Skeletal muscle oxidative capacity related to intramyocellular lipid in young but not in older individuals.

Abstract

Skeletal muscles contain lipids inside and outside cells, namely intramyocellular lipids (IMCL) and extramyocellular lipids (EMCL), respectively; lipids have also been found to be interspersed between these muscles as adipose tissue, namely intermuscular adipose tissue (IMAT). Metabolized IMCL has been recognized as an important substrate for energy production and their metabolism is determined by the muscle oxidative capacity. Therefore, it has been speculated that muscle oxidative capacity is related to muscle lipid content. Excessive accumulation of EMCL and IMAT has been confirmed in older individuals, leading to metabolic disorders and a decline in muscle strength. However, whether EMCL and IMAT contribute to muscle lipid metabolism remains unknown. This study aimed to investigate whether muscle oxidative capacity is related to IMCL, EMCL, and IMAT in young and older individuals. A total of 18 young and 14 older individuals were included and their muscle oxidative capacity was assessed based on the recovery rate of muscle oxygen saturation after exercise, using near-infrared spectroscopy of the medial gastrocnemius. IMCL, EMCL, and IMAT were assessed using magnetic resonance spectroscopy and imaging. A relationship between muscle oxidative capacity and IMCL was confirmed in young (r = -0.47, P < 0.05) but not older individuals (r = 0.22, P = 0.45). Muscle oxidative capacity was not related to EMCL or IMAT in either young or older individuals. These results suggest that IMCL in young individuals can contribute to muscle lipid metabolism, but not EMCL and IMAT, and this relationship differs with aging.