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Body Composition in Sport: DXA

Monitoring body composition in athletes is beneficial for a myriad of reasons. DXA, or dual-energy X-ray absorptometry, is one of many methods that can be used to assess body composition in athletes. How DXA works, its popularity, and sources and margin of error are reviewed in this article.

How DXA Works

Dual-energy X-ray absorptiometry (DXA) uses attenuation of calibrated X-ray beams with dual photon energy (emitting a very small dose of ionizing radiation) to produce pictures of the inside of the body. The DXA machine sends these thin, low-dose x-rays with two distinct energy peaks through the body. One peak is absorbed mainly by soft tissue and the other by bone. Therefore, DXA is a three-compartment model of body composition as it segregates the body into 3 compartments: fat mass, lean mass, and bone [1]. The fact that DXA can quantify regional bone mass (and bone density) is what sets DXA apart from the remainder of the body composition methods.

Popularity

DXA is the most widely utilized technique for measuring body composition [2], and is considered the “gold standard” body composition measurement tool for athletes [3-7]. In a 2013 survey, 60% and 40% of international and national sport professionals, respectively, were found to use DXA for measuring body composition [8]. DXA is frequently used as the reference measure to assess the validity of other body composition assessment protocols [9-15]. 


Nana et al. (2015). Methodology review: using dual-energy X-ray absorptiometry (DXA) for the assessment of body composition in athletes and active people. International Journal of Sport Nutrition and Exercise Metabolism, 25(2), pp.198-215.

Biological Error Considerations

It must be noted that the validity of DXA is reduced in very lean, or highly obese subjects, as several researchers have shown underestimation and overestimation of fat mass in very lean and obese individuals, respectively [16]. There are a few key biological (day-to-day) aspects of assessment that can affect measurement variability, or error. Biological variation, including differences in food [17] or fluid [18] intake, and previous exercise [19, 20] can influence DXA results. Alteration of intramuscular solutes (i.e. glycogen, creatine, carnosine), and their associated water-binding properties is another source of biological variation [21-23]. To avoid misinterpretation of the data, it’s essential to understand the biological context of the athlete during data collection, and, if possible, ensure the context remains consistent for the squad of athletes, as well as for each athlete, longitudinally.

For example, an athlete that commences creatine supplementation, or embarks on a higher-carbohydrate diet may have artificially elevated lean mass via DXA, whereas an athlete that embarks on a lower-carbohydrate diet or abstains from creatine supplementation, may have artificially lower lean mass values [21]. If measurements are taken the day following a game or match, athletes who participated in the event may have artificially lower lean mass values due to decrements in muscle glycogen stores, and/or dehydration, compared with athletes that did not participate in such high volumes of high-intensity activity [21, 24]. Although oftentimes uncontrollable, it’s important to understand the impact that these biological contextual factors can have on the DXA results, at the very least.

Technical Error Considerations

Additionally, there are a few key technical (machine/technician) aspects that can affect measurement validity (accuracy) and reliability (consistency). Prone vs. supine subject positioning [25, 26], and even inconsistent clothing worn during assessment [20] can impact results. Using different DXA machine manufacturers [27-29] or scan speeds [30], changing software platforms [31, 32], or altering beam technology within the same manufacturer’s product [33, 34] are not advised because these factors can also influence results.

The different technologies used to scan the body amongst DXA devices include pencil and fan beam technologies. These different technologies, or beams, are pretty easy to comprehend; pencil beam technology uses narrow x-ray beams (i.e. like a pencil), whereas fan beam technology uses wider x-ray beams (i.e. spread out, like a fan). Although fan beams allow faster scanning and better image resolution, these benefits come at a slightly higher cost of radiation exposure [35]. As stated earlier, it’s important to understand the beam technology being used and remain consistent with the beam type to ensure that longitudinal results are reliable.

Biological Sources of ErrorTechnical Sources of Error
Inconsistent food and fluid intakeChanging DXA machine manufacturer or changs in product specs
Inconsistent previous exerciseInconsistent scanning speed
Supplementation that may impact intramuscular solutes with water-binding properties  (i.e. creatine, carnosine, glycogen)Altering beam technology, even within same manufacturer’s product
Changes in body size (may overestimate fat mass in very large subjects; underestimate fat mass in very lean subjects)Inconsistent clothing worn during assessment
Inconsistent subject positioning (prone vs. supine)Changing software platforms

General Margin of Error

Although DXA has been reported to accurately measured body fat percentage within 1% margin of error compared with a four-compartment model in ethnically diverse male and female athletes from various sporting backgrounds [36], DXA typically has a 2-3% standard error estimate (SEE) when predicting body fat percentage [37].

The standardization of body composition assessment is the most critical aspect for accurate result interpretation and quantification, and must not be overlooked [38]. To minimize preventable error when using DXA to assess body composition, I advise utilizing the best-practice protocol outlined by Nana et al. (2015), [20].

This research group also provides a nice list of advantages and disadvantages for practically using DXA with athletes:

AdvantagesDisadvantages
Suitable for most athletesExpensive equipment
Fast (~5 mins for fan beam, up to 15 mins for pencil beam)Generally not portable
Provides regional body compositionMost scanning beds too small for typical physique of larger athletes
Radiation dose is low dose (~0.5 µSv) and safe for sequential measurementsTrained technician required
Non-intrusiveManufacturers’ body composition algorithms may not be appropriate for athletic population

Summary

Dual energy X-ray absorptiometry (DXA) is one of the most widely used body composition assessment methods available, and for good reason. It’s non-intrusive, extremely precise, and enables for regional body composition assessment, including bone mass. However, it’s also expensive, emits a small dose of radiation with each use, requires a technician to operate, and is, generally, not portable. Various factors affect its accuracy and reliability, including previous exercise, hydration, and eating habits. As is the case with any body composition assessment method, understanding the factors that affect the accuracy and reliability of DXA, and applying a standardized approach for data collection is advised to avoid measurement artifact caused by these factors. In any case, the most important aspect of body composition assessment is the standardization of data collection. If standardization is lacking, misinterpretation of athlete physique is near-definite [38].

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