Articles in PresS. J Appl Physiol (May 23, 2003). 10.1152/japplphysiol.00366.2003 PSOAS MUSCLE ATTENUATION MEASUREMENT WITH COMPUTED TOMOGRAPHY INDICATES INTRA-MUSCULAR FAT ACCUMULATION IN PATIENTS WITH THE HIV- LIPODYSTROPHY SYNDROME Martin Torriani 1, Colleen Hadigan 2, Megan E. Jensen 1, Steven Grinspoon 2 1 Division of Musculoskeletal Radiology and 2 Program In Nutritional Metabolism, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts 02114 Funded in part by NIH Grants DK59535, RR300088, and RR01066. Contact information Martin Torriani, M.D. Division of Musculoskeletal Radiology Department of Radiology, Massachusetts General Hospital 15 Parkman Street WACC 515 Boston, MA 02114 Phone: 617-724-0548 Fax: 617-726-5282 E-mail: mtorriani@hms.harvard.edu Running head PSOAS MUSCLE ATTENUATION VALUES IN HIV-LIPODYSTROPHY Copyright (c) 2003 by the American Physiological Society.
JAP-00366-2003 ABSTRACT The HIV-lipodystrophy syndrome is characterized by abnormalities of lipid metabolism, glucose homeostasis and fat distribution. Overaccumulation of intra-muscular lipid may contribute to insulin resistance in this population. We examined 63 men: HIV-positive with lipodystrophy (n=22), HIV-positive without lipodystrophy (n=20), and age- and body mass indexmatched HIV-negative controls (n=21). Single slice computed tomography (CT) was used to determine psoas muscle attenuation and visceral fat area. Plasma free fatty acids (FFA), lipid profile, and markers of glucose homeostasis were measured. Muscle attenuation was significantly decreased in subjects with lipodystrophy [median (inter-quartile range), 55.0 (51.0-58.3)] compared to subjects without lipodystrophy [57.0 (55.0-59.0); P=0.05] and HIV-negative controls [59.5 (57.3-64.8); P<0.01]. Among HIV-infected subjects, muscle attenuation correlated significantly with FFA (r=-0.38; P=0.02), visceral fat (r=-0.49; P=0.002), glucose (r=-0.38; P=0.02) and insulin (r=-0.60; P=0.0001) response to 75-g OGTT. In forward stepwise regression analysis with psoas attenuation as the dependent variable, visceral fat (P=0.02) and FFA (P <0.05), but neither BMI, subcutaneous fat nor antiretroviral use, were strong independent predictors of muscle attenuation (r 2 =0.39 for model). Muscle attenuation (P=0.02) and visceral fat (P=0.02), but not BMI, subcutaneous fat, FFA, or antiretroviral use, were strong independent predictors of insulin response (area under the curve) to glucose challenge (r 2 =0.47 for model). These data demonstrate that decreased psoas muscle attenuation due to intra-muscular fat accumulation may contribute significantly to hyperinsulinemia and insulin resistance in HIVlipodystrophy patients. Further studies are needed to assess the mechanisms and consequences of intra-muscular lipid accumulation in HIV-infected patients. KEYWORDS Muscle attenuation; insulin resistance; protease inhibitor; acquired immunodeficiency syndrome
JAP-00366-2003 1 INTRODUCTION HIV-infected patients receiving highly active antiretroviral therapy often demonstrate fat redistribution, characterized by subcutaneous fat loss and visceral fat hypertrophy. Changes in fat distribution are often seen in association with insulin resistance and dyslipidemia and overall lipolysis rates are increased among such patients (17). Excess free fatty acids from lipolysis may accumulate in muscle and thereby affect glucose entry and subsequent phosphorylation. Recent studies in this population using proton magnetic resonance (MR) spectroscopy demonstrate increased intramyocellular lipid concentrations (13; 22). Muscle attenuation values obtained with computerized tomography (CT) decrease as a function of augmented lipid concentrations, and are important independent markers of insulin resistance in non-hiv-infected patients with obesity and type 2 diabetes (15; 16). However, prior studies using CT scan have not compared muscle attenuation as a surrogate index of intramyocellular fat content in relationship to body composition and metabolic variables in HIV-infected individuals. MATERIALS AND METHODS Experimental Subjects and Protocol Attenuation values of psoas muscles were obtained by computed tomography (CT) in 22 HIV-infected men with lipodystrophy syndrome (LIPO), 20 HIV-infected men without lipodystrophy (NONLIPO), and 21 HIV-negative male control subjects (CTRL) recruited from the multidisciplinary HIV practice at the Massachusetts General Hospital. Subjects were referred for evaluation of observed changes in fat distribution, and also recruited from advertisements seeking HIV-infected patients with and without evidence of fat redistribution. HIV status was confirmed by enzyme-linked immunosorbent assay and Western blot testing in all subjects. Lipodystrophic subjects were selected based on a waist-to-hip ratio more than 0.95 and a history of significant change in fat distribution in the trunk, extremities, neck, or face. In all LIPO cases, the presence of changes in fat distribution was confirmed by physical
JAP-00366-2003 2 examination and scored by a single investigator as severe (1.5 on a scale of 0 2) in 1 or more areas. Severe LIPO was scored for changes obvious to the casual observer and mild-tomoderate LIPO for changes noticeable to the patient and confirmed by the single investigator. Objective criteria used in the determination of severe LIPO included, but were not limited to, prominent peripheral venomegaly and a palpable dorsocervical fat pad. In contrast, HIVpositive, NONLIPO subjects were recruited from advertisements seeking HIV-infected men without changes in fat distribution. NONLIPO subjects were selected based on a waist-to-hip ratio less than 0.95 and did not demonstrate significant fat redistribution in any area on physical examination. Lipodystrophy patients were classified as having significant peripheral lipoatrophy if they demonstrated moderate or severe fat loss in the arms or legs. HIV-infected subjects receiving antiretroviral medications were on a stable regimen for more than 6 weeks. One subject in the LIPO group was receiving stable thyroid hormone replacement. No other subjects were known to have thyroid disease. To prevent enrollment of subjects with primary HIV-related wasting, patients with a BMI less than 20 kg/m 2 were excluded from all groups. Subjects receiving testosterone, growth hormone, anabolic hormones, glucocorticoid, antidiabetic agents, and megestrol acetate were excluded. Exclusion criteria also included known diabetes mellitus, hemoglobin level less than 9.0 g/dl, and age more than 60 and less than 18 years. The non-hiv-infected control subjects were in good health, using no medications, with a waist-to-hip ratio less than 0.95. Written informed consent was obtained from each subject before testing, in accordance with the Committee on the use of Humans as Experimental Subjects of the Massachusetts Institute of Technology and the Subcommittee on Human Studies at the Massachusetts General Hospital. Body composition by CT scan and dual energy X-ray absorptiometry (DEXA), and metabolic indices such as fasting lipid levels, insulin, glucose, oral glucose tolerance test (OGTT), plasma FFA, CD4 count, and HIV viral load were also assessed. Body composition and other endocrine data have previously been reported in this subpopulation (25; 31).
JAP-00366-2003 3 Experimental methods CT attenuation of psoas muscle. All scans were performed with a LightSpeed CT scanner (General Electric, Milwaukee, WI). A lateral scout image of the abdomen was obtained to identify the L4 pedicle, which served as a landmark for a single-slice image at this level. Scan parameters for each image were standardized (144 cm table height, 80 kv, 70 ma, 2 seconds, 1 cm slice thickness, 48 cm field of view). Bilateral psoas muscle attenuation values were measured utilizing Impax workstations (AGFA Diagnostic Software, version 4; Agfa, Ridgefield Park, NJ). A circular region of interest (ROI) with area of 1.5 cm 2 was placed in the most homogenous area of the muscle, avoiding visible intra-muscular fat (Figure 1). A single measurement was obtained from each side, and average psoas muscle attenuation value was calculated and expressed in Hounsfield units (HU). Body composition analysis. The single cross-sectional CT image at L4 was utilized to assess distribution of subcutaneous and visceral abdominal fat. Fat attenuation values were set at -50 to -250 HU as described by Borkan et al. (5) and intra-abdominal visceral and subcutaneous fat areas were determined based on tracings obtained utilizing commercial software (Alice, Parexel Inc., Waltham, MA). Abdominal visceral (VAT) and subcutaneous fat (SAT) and the ratio of VAT: total abdominal cross-sectional area were determined. Fat and fat-free mass were determined by dual-energy x-ray absorptiometry (DEXA) using a Hologic 4500 densitometer (Hologic, Inc., Waltham, MA). The technique has a precision error of 3% for fat and 1.5% for fat-free mass (23). Baseline weight was determined after an overnight fast and percent IBW was calculated based on standard height and weight tables (26). Waist-to-hip ratio was determined from the circumferential measurements of the waist at the level of the umbilicus and the hips at the level of the iliac crest taken with the patient in an upright standing position.
JAP-00366-2003 4 Hormonal assessment and laboratory methods. Serologic assessment included fasting plasma FFA, triglycerides, low-density lipoprotein (LDL), high-density lipoprotein (HDL), and total cholesterol. A standard 75-g oral glucose tolerance test with insulin and glucose levels, CD4, and HIV viral load were also performed. All parameters were determined using previously published methods (20; 31). Statistical analysis. Comparisons were made between the groups (LIPO vs. NONLIPO, LIPO vs. CTRL, and NONLIPO vs. CTRL) by the Wilcoxon rank sum test. Chi-square analysis was used to assess group differences for categorical variables. Univariate regression analyses were performed, comparing CT attenuation values and indices of body fat and composition among all HIV-infected patients. Forward stepwise regression analysis, P = 0.1 to enter the model, was performed to determine relevant factors contributing to muscle attenuation and hyperinsulinemia. Statistical significance was defined as P < 0.05. Results are median plus interquartile range. Statistical analyses were made using JMP Statistical Database Software (SAS Institute, Inc., Cary, NC). RESULTS Age and BMI were not significantly different between the groups (Table 1). As previously reported, no significant difference in whole-body fat mass measured by DEXA was detected between groups (31). Regional trunk fat was increased, and extremity fat decreased in LIPO compared to NONLIPO and CTRL subjects. The NONLIPO and CTRL groups showed no significant difference in either truncal or extremity fat. LIPO subjects demonstrated significantly increased visceral fat area and visceral fat to total abdominal cross-sectional area determined by CT, compared with NONLIPO and CTRL subjects. No significant differences in visceral fat area were observed between NONLIPO and CTRL subjects. CD4 cell count and HIV viral load were not significantly different between HIV-infected groups, although duration of HIV, protease
JAP-00366-2003 5 inhibitor (PI) use and nucleoside reverse transcriptase inhibitor (NRTI) use were greater in lipodystrophic patients when compared to the NONLIPO group (Table 1). The mean psoas muscle attenuation value measured by CT was significantly lower in the LIPO subjects, compared with NONLIPO and CTRL subjects (Figure 2). A significant difference was also seen between attenuation values of NONLIPO and CTRL (P < 0.05). Among HIV-infected subjects, BMI, waist-to-hip ratio, and body composition measurements were highly inversely related to psoas muscle attenuation (Table 2). Trunk fat measured by DEXA (r = -0.50, P = 0.002), visceral fat by CT (r = -0.49, P = 0.002) and whole-body fat by DEXA (r = -0.32, P = 0.05) correlated inversely with psoas muscle attenuation. In contrast, subcutaneous fat by CT was not significantly associated with psoas attenuation values. Additionally, plasma FFA, LDL, total cholesterol, and glucose and insulin area under the curve during OGTT were significantly inversely related to muscle attenuation determined by CT scan (Table 2). A forward stepwise regression analysis was performed with psoas muscle attenuation value as the dependent variable (Table 3). Visceral fat, BMI, plasma FFA, and subcutaneous fat area, PI and NRTI use were tested as independent variables utilizing a probability to enter of 0.1. The resulting whole model r 2 value was 0.39, and demonstrated that visceral fat (P = 0.02) and plasma FFA (P <0.05), but not BMI, subcutaneous fat or antiretroviral use, were strong predictors of psoas muscle attenuation. To determine the effect of muscle attenuation on hyperinsulinemia we also performed forward stepwise regression analysis for insulin AUC (Table 3). Psoas attenuation, visceral fat, BMI, subcutaneous fat area, FFA, PI and NRTI use were tested as independent variables utilizing a probability to enter of 0.1. Both muscle attenuation (P = 0.02) and visceral fat (P = 0.02), but not BMI, subcutaneous fat, FFA or antiretroviral use were strong independent predictors of insulin area under the curve (r 2 = 0.47 for model). This model indicated that a decrease of 1 HU in psoas attenuation (2% of median, 56 HU) predicted an increase in insulin AUC of 776 µiu/ml (14% of median, 5491 µiu/ml).
JAP-00366-2003 6 DISCUSSION The HIV-lipodystrophy syndrome is characterized by increased visceral adiposity, loss of subcutaneous extremity fat, insulin resistance and dyslipidemia (9; 10; 27; 36). The mechanisms of the syndrome are unknown and may be related to protease inhibitor or nuclease reverse transcriptase inhibitor effects (8; 21) or an interaction of drug and non-drug factors. In addition, it is not known whether the HIV-lipodystrophy syndrome represents a single pathophysiological entity or several distinct subsyndromes. Hypertriglyceridemia is associated with increased concentrations of muscle lipid and insulin resistance in animal models (11), healthy subjects (3), obese and type 2 diabetics (15; 29). Intra-muscular lipid may contribute to skeletal muscle insulin resistance through inhibition of insulin signaling at PI-3 kinase and related inhibition of glucose transport (30). The extent and metabolic consequences of intramuscular lipid accumulation in HIV-infected patients with fat redistribution remains unknown. In non-hiv-infected patients, data obtained with CT suggest skeletal muscle attenuation decreases as a function of augmented lipid concentrations (14), and is an important independent marker of insulin resistance in obesity and type 2 diabetes (15; 16). Goodpaster et al. (14) have validated CT attenuation values of psoas, thigh and calf muscles as reliable indicators of lipid content. CT attenuation values of skeletal muscle have not previously been assessed in HIV lipodystrophy, and the relationship between muscle attenuation value, body composition, plasma FFA and glucose homeostasis remains unknown in this syndrome. Our study demonstrates low CT attenuation values of the psoas muscle in HIV lipodystrophic patients. Furthermore, CT attenuation values correlate significantly with insulin response to OGTT and plasma FFA. The LIPO subjects in our study had a mixed lipodystrophy pattern with predominant visceral hypertrophy. Visceral fat was increased to nearly twice the level of controls, whereas subcutaneous fat area was decreased by only 23% in comparison to BMImatched control subjects. Visceral fat area was a significant predictor of muscle attenuation values in HIV-infected individuals, whereas subcutaneous fat area was not significantly
JAP-00366-2003 7 associated with psoas muscle attenuation in either univariate or multivariate regression analysis. Increased basal rates of lipolysis and plasma FFA have been demonstrated in patients with HIV infection and fat redistribution receiving highly active antiretroviral therapy (17; 24; 32; 33; 36). Increased lipolysis was recently shown to be a strong predictor of insulin resistance in HIV-infected men (17), and improved insulin sensitivity was demonstrated after acute inhibition of lipolysis with acipimox in this population (18). Results from the current study extend these prior observations and demonstrate increased intra-muscular lipid in HIV-infected patients, in association with fat distribution and FFA. Furthermore, our data is in agreement with the results of Behrens et al. (1) demonstrating hyperinsulinemia and reduced insulin-mediated glucose uptake of skeletal muscle in HIV-lipodystrophic patients with increased levels of FFA. Visceral fat area, but not subcutaneous fat area, was highly related to CT attenuation. In stepwise regression modeling, visceral fat remained strongly associated with muscle attenuation, in a model including PI and NRTI use as well as BMI. These data are consistent with and extend those of Gan et al. (13), demonstrating a relationship between visceral, but not subcutaneous, fat and intramyocellular lipid by proton MR spectroscopy in this population. Our results support the hypothesis that the lipodystrophy syndrome represents several distinct subsyndromes in which lipoatrophy per se is not associated with intra-muscular lipid accumulation and is related to use and duration of NRTI therapy (2; 12). In contrast, our data suggest that visceral adiposity and a marked metabolic syndrome is associated with intramuscular lipid accumulation. Although the mechanism of visceral hypertrophy is not known, preliminary data suggest that use of protease inhibitors may contribute to visceral fat accumulation, and related insulin resistance and dyslipidemia (10). Of note, the NONLIPO group also demonstrated decreased psoas muscle attenuation, although not to the same degree as the LIPO patients. This suggests a spectrum of metabolic derangement among HIVinfected patients that is not entirely defined by the presence of fat redistribution.
JAP-00366-2003 8 We demonstrate that muscle attenuation is a strong predictor of hyperinsulinemia, suggesting significant metabolic consequences of intra-muscular lipid accumulation in this population. Our data suggest an overall schema whereby visceral fat hypertrophy may contribute to intra-muscular fat accumulation and hyperinsulinemia. Increased lipolysis in association with visceral adiposity may result in increased FFA levels that accumulate in the muscle, particularly if there is relatively less subcutaneous or extremity fat to serve as a depot for fat substrate. In turn, excess intramyocellular lipid may reduce glucose uptake into the muscle through effects on PI-3 kinase and thereby contribute to insulin resistance. Because of the cross-sectional nature of this study, we cannot determine causality, and it is also plausible that insulin resistance as a primary event impairs insulin mediated inhibition of lipolysis, resulting in excess free fatty acids and lipid accumulation in the muscle. Furthermore, Luzi et al. (22) demonstrated decreased lipid oxidation rates in association with increased intramyocellular lipid accumulation among HIV-infected patients receiving antiretroviral therapy, but regional and whole body lipolytic rates were not quantified. Although skeletal muscle fat content can be estimated on the basis of X-ray attenuation value by CT (6; 7; 14; 28), this methodology is incapable of directly measuring muscle lipid content and distinction of intramyocellular and extramyocellular fat is not possible. Proton magnetic resonance (MR) spectroscopy has been recently validated as an effective and specific method to directly quantify skeletal muscle lipids (4; 35), with reliable results correlating intramyocellular lipids with biopsy samples (19) and insulin sensitivity (34). This technique has been successfully employed by Gan et al. (13), who found increased intra-muscular lipid in association with insulin resistance in HIV-lipodystrophy patients. Nevertheless, single-slice CT scanning as performed in this study, appears to be effective for assessment of body composition and overall psoas muscle adiposity overcoming anatomical limitations of MR spectroscopy that is limited to two muscles of the lower leg. The capability of quantifying attenuation in a variety of muscles as well as fat distribution around muscle represent a
JAP-00366-2003 9 significant advantage of CT methodology. The widespread availability of CT, relative low-cost and prompt acquisition of data also makes it well suited for large-scale clinical investigation. In summary, this study demonstrates decreased psoas muscle attenuation in HIVinfected patients with evidence of the lipodystrophy syndrome. Diminished psoas attenuation is associated with abnormal body composition, lipid and glucose metabolism in this population. Our data support the hypothesis that overaccumulation of intra-muscular lipids contributes to decreased insulin sensitivity and hyperinsulinemia in this population. Further studies investigating the pathogenesis of intra-muscular lipid accumulation and effects on glucose homeostasis are necessary to determine optimal treatment strategies for dyslipidemia and insulin resistance in HIV-infected patients.
JAP-00366-2003 10 ACKNOWLEDGEMENTS We thank the nursing and bionutrition staff of the General Clinical Research Center at the Massachusetts General Hospital for their dedicated patient care.
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32, 1999. JAP-00366-2003 16
JAP-00366-2003 17 Figure 1. Axial computerized tomography of the abdomen obtained at level of L4 showing subcutaneous fat (SC) and intra-abdominal (visceral) fat (IA) interspersed between bowel loops. Placement of regions of interest used to measure psoas (P) attenuation values is shown.
JAP-00366-2003 18 Figure 2. Median and interquartile range for psoas muscle attenuation in HIV LIPO (n = 22), HIV NONLIPO (n = 20), and 21 HIV-negative age- and BMI-matched control subjects. *, P = 0.05 vs. NONLIPO;, P = 0.003 vs. controls;, P = 0.04 vs. controls
JAP-00366-2003 19 Table 1. Group comparison by lipodystrophy and HIV status HIV-infected lipodystrophic (n = 22) HIV-infected nonlipodystrophic (n = 20) Normal controls (n = 21) Age, anthropometrics and disease status Age (yr) 47 (38-50) 41 (37-44) 43 (37-49) Waist- to-hip ratio 0.99 (0.97-1.04) a,c 0.91 (0.88-0.93) 0.92 (0.86-0.95) CD4+ T-cells (#/mm 3 ) 371 (144-565) 313 (153-434) NA HIV Viral load (copies/ml) 89 (50-13150) 10475 (73-42925) NA Duration of HIV (yr) 9 (6-10) c 6 (4-8) NA NRTI use (%) 100 c 60 NA PI use (%) 90 c 45 NA Body composition BMI (kg/m 2 ) 24.6 (22.2-26.6) 24.4 (23.3-25.9) 24.8 (22.7-26.1) Whole-body fat by DEXA 14.1 (12.0-18.4) 14.6 (11.0-18.9) 16.7 (13.4-19.4) (kg) Trunk fat : Total Fat by 0.61 (0.56-0.65) a,c 0.46 (0.41-0.49) 0.49 (0.42-0.52) DEXA (kg) Extremity fat : Total Fat by 0.31 (0.27-0.35) a,c 0.46 (0.44-0.51) 0.44 (0.42-0.51) DEXA (kg) Visceral fat area by CT 158 (114-196) a,c 76 (51-112) 88 (62-116) (cm 2 ) Subcutaneous fat area by 118 (69-184) 118 (99-181) 159 (124-235) CT (cm 2 ) VAT: abdominal crosssectional 0.25 (0.18-0.31) a,c 0.15 (0.11-0.18) 0.14 (0.12-0.20) area by CT Psoas muscle density by 55.0 (51.0-58.3) a,d 57.0 (55.0-59.0) e 59.5 (57.3-64.8) CT (HU) Metabolic status FFA (mmol/l) 0.62 (0.39-0.73) 0.56 (0.52-0.76) 0.58 (0.49-0.78) HDL (mg/dl) 36 (30-41) a,d 44 (36-50) 47 (40-58) LDL (mg/dl) 114 (82-143) 92 (76-123) 96 (77-122) Triglycerides (mg/dl) 189 (124-378) a,c 117 (74-182) e 75 (50-101) Cholesterol (mg/dl) 193 (160-223) b,d 164 (141-198) 169 (139-193) Fasting glucose (mg/dl) 93 (85-98) 94 (89-97) 90 (85-95) 14475 (13155- OGTT glucose response, 18330 (15435-19883) a 16493 (14760- AUC [mg/dl (120 min)] 18518) e 16478) Fasting insulin (µiu/ml) 11.5 (8.4-21.3) a 8.7 (7.4-12.8) 7.8 (6.6-8.5) OGTT insulin response, AUC [µiu/ml (120 min)] 8389 (3784-20539) b,d 4781 (3856-6338) 4748 (3508-6190) NRTI, nucleoside reverse transcriptase inhibitor; PI, protease inhibitor; FFA, Plasma free fatty acids; OGTT, Oral glucose tolerance test, NA, not applicable. For HIV-infected lipodystrophic patients vs. controls, a indicates P < 0.01 and b indicates P < 0.05. For HIV-infected lipodystrophic vs. HIV infected nonlipodystrophic patients, c indicates P < 0.01 and d indicates P < 0.05. For HIV-infected nonlipodystrophic patients vs. controls, e indicates P < 0.05. Results are median (interquartile range).
JAP-00366-2003 20 Table 2. Univariate correlations of body composition, WHR, and metabolic status with psoas muscle density in HIV-infected subjects Psoas muscle density (HU) r value P value Body composition and anthropometrics BMI -0.43 0.008 WHR -0.54 0.0006 Visceral fat by CT -0.49 0.002 Subcutaneous fat by CT -0.23 0.17 Whole-body fat by DEXA -0.32 0.05 Trunk fat by DEXA -0.50 0.002 Metabolic status FFA -0.38 0.02 LDL cholesterol -0.42 0.02 Triglycerides -0.30 0.07 Cholesterol -0.47 0.004 Glucose response to OGTT -0.38 0.02 (AUC) Fasting insulin -0.32 0.06 Insulin response to OGTT (AUC) -0.60 0.0001 FFA, plasma free fatty acids; OGTT, oral glucose tolerance test.
JAP-00366-2003 21 Table 3. Forward stepwise regression analysis of HIV-infected patients Dependent variable Parameter estimate Stepwise P Psoas muscle attenuation (r 2 = 0.39 for whole model, P = 0.1 to enter) Visceral fat by CT (cm 2 ) -0.024 0.02 Plasma FFA (mmol/l) -6.14 0.049 BMI (kg/m 2 ) -0.37 0.10 Subcutaneous fat by CT (cm 2 ) NA 0.89 PI Use NA 0.49 NRTI Use NA 0.82 Insulin AUC (r 2 = 0.47 for whole model, P = 0.1 to enter) Visceral fat by CT (cm 2 ) 53.05 0.02 Psoas muscle attenuation (HU) -776.25 0.02 Plasma FFA (mmol/l) NA 0.31 Subcutaneous fat by CT (cm 2 ) NA 0.38 BMI (kg/m 2 ) NA 0.21 PI Use NA 0.53 NRTI Use NA 0.67 PI, Protease inhibitor; NRTI, Nucleoside reverse transcriptase inhibitor; NA, not applicable.