INTRODUCTION

Assessment of body composition corresponds to a branch of human biology^1,2 and traditionally, body composition models comprise two compartments,³ dividing body mass into fat and fat free mass. Technological advancements have allowed the appearance of nondestructive methods of evaluating body composition, being, in general, considered in vivo methodologies. Dual energy X-ray absorptiometry (DXA) assessment is based on a model of body composition evaluation, allowing quantification of fat tissue, lean soft tissue and bone mineral content (BMC), both for the whole body and for standardized segments (head; trunk, subdivided into ribs, pelvis and spine; upper limbs and lower limbs), and it is also possible to obtain the aforementioned components in regions of interest, termed ROI. From the clinical point of view, there is considerable interest in assessment of the proximal femoral area, often the target of prosthesis, by surgery, and additionally the lumbar spine (especially L1-L4), a segment with high informative value regarding the general state of the skeletal structure.

DXA technology has gained popularity over other technologies for its speed, low cost and reduced radiation exposure.^4,5 Previously, the available technology would use iodine-125 as a source of radiation, the methodology being single-photon absorptiometry (SPA), aimed to estimate the mineral content of long bones distal portion, such as radius and ulna, being described as valid.⁶ The SPA technology was used to obtain one of the most popular equations to determine the percentage of fat mass from the triceps and subscapular fat folds.⁷ DXA technology measures the attenuation of X-rays emitted at frequencies of two different energies, with iodine-125 being replaced by gadolinium-153, which emits radiation at 44 and 100 KeV. There are currently several DXA devices and manufacturers Hologic and Lunar are the most mentioned ones.^8-10 DXA has been used mainly in the determination of BMC and, consequently, bone mineral density (BMD), by combining BMC and bone area. In fact, this is a criticism of the method, since the BMD should consider BMC by bone volume and not so much by area, assuming that there are inter-individual variations of those observed in relation to thickness measurement. Each of the manufacturers has developed models for reduction of radiological exposure, scanning time and precision gains [either by improving detectors resolution or by the technology of X-ray emission tubes, highlighting procedures “pencil beam” (PB) and “fan beam” (FB)]. DXA, like the generality of the technologies, has assumptions, such as the heating of tissues during the procedures of application of the method, and values of water and carbon dioxide are lost, making it necessary to transform the BMC quantified to obtain the bone mineral itself.⁵ Additionally, it is assumed that fat and lean mass have attenuation constants and the exposure of said tissues to low and high voltages allows determining the proportions per unit area, assuming that the thickness in the antero-posterior plane does not affect the estimates.

Most of the studies focus on the comparisons between equipment from different manufacturers (Hologic vs. Lunar) or between equipment of the same manufacturer adopting different technologies for X-ray emission, i.e., PB as opposed to FB¹¹ or even among methodologies: DXA vs. computed tomography scan^12-14 or by the methodology of corporal potassium measurement.¹⁵ As a general rule, studies point to an error associated with DXA technology ranging from 1% to 3%.^16,17 The interest of Sports Sciences for body composition is due, in large part, to the search for indices of metabolic efficiency. Recently, studies have emerged to express the oxygen uptakes per unit of fat-free body mass¹⁸ since the fat corresponds to a nonrelevant metabolic tissue, or even considering oxygen consumption in the running pattern expressed per liter of lower limb volumetry (or even mL.L^-1.kg^-1). The expression per appendicular mass of the lower limb is especially relevant when the movement pattern does not oblige the athlete under evaluation to support the whole body mass and this happens in the cycle ergometer to obtain maximum and average mechanical power¹⁹ or in the dynamometer isokinetic technique to evaluate the moments of maximum strength (peak torques) of the muscles responsible for knee extension and flexion actions.²⁰ However, the search for informative and noninvasive protocols on limbs volume and body mass has justified a line of study^21-23 devoted to more parsimonious methodologies in terms of costs and morosity (anthropometry) based on geometric models.²⁴ The interest in total body composition and of particular segments has been especially emphasized for the lower limb or only for the thigh. Despite the popularity of the DXA technology, studies about competing equipment to estimate the determination accuracy of fat tissue, lean soft tissue and BMC are not abundant for the whole body and particular segments. In fact, the possible absence of agreement among equipments shall partially compromise proposals of calibration of new equations.^21-23

The present study aims to examine agreement among indicators resulting from the application of competing DXA from manufacturer Lunar, namely taking into account Lunar DPX-MD+ (PB technology) and Lunar iDXA (FB technology), having been carried out with healthy young adults from multiple sports.

METHODS

RESULTS

Table 1 summarizes the characteristics of the sample. BMC, bone area used in determination of BMD, whole body lean soft tissue and whole body fat tissue, are presented in Table 2. Also the out-puts related to BMD in the proximal femoral area are presented in Table 2 [femur neck, triangle of ward, trochanter and shaft]. Table 2 includes data from Lunar DPX-MD+ and Lunar iDXA. Violation of assumptions of normal distribution was uniquely noted for fat tissue with regard to the whole body.

Comparison of results by two competing pieces of equipment (Table 3) suggests a substantial intra-individual difference for whole body BMC (mean of intraindividual difference = 110 g, magnitude of the wide effect: d = 1,312) and also for the bone area used in calculating BMD (mean intraindividual difference = 65 cm², effect: d = 1.761). However, for the BMD intraindividual differences were negligible (–0,001 g.cm-²) and the magnitude effect was trivial. In addition, magnitude of the intraindividual difference was large for the fat tissue, with the average being 11.87 kg for equipment Lunar DPX-MD+ and 13.56 kg for equipment Lunar iDXA, corresponding to a mean of intraindividual differences of 1.70 kg (d = 1.612, magnitude of the wide effect). Differences for the lean tissue were trivial, that is, 0.04 kg.

BMD values for the proximal femoral area showed that for the femoral neck. The variation associated with the equipment corresponded to a trivial intraindividual difference of 0.001 (d = 0.031) and the magnitude of the differences among intrain-dividual means for the Ward triangle and trochanter was respectively, d = 0.512 (small magnitude) and d = 0.495 (small magnitude). For the shaft of the femur, the magnitude of the differences was moderate (d=0.656).

For lean soft tissue indicators, differences were observed for all segments and ROI: d = 1.687 (large differences) for the trunk; d = 1.237 (large effect size) for upper limbs; d = 1.402 (also large effect) for the lower limbs. For the ROI, the lean soft tissue showed a vary large variation between equipments (d = 4.014).

With regard to the ICCs, for all measures above, ICC > 0.900 was obtained. The CV % fluctuated between 2.3% and 2.7% for the measures used in calculation of BMD. For tissue, CV % is only 1.5% for the lean soft tissue component and 11.2% for the fat tissue component. For the variables of the proximal femoral area, TEM was always less than 5% of the combined mean [i.e., CV % equal to 2.4% for the femoral neck, 2.5% for the Ward triangle, 2.0% for the trochanter and 4.5% for the shaft], the data quality being corroborated by ICC coefficients always higher than 0.950. For lean soft tissue, CV % = 2.1 and ICC = 0.972 for the trunk, CV % = 4.2 and ICC = 0.967 for the upper limbs; CV % = 3.2 and ICC = 0.966 for the lower limbs were observed. For the ROI, the lean soft tissue showed a higher variation between equipment (CV % = 8.43, ICC = 0.931).

DISCUSSION

In the present study, agreement among indicators resulting from the application of competing equipments used in DXA, one being a PB technology (Lunar DPX-MD+) and another, FB (Lunar iDXA) was examined. Regarding BMC, the bone area for determining BMD, BMD, fat tissue and lean soft tissue, healthy adults and sportsmen of various sports were assessed. Negligible differences were found for BMD, despite a trend for Lunar DPX-MD+ to produce higher values for BMC and also for the bone area. In a study of women between the ages of 21 and 80,¹¹ there was a trend for the FB mode to underestimate (by comparison to the PB mode) the bone area used to calculate BMD. This study, previously mentioned, was carried out with the equipment Hologic QDR-2000, that has the possibility of adopting the two modes mentioned above (FB and PB).⁵ However, other studies^28,29 shoed that among FB technology equipment, the subject’s thickness constitutes a source of discrepancy. The Lunar manufacturer’s first equipment had a beam angle of about 30 degrees, considering wide-angle FB,³⁰ having been replaced by narrow-angle FB equipment (in the Lunar Prodigy equipment the angle is 4.5 degrees), and considered several overlapping scans, which takes place in the Lunar iDXA (equally a narrow-angle FB, with the added advantage of being equipped with a CZT-HD high resolution detector). In the present study, a high ICC was always obtained between the aforementioned narrow-angle FB (iDXA) equipment and the PB (Lunar DPX-MD+) equipment.

For measurements of the proximal femoral area, which are widely used in clinical settings, BMD presented differences between equipment which fluctuated between trivial and moderate, although CV % and ICC confirmed a certain idea of data quality, especially for the femur neck, trochanter and Ward triangle, revealing shaft as a more problematic parameter. The literature confirms this trend for variation in the discrepancy between FB and PB modes, namely in a study of 63 women¹¹ which made it possible to conclude that there was an overestimation of +1.5% by FB in the lumbar spine, in parallel with an underestimation of -0.7% in the femoral neck and -1.8% in the trochanter. In the latter region, a correlation of +0.36 was found among the residuals of the two modes (FB/PB) and the body mass of the women evaluated. Also, in another study with 40 postmenopausal women, accuracy of repeated BMD measurements was 1.1-1.6% for the lumbar spine and 2.2-2.5% for the femoral neck, with intraindividual variation being highest for obese women.³¹

For fat tissue, it is possible that variations in energy and data processing associated with each of the equipments contribute to a substantive intraindividual difference and caution is recommended for the acceptance of fat mass data, expressed in kg, from the DXA technology. In the present study, this variable showed the highest value for CV % (11.2%) and the magnitudes of intraindividual differences were large. Regarding lean soft tissue, the data between DXA-MD+ and iDXA appeared to be in agreement, except for ROI, where it is considered an additional source of error, that is, the error introduced by the observer, also involving data processing and not just acquisition. Use of DXA for determination of trunk fat, combined with measurements and thickness of subcutaneous fat folds, was tested as a protocol to obtain an intra-abdominal fat quantification using computed tomography¹⁴ having explained 91% of the interindividual variance, although with a CV % of 14.8%.

Appendicular composition has also been of interest in several studies using DXA technology. For example, a study of 41 male rugby players (16.3 to 20.7 years old) calibrated the geometric models by anthropometry to determine the lower limb volumetry, based on data obtained by DXA (Hologic, Explorer W, Waltham, Massachusetts, USA, software QDR version 12.4) to obtain data on fat mass and fat-free mass of the lower limb.²¹ Thus, it was possible to determine correlation coefficients of 0.81 and 0.90 between the anthropometric method and the DXA reference. More recently, the same researchers²² has calibrated the geometric models based on two conical structures (only for the thigh) in 168 school-age children using the DXA equipment (Hologic Explorer W, Waltham, Massachusetts, USA, software QDR version 12.4). In this last study, the appendicular thigh volume corresponded to an ROI defined between the transverse planes that pass between two anthropometric references: ischium and suprapatellar. Finally, another study²³ has been carried out with 42 adolescent volleyball players (14.0-17.9 years) aiming at anthropometric calibration of thigh volumes obtained by anthropometry and DXA (Lunar DPX NT/Pro/MD+/Duo/Bravo). However, intra-observer reliability for the same equipment has not been determined, particularly with regard to ROI, which requires more expertise from the observer.

CONCLUSIONS

In general, the various parameters revealed good reproducibility and allowed to confirm a certain idea of the quality of the indicators resulting from the application of competing DXA equipment (Figure 1). Negligible differences were found for BMD, despite a trend for equipment Lunar DPX-MD+ to produce higher numbers for BMC and also for the area. It is recommended, however, that measurements of whole body fat tissue and in the case of lean soft tissue in the thigh, when obtained by DXA, be not taken as a criterion, but rather as a reference. Such an understanding has implications for the interpretation of intraindividual discrepancies that would comprise measurement error in each of the competing variables and not only in the predictive variable.