Importance of skeletal muscle lipid levels for muscle function and physical function in older individuals

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

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.

Aging-Related Sarcopenia: Metabolic Characteristics and Therapeutic Strategies

The proportion of the elderly population is gradually increasing as a result of medical care advances, leading to a subsequent surge in geriatric diseases that significantly impact quality of life and pose a substantial healthcare burden. Sarcopenia, characterized by age-related decline in skeletal muscle mass and quality, affects a considerable portion of older adults, particularly the elderly, and can result in adverse outcomes such as frailty, fractures, bedridden, hospitalization, and even mortality. Skeletal muscle aging is accompanied by underlying metabolic changes. Therefore, elucidating these metabolic profiles and specific mechanisms holds promise for informing prevention and treatment strategies for sarcopenia. This review provides a comprehensive overview of the key metabolites identified in current clinical studies on sarcopenia and their potential pathophysiological alterations in metabolic activity. Besides, we examine potential therapeutic strategies for sarcopenia from a perspective focused on metabolic regulation.

Association of skeletal muscle oxidative capacity with muscle function, sarcopenia-related exercise performance, and intramuscular adipose tissue in older adults

Muscle function and exercise performance measures, such as muscle endurance capacity, maximal strength, chair stand score, gait speed, and Timed Up and Go score, are evaluated to diagnose sarcopenia and frailty in older individuals. Furthermore, intramuscular adipose tissue (IntraMAT) content increases with age. Skeletal muscle oxidative capacity determines muscle metabolism and maintains muscle performance. This study aimed to investigate the association of skeletal muscle oxidative capacity with muscle function, exercise performance, and IntraMAT content in older individuals. Thirteen older men and women participated in this study. Skeletal muscle oxidative capacity was assessed by the recovery speed of muscle oxygen saturation after exercise using near-infrared spectroscopy from the medial gastrocnemius. We assessed two muscle functions, peak torque and time to task failure, and four sarcopenia-related exercise performances: handgrip strength, gait speed, 30-s chair stand, and Timed Up and Go. The IntraMAT content was measured using axial magnetic resonance imaging. The results showed a relationship between skeletal muscle oxidative capacity and gait speed but not with muscle functions and other exercise performance measures. Skeletal muscle oxidative capacity was not related to IntraMAT content. Skeletal muscle oxidative capacity, which may be indicative of the capacity of muscle energy production in the mitochondria, is related to locomotive functions but not to other functional parameters or skeletal fat infiltration.