Adipose tissue infiltration in skeletal muscle of healthy elderly men: relationships with body composition, insulin resistance, and inflammation at the systemic and tissue level.

Background: Association between inflammatory markers and intermuscular adipose tissue (IMAT) has been reported. We hypothesized that subclinical inflammation of adipose tissue surrounding and infiltrating muscle could be related to the metabolic and functional abnormalities of the "aging muscle."

Methods: In 20 healthy elderly men undergoing elective vertebral surgery, IMAT within erector spinae was evaluated by magnetic resonance imaging and body composition by dual-energy x-ray absorptiometry. Fasting glucose, insulin, high-sensitive C-reactive protein (hs-CRP), leptin, adiponectin, and interleukin 6 (IL-6) were measured, and insulin resistance was estimated by homeostasis model assessment (HOMA) index. In subcutaneous adipose tissue (SAT) biopsies near the erector spinae, quantification of gene expression was performed.

Results: IMAT showed a significant association with body mass index and total and regional body fat, even after adjustment for age. Insulin, HOMA, and leptin were significantly correlated with IMAT, whereas hs-CRP presented an association of borderline significance. IL-6 expression in SAT was significantly associated with IMAT; IL-6 messenger RNA (mRNA) was negatively associated with adiponectin and peroxisome proliferator-activated receptor gamma expression. In multivariate regression analysis, 68% of IMAT variance was explained by fat mass and age, independent of waist circumference, leptin, HOMA, and IL-6 mRNA.

Conclusion: IMAT was primarily related to age and total body adiposity; subclinical inflammation in fat significantly contributes to IMAT.