Assessment of body composition by whole-body densitometry: what radiologists should know (2025)

Abstract

The analysis of body composition is a fundamental part of a nutritional statusassessment, and the use of diagnostic imaging methods has been increasinglyrequired for an adequate characterization of the lean body mass and fat mass.Body composition measurements are useful in evaluating the effectiveness ofnutritional interventions and monitoring changes associated with aging andchronic diseases. Whole-body densitometry using dual-energy X-ray absorptiometry(DEXA) is one of the most widely used methods in clinical practice, allowinghighly accurate assessment of the bone mineral content, lean body mass, and fatmass. Although a DEXA examination provides a lot of information, there is stillno universal standardization of the parameters to be included in radiologyreports. The aim of this study was to review the most relevant information forassessing body composition by whole-body densitometry.

INTRODUCTION

The human body is primarily composed of four molecular-level components-water, fat,protein, and minerals-typically in descending order by quantity. Body compositionanalysis is a fundamental part of a nutritional assessment, allowing the accuratediagnosis of conditions such as visceral obesity, as well as being useful in thediagnostic investigation of sarcopenia, which may be related to higher risk andworse prognosis in various types of clinically and surgically treated diseases. Inaddition, changes in body composition are known to be associated with severaldiseases, such as cardiovascular diseases, diabetes, cancer, osteoporosis, andosteoarthritis⁽¹⁾.

Anthropometric parameters-including body mass index, waist circumference, andwaist-to-hip ratio, and skinfold thickness-have been used for the indirectassessment of body composition in clinical practice, although those measures havelimitations, especially in patients who are elderly or obese^(2,3). Another technique routinely used in clinical practice isbioelectrical impedance, which is quite accessible and affordable, allowing theevaluation of multiple parameters such as total body water, fat mass, and lean bodymass. However, that technique also has limitations, related to the variation inresults among different devices, as well as inter- and intra-individual variability,which can be attributed to nutritional status, hydration, physical activity, diet,age, and comorbidities^(3-6).

When comparing methods of body composition analysis, it is important to distinguishfat mass from adipose tissue, which is approximately only fat mass, the remainderbeing water, protein, and minerals. Although most body fat is stored in adiposetissue, fat is also present in organs such as the liver (potentially leading tohepatic steatosis) and skeletal muscle (potentially leading to myosteatosis). It isnow well known that the metabolic risk related to fat accumulation is stronglydependent on its distribution within the body^(1,2,7). In addition to fat, which functions as long-termenergy storage, the study of skeletal muscle mass is of great interest in clinicalpractice, and knowledge of the balance between the energy consumed by muscles andthat stored in fat compartments is therefore highly relevant to the understanding ofmetabolic balance⁽¹⁾.

Imaging methods have been increasingly used in order to facilitate the evaluation ofbody composition, as well as the monitoring of the different body compartments andtheir distribution, allowing the appropriate characterization of lean body mass andfat mass. Several imaging methods have been studied, including ultrasound, magneticresonance imaging (MRI), computed tomography (CT), and whole-body densitometry usingthe dual-energy X-ray absorptiometry (DEXA) technique, the last two being the mostcommonly used in clinical practice. Due to the high radiation dose, the use of CTfor body composition assessment is reserved for patients who are undergoing CT foranother clinical indication (“convenience imaging”). The main advantages of DEXAinclude the fact that it is a rapid method, is widely available, and is affordable;it allows a highly accurate assessment of bone mineral content, fat mass, and leanbody mass, with well-established reference values^(7-11).

The analysis of body composition, especially the estimation of bone mineral contentand total body fat, has been shown to be more accurate with DEXA than with otherbody density-based methods^(12-15). Although DEXA assumes constanthydration of the lean body mass, it should be borne in mind that hydration variesdepending on the age and sex of the patient, as well as in the presence of a chronicdisease, which could be a limitation in some specific groups of patients, especiallyelderly patients with comorbidities^(12,13). Therefore, thedegree of hydration can be a confounding factor in the assessment of lean mass byDEXA and variations in hydration must be taken into account in the analysis ofchanges related to nutritional interventions or physical activity over time,especially in athletes^(14,15).

Although DEXA has been used with increasing frequency in clinical practice, there isstill no standardization specific for reports of densitometry tests of bodycomposition in Brazil. The objective of this article is to review the most relevantinformation provided by DEXA for the assessment of body composition.

INDICATIONS FOR DEXA

According to the International Society of Clinical Densitometry⁽¹²⁾, the main indications for theassessment of body composition by DEXA are as follows: to assess lean body mass andfat mass in patients who have been treated for obesity, either clinically (with dietor medications) or surgically (with bariatric surgery), and who have achieved a≥ 10% weight loss; to quantify appendicular lean body mass in patients atrisk for sarcopenia and in patients presenting with muscle weakness or poor physicalperformance; and to assess body fat, because of the risk of lipodystrophy, inHIV-infected patients on antiretroviral therapy (with zidovudine or stavudine). ADEXA body composition assessment may also be indicated for athletes or anyindividual as part of the assessment of nutritional status, as well as to monitorthe results of weight loss interventions such as diet, physical activity, and drugtreatment⁽¹²⁾.

According to the latest update of the consensus statement issued by the EuropeanWorking Group on Sarcopenia in Older People⁽¹⁶⁾, sarcopenia is defined as a syndrome characterized byprogressive, generalized loss of muscle strength, together with a quantitative orqualitative loss of skeletal muscle mass, which is associated with adverse eventsand worse clinical outcomes, such as a loss of physical independence and reducedquality of life, as well as an increased risk of falls/fractures and death.Sarcopenia can be primary, when associated with the aging process, or secondary,when associated with other triggers, such as inadequate protein intake,gastrointestinal malabsorption disorders, critical illness, cancer, and variouschronic diseases (e.g., chronic kidney disease, chronic obstructive pulmonarydisease, and severe congestive heart failure). The current consensus recommends thatthe diagnosis of sarcopenia be based on a number of factors^(7,16): reduced muscle strength alone is indicative of probablesarcopenia; the diagnosis of sarcopenia can be confirmed if there is also a lowquantity/quality of skeletal muscle mass; and patients who progress to poor physicalperformance are categorized as having severe sarcopenia.

PREPARATION FOR AND TECHNIQUE EMPLOYED IN DEXA

It is necessary to differentiate between a physician order for DEXA in which theobjective is the assessment of body composition and one in which the objective isconventional bone densitometry. Although both tests are performed in the samescanner, bone densitometry, which is routinely used for the diagnosis and monitoringof osteopenia and osteoporosis, evaluates specific bone sites, including the lumbarspine, proximal femur, and forearm. The evaluation of body composition by DEXArequires a whole-body scan, which is not routinely performed to assess bone densityin adults (except in those under 20 years of age).

Although there is no specific preparation for DEXA, the examination should not beperformed after any other imaging examination in which contrast was administered(e.g., contrast-enhanced X-ray or CT scan), especially one in which oral contrastwas used. Because it uses ionizing radiation, DEXA is contraindicated in pregnantwomen. It also cannot be performed if the weight of the patient exceeds the capacityof the equipment, which ranges from 160 kg to 225 kg among devices. The dose ofionizing radiation employed in DEXA is quite low (0.001 mSv), approximately onetenth of that employed in a simple chest X-ray. In DEXA, the X-ray source generatesa dual-energy beam that is attenuated during its passage through the body, beinginfluenced by the intensity of the energy, as well as by the density and thicknessof the tissues^(17,18).

A DEXA examination is performed with the patient in the supine position (Figure 1), with conventional densitometryequipment (the same used for bone densitometry examinations), and takes 2-10 min,the examination time varying depending on the scanner used and the size of thepatient. However, it is still possible to assess body composition by region (trunk,arms, and legs) through proper positioning of the reference lines (Figure 2). For patients who are very tall or verywide, in whom it is not possible to acquire a whole-body scan in a singleacquisition, the devices rely on specific mirroring techniques that allow the“reconstruction” of the image of a limb based on the image of the contralaterallimb, for example^(8,11).

ANALYSIS AND INTERPRETATION OF DEXA IMAGES

The various components of body composition can be analyzed by DEXA, which canidentify differences in density among bone mineral content, lean body mass, and fatmass. Bone mineral density values obtained from whole-body densitometry are not usedfor the diagnosis of osteopenia or osteoporosis in adults, which requires a targetedbone densitometry examination to assess specific sites (the lumbar spine, proximalfemur, and forearm). Therefore, in a DEXA examination performed for the evaluationof body composition, the bone mineral content data are less important than are thelean body mass and fat mass data. Table 1summarizes the lean and fat mass data obtained with DEXA.

The use of DEXA allows adipose tissue to be detected with high accuracy, making itpossible to calculate the percentage of body fat and the fat mass index (FMI), aswell as the android/gynoid fat ratio (Figures 3and 4). Unlike the body mass index, which isbased on total body weight, the FMI is based on body fat only and haswell-established reference values for both sexes⁽¹⁹⁾, therefore being considered a better tool for theassessment of overweight and obesity (Table2).

Gynoid fat, also known as peripheral or gluteofemoral fat, is concentrated in thepelvis and thighs and is associated with a lower cardiovascular risk than is androidfat, also known as central or truncal fat, which is concentrated in the abdominalregion and is associated with a higher risk of metabolic complications. Therefore,an android/gynoid fat ratio greater than 1 (android fat predominance) increases therisk of cardiovascular disease, dyslipidemia, insulin resistance, type 2 diabetes,and metabolic syndrome⁽²⁰⁾.

Modern DEXA devices also allow the quantification of visceral adipose tissue (VAT)and subcutaneous adipose tissue (SAT) in the abdominal region. The measurement ofVAT is traditionally performed with axial imaging methods, such as CT and MRI,either by volumetric evaluation or by assessing the area of VAT in an axial slice,most commonly acquired at the L3 level⁽²¹⁾. The DEXA-based estimation of the VAT area has been foundto correlate well with the CT-based estimation and has been routinely used in theassessment of body composition^(22-24). A VAT area ≥ 100cm² is associated with high cardiovascular risk, whereas a VAT area≥ 160 cm² is associated with very high cardiovascularrisk⁽²⁵⁾. In addition, theVAT/SAT ratio provides a relative index for the accumulation of abdominal fat, aVAT/SAT ratio ≥ 0.4 (predominance of VAT) being a major risk factor fordisorders of glucose and lipid metabolism⁽²⁶⁾.

The assessment of lean body mass can be used for the diagnosis and monitoring ofsarcopenia. With DEXA, body composition can be assessed with good accuracy and lowradiation exposure. However, it should be borne in mind that although DEXA assessesoverall lean body mass, it does not assess skeletal muscle mass separately. The leanbody mass evaluated by DEXA includes skeletal muscle mass, viscera, and fluids,which have similar radiological density. Therefore, it is not possible todifferentiate among them in the densitometry examination. Consequently, the measureused for the analysis of lean body mass in DEXA is the ALM; that is, the sum of thelean body mass of the arms and legs, excluding the trunk region where there isgreater overlap with viscera and liquids. Because DEXA does not assess skeletalmuscle mass directly, some physicians are reluctant to accept it as the goldstandard for this purpose. Despite those limitations, specific cutoff points havebeen proposed for specific populations, with the aim of identifying low muscle mass,using the ALMI devised by Baumgartner et al.⁽²⁷⁾. The ALMI is calculated as the ALM divided by the heightin meters squared. In the investigation of sarcopenia, a diagnosis of low lean bodymass is confirmed if the ALMI is < 5.5 kg/m² in women or < 7kg/m² in men (Figures 3 and4, respectively).

LIMITATIONS OF DEXA

One of the main limitations of DEXA is the exposure to ionizing radiation, which,albeit low, can limit the performance of serial examinations. In addition, it can bedifficult to position the patient correctly to perform the examination, especiallyif the patient is obese or has some functional limitation. Despite its low cost andbroad availability in comparison with other imaging methods (especially CT and MRI),DEXA is not routinely used for all patients, being reserved for selected cases. Inmost patients, it is possible to assess nutritional status with more easilyavailable, faster, lower-cost methods that can be performed in the office, includingthe measurement of anthropometric parameters, including skinfold thickness, andbioelectrical impedance analysis.

CONCLUSION

The use of DEXA for the analysis of body composition provides important complementaryinformation for assessing nutritional status, especially in patients at risk forsarcopenia. Despite its high accuracy and relatively low cost, DEXA is still notwidely used in Brazil. It should be more extensively disseminated, so that morepatients have access to and benefit from the use of this tool.

REFERENCES