DEXA scanning

Dual energy x-ray absorptiometry (DEXA) is a measurement technique used to determine bone mineral density. DEXA is increasingly being used to determine body composition in a variety of settings, including obesity [1], sarcopenia [2], and elite sport [3]. A DEXA scan involves the passing of low-radiation x-ray beams through bones, and the amount of energy that is absorbed allows for the determination of the density of the bone. Radiation energy per pixel is established and converted to g/cm for a given area. The number of pixels in each area is determined, and the amount of bone in each pixel is calculated, allowing for the calculation of bone density in specific bones [4]. Two energies are used to allow estimates of soft tissue absorption, separate to the bone. The DEXA scan creates a two-dimensional image, preventing the measure of bone depth.

DEXA scan – body composition
There are several methods used to determine body composition, and each method has pros and cons regarding accuracy, financial cost, practicality, and other factors (for more information on the methods of body composition assessment and their uses, click here). DEXA scans provide a 3-compartmental model for body composition estimation. The three compartments being;
1. Bone mineral
2. Fat
3. Other fat-free mass that isn’t bone
A criticism of 2-compartment models (dividing weight into fat mass and fat-free mass) is that errors occur due to assumptions of the density in fat mass and fat-free mass (e.g. variations in bone density have been identified between different ethnicities [5]). This, therefore, demonstrates a potential advantage of DEXA scan use.

DEXA scanning offers a number of potential benefits for body composition assessment. The scan itself involves the movement of a ‘machine arm’ across the length of the body being measured. A full body scan takes approximately 8-minutes, making it a time-efficient, non-invasive measure of body composition. The scanner can also be adjusted to measure a single body part or segment, providing regional estimations of body composition.

DEXA scans are used to determine bone mineral density. DEXA scanning is considered the gold standard method for determining bone mineral density and is thus used in the assessment of osteopenia (lower than average bone density) and osteoporosis (much lower bone density, risk of bone fracture). This can be important in certain athletic populations with a greater risk of low bone density and osteoporosis, such as long-distance runners (predominantly females). Therefore, the use of serial DEXA body composition measurement may provide data regarding athlete conditioning, training outcomes, rehabilitation progress, as well as indicating increases in fat mass, or decreases in skeletal muscle mass or bone mineral density that could be detrimental to athletic performance [6].

Validity and Reliability
The validity (the agreement between the true value and a measurement value) of body composition is key to determine the precision of a measurement. A multi-compartment model, specifically the four-compartment model (fat mass, total body water, bone mineral mass, residual mass), is regarded as the criterion method in determining body composition, and thus this model is often used in comparison when assessing the validity of DEXA measurements [7]. Despite this, there is a relative lack of validation studies using these comparisons, predominantly due to the time, labour, and financial cost involved [7].

The validity of DEXA scans when compared to the four-compartment model is currently equivocal. A study by Arngrimsson et al., (2000) in male and female distance runners reported that DEXA underestimated body fat measurements by around 2% [8]. This finding was replicated in a study by Van der Ploeg et al., (2003) in a healthy adult population [9], and slightly less than the 4% difference in trained men found by Withers et al [10]. Fat-free mass was also reported to be overestimated by approximately 2.5kg [10].

Conversely, other research has shown an overestimation of body fat by 3-4%, and an underestimation of fat-free mass by up to 3kg [3, 11]. The differences in the reported validity of DEXA scans for body composition may partly be due to differences in software, manufacturer, and the wide range of body composition values across various cohorts. It is worth noting that although current research is conflicting, it is commonly assumed that DEXA is a valid technique for the assessment of body composition.

It is also important to consider the validity of DEXA results not only for cross-sectional data (observational data at one given time point), but also when assessing changes in body composition over time, as this is often of interest to the sport science practitioner. Early research in this area used the novel method of scanning participants, before placing lard samples on the participants to simulate a gain in adipose tissue. One study showed that fat mass and lean body mass measurements were not significantly different from expected change following the placement of 8.8kg of lard on healthy female subjects [11]. Another study also reported measurements in line with expectations when 11.1kg was added, however, there was evidence that fat mass was underestimated by 1.9% when the amount of lard added was increased to 22.2kg [12].

Data on DEXA use to assess body composition in athletic populations is scarce. Van Marken et al., [13] showed no difference in mean body composition change between the four-compartment model and DEXA scans in male bodybuilders. This finding was replicated by Santos et al., [3] in male judo competitors. Despite these findings, both studies reported that individual error rates of 4% for body fat measurement, highlighting the large individual differences within measurements.

The reliability (the reproducibility of the observed value when the measurement is repeated) of DEXA scanning is important for determining the precision of a single measurement, as well as the ability to detect change over multiple measurements. Despite the various benefits of DEXA scanning for body composition assessment, it is not without error. Inconsistent results can occur when using scanners from different manufacturers [14], as well as two scanners produced by the same manufacturer [15]. Software updates may also alter body composition results due to algorithm variation [16]. Differences in the x-ray beam used can also contribute to error; pencil beams and fan beam DEXA systems have been shown to provide significantly different body composition test results, limiting comparison between the two [17].

Biological variation may also affect reliability in DEXA measurements, particularly in athletic populations. The effects of fluid intake on body composition has been studied previously, showing that fluid intake of 0.8-2.4L of water can significantly increase lean mass estimates in the trunk region [18]. Similar results were reported by Thomsen et al., [19], showing that estimates of lean body mass were increased by over 1kg per hour following a standard meal weighing 1311g, as well as after drinking 1L of water.

Changes in any of these variables are not uncommon in athletic populations and may mask or give the false impression of changes in skeletal muscle mass. Bone et al., [20] reported that glycogen and creatine loading increased lean mass values by 2.1% and 1.3%, respectively. Variation in hydration status of 5% has also been shown to alter DEXA predictions of body fat by 3% [21]. As manipulation of these variables is commonplace with athletes, standardization is key to accurately detect meaningful change.

Issues with DEXA scanning
Practically, DEXA scanning may also have some issues. The cost and accessibility of DEXA scanning can be an issue, particularly for sports teams that have large squad numbers and limited budgets. Furthermore, DEXA scans do use ionizing radiation, and although the absolute dose per scan is minimal, it may be advisable to minimise the number of scans performed per year on one individual; limiting the quantity of body composition data that can be acquired [22]. In these instances, it may be prudent to use more basic, cost-effective methods of body composition assessment such as skinfold calipers.

A further issue (particularly with some athletic populations) can be the size of DEXA scanners, as they may not be able to accommodate individuals that are taller or broader than the scanning area. For tall athletes, one technique is allowing the athlete to bend their knees, thus allowing the head and feet to be included in the scan. However, this has been shown to lead to significant error, with fat mass (9.2%) and lean body mass (4%) both vastly overstated when compared to a standard measure [23]. Other methods involve summation of separate scans to achieve a total body measurement, however, this method also leads to substantial error [24].

As mentioned previously, reliability may be an issue if best practice is not observed, and measurements are not correctly standardised [25]. Measurements of fat and fat-free tissue are also made on the assumption that fat-free mass hydration is constant at 73% [26], however, the hydration of fat-free mass has been suggested to be 72-74.5% [27]. Although not necessarily an issue, as all measurement methods have an inherent error due to assumptions, these variations may potentially cause detectable variability in the estimation of fat-free mass.

Future Research
In the previous article on body composition assessment (read here), the need for a greater dataset of reference body composition data for athletes in various sports (and various positions/roles within these sports) was highlighted. To this end, the ability of DEXA scans to also provide segmental analysis may provide practitioners to identify ‘regions of interest’ that may determine successful performance in these sports or roles. This also may allow greater insight into the effects of an intervention that has been utilised. For example, if lower-body musculature is identified as important, the ability to track changes in skeletal muscle at this site could be useful.

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