Muscle Type Dependent Increase in Intramyocellular Lipids during Prolonged Fasting of Human Subjects: A Proton MRS Study

Abstract

The amount of intramyocellular lipids in skeletal muscle was assessed by proton magnetic resonance spectroscopy during a voluntary fasting period of 120 h in four healthy lean volunteers. The aim of the study was to determine whether muscular lipid uptake in the presence of high plasma lipid levels, or lipid oxidation due to lacking glycogen as a source of energy in musculature, are the dominant effects on intramyocellular lipid levels under fasting conditions in various muscle types. Intramyocellular lipids were quantified in the tibialis anterior (mixed type I and type II fibers, predominantly type II) and the soleus muscle (predominantly type I fibers) before and after 24 h, 72 h, and 120 h of fasting. An extreme increase in intramyocellular lipids to levels of 369 % (median) was found in the tibialis anterior muscle compared to baseline value (intramyocellular lipid level prior to fasting, set to 100 %; p = 0.02). The soleus muscle with clearly higher baseline content of intramyocellular lipids (2 - 4-fold compared to tibialis anterior) revealed slightly delayed and less pronounced uptake of intramyocellular lipids during fasting to 152 % (median) after 120 h (p = 0.02). The absolute increment in intramyocellular lipids (in terms of ratios between lipid and creatine signals) was also higher in tibialis anterior than in soleus (not statistically significant). These findings indicate augmentation of the intramyocellular lipid pool during long-term elevation of plasma FFA in the presence of low plasma insulin concentrations in both muscles investigated. The rate of muscular lipid oxidation during fasting is clearly lower than the increased uptake of FFA by myocytes.