Intramyocellular Lipid Reduction During Hindlimb Unloading Assessed by In Vivo ¹H-MRS

This study aimed to investigate longitudinal changes in intramyocellular (IMCL) and extramyocellular lipids (EMCL) in skeletal muscle during hindlimb unloading using in vivo proton magnetic resonance spectroscopy (¹H-MRS). Hindlimb unloading was performed in male Wistar rats (n = 11) for 14 days. ¹H-MRS measurements were acquired in the tibialis anterior (TA) and plantaris muscles at baseline and after unloading. In addition, control animals (n = 5) were analyzed over the same period under standard housing conditions. IMCL and EMCL were quantified using LCModel. IMCL levels decreased in both the TA (2.88 ± 2.08 to 1.57 ± 0.50 × 10^-3 mmol/L, p = 0.0295) and plantaris muscles (2.70 ± 0.99 to 1.32 ± 0.83 × 10^-3 mmol/L, p = 0.0019) following hindlimb unloading. EMCL levels showed an increase in the plantaris muscle (1.80 ± 1.44 to 3.78 ± 2.22 × 10^-3 mmol/L, p = 0.0234), whereas no significant change was observed in the TA. In control animals, no consistent directional changes were observed in either IMCL or EMCL over the same interval. Hindlimb unloading was associated with a reduction in IMCL content in skeletal muscle. These findings provide longitudinal in vivo evidence of IMCL alterations under disuse conditions. Changes in EMCL were observed in the plantaris muscle; however, these should be interpreted with caution. ¹H-MRS may serve as a useful noninvasive tool for assessing muscle lipid content.