Interpretation of Dual-Energy X-Ray Absorptiometry-Derived Body Composition Change in Athletes: A Review and Recommendations for Best Practice

Highlights

• DXA provides measures of whole body, regional fat and lean mass, plus indices of bone health.

• It is becoming a more widely used technique for the assessment of physique traits of athletes.

• There is a need to implement standardized pre-scan protocols that assist in minimizing technical and biological error, to allow recognition of the smallest meaningful changes.

• When technical and biological factors that influence precision and reliability are accounted for, DXA is an excellent method for tracking longitudinal changes in physique traits of athletes.

Abstract

Dual-energy X-ray absorptiometry (DXA) is a medical imaging device which has become the method of choice for the measurement of body composition in athletes. The objectives of this review were to evaluate published longitudinal DXA body composition studies in athletic populations for interpretation of “meaningful” change, and to propose a best practice measurement protocol. An online search of PubMed and CINAHL via EBSCO Host and Web of Science enabled the identification of studies published until November 2016. Those that met the inclusion criteria were reviewed independently by 2 authors according to their methodological quality and interpretation of body composition change. Twenty-five studies published between 1996 and November 2016 were reviewed (male athletes: 13, female athletes: 3, mixed: 9) and sample sizes ranged from n = 1 to 212. The same number of eligible studies was published between 2013 and 2016, as over the 16 yr prior (between 1996 and 2012). Seven did not include precision error, and fewer than half provided athlete-specific precision error. There were shortfalls in the sample sizes on which precision estimates were based and inconsistencies in the level of pre-scan standardization, with some reporting full standardization protocols and others reporting only single (e.g., overnight fast) or no control measures. There is a need for standardized practice and reporting in athletic populations for the longitudinal measurement of body composition using DXA. Based on this review and those of others, plus the official position of the International Society for Clinical Densitometry, our recommendations and protocol are proposed as a guide to support best practice.

Introduction

Dual-energy X-ray absorptiometry (DXA) is a medical imaging technology that uses 2 X-ray beams of different energies that are diversely attenuated by bone and soft tissue. Although the primary application of DXA has historically been for osteoporosis assessment using bone mineral density (1), there has been a rapid growth in its application for the measurement of fat and lean soft tissues over the last 2 decades. This largely followed the introduction of fan beam densitometers with quicker scan acquisition times, better image resolution and quality, and lower radiation exposure. The radiation dose from a total body scan is now relatively small, facilitating its application in longitudinal monitoring of physique traits. As an example, the effective dose from 1 standard mode total body scan on the GE Lunar iDXA is around 2 µSv, and 1 d of natural background radiation in the United Kingdom is around 5–8 µSy (2).

Of all fields, sports science and medicine has witnessed a considerable rise in the number of published research studies utilizing DXA for the assessment of total and regional body composition. These studies have included the body composition profiling of competitive athletes 3, 4, in relation to bone density 5, 6, 7 and nutritional status 8, 9. Serial DXA measurements are increasingly being used to explore the effects of training and diet on athlete body composition changes 10, 11, 12, 13, 14, 15, 16, 17. DXA is regarded as the preferred method for measuring absolute body composition by the International Olympic Committee (18) and is gaining popularity across elite sport centers of excellence, professional sports clubs, and the wider public commercially.

There are a number of alternative imaging modalities for the assessment of body composition, including magnetic resonance imaging, which offers an accurate and reliable alternative to DXA but is limited because of expense and longer scanning times. The reproducibility of DXA is excellent 19, 20, 21 and is frequently used as the criterion method, from which other measurement methods are validated. Nonetheless, it is also now known to be impacted by a number of factors that can affect results and the repeatability of the measurement. Furthermore, knowledge of measurement precision is required for interpreting what constitutes a true and meaningful change—defined as a change that is biological and not arising from technical error. This is particularly pertinent when monitoring body composition in highly trained athletic populations because of their reduced potential for adaptation over a defined period (16). Once the precision error of a device is established, the least significant change (LSC) value can be calculated and applied as follows:

$$\text{LSC}=2.77\times \text{Precision Error}$$

The LSC is the change between 2 DXA measurements that is required for 95% confidence that an actual change has occurred 22, 23. The International Society for Clinical Densitometry (ISCD) official position stand recommends the application of LSC for interpreting longitudinal body composition measurements (23).

It is important to note that the precision of DXA is influenced by both technical and biological factors. Technical factors include instrument model, reference database (24) and scan mode 25, 26, 27, adjustment of regions of interest, and subject positioning, whereas biological factors include subject preparation (28), age (29), sex (21), and body size 21, 30, 31, 32, which can vary greatly between athletes from different sports. In the largest DXA longitudinal precision study to date, Powers and colleagues demonstrated differences in precision error of tissue measurements across the body mass index categories (21). In addition, we have previously found that athleticism is an important factor for precision, with increased precision error for body composition measurements in rugby players (31) compared to precision errors reported for a heterogeneous study sample (20). The distribution of lean and bone tissues are different between athletes according to sport, and differences in body sizes and thickness may affect total and regional X-ray attenuation characteristics. For example, a larger fat free mass (FFM) might predispose an individual to greater error in FFM estimates, given the range of factors influencing FFM such as a larger total body water flux. As such, knowledge of precision error that is population-specific is particularly important for body composition studies of athletic groups.

The purpose of this review was to evaluate the methodologies of all published athlete longitudinal DXA-based body composition studies between 1996 and 2016, according to the assessment of body composition change. On the basis of this review and that of others (33), plus the ISCD official position (23), we provide a guide for consistency and best practice in the interpretation and presentation of DXA-derived body composition change in sports science research and practice.