Therefore, the intra-rater reliability of the test is extremely important.
Whole Body Plethysmography (BodPod)
Air-displacement plethysmography (ADP) can allow for the calculation of body composition through a 2-compartment model, based on assumptions of value constants for FM and FFM densities. ADP determines body volume by measuring the reduction in chamber volume caused by the introduction of a subject/athlete into a chamber with a fixed air volume . Body weight and body volume are determined by this method, with mass divided by volume providing a measure of density.
Again, this measurement is a non-invasive and quick method, with the advantage of not requiring exposure to radiation. BodPod has been shown to be a valid measure of group average body composition when compared to DXA in female collegiate athletes . However, research has suggested a difference of 5.3% between the BodPod and validated four compartment models, with an error rate of 15% . BodPod has also shown limited accuracy when attempting to determine changes over time , a primary consideration when choosing an assessment method for athletes.