Myosteatosis and myofibrosis: relationship with aging, inflammation and insulin resistance.

The mechanisms impairing muscle quality and leading to myofibrosis (MF) and myosteatosis (MS) are incompletely known. In biopsies of paraspinous muscle (PM) of 16 elderly men undergoing elective vertebral surgery, we histologically determined the area of MF and MS expressed as muscle quality index (MQI), in order to investigate the relation between them, as well as the main predictors of muscle quality. Total PM area and intermuscular adipose tissue (IMAT) were evaluated by MRI and body composition by DXA. Circulating fasting glucose, insulin, hs-CRP, leptin, adiponectin and IL-6 were measured and HOMA index calculated. Quantification of gene expression in PM and in subcutaneous adipose tissue (SAT) overlying the muscle was performed by rt-PCR. The degree of MS and MF was significantly and positively related to each other and positively associated with BMI, waist, FM and FM\% as well as with IMAT. The area of PM was negatively related with MF even after adjustment for weight. Leptin was positively associated with MF and MS, whereas hs-CRP to MF. In backward regression analyses, larger waist and smaller PM area explained 90\% of MF variance, whereas leptin about 80\% of MS variance. IL-6 expression in SAT was significantly higher in participants with higher MQI values. In PM biopsies we found significantly higher expression of SOCS-3 and a trend toward higher expression of myostatin with greater degrees of MQI. MS and MF are related phenomena that concur to alter muscle quality and both should be considered in further studies on the evolution of sarcopenia.