Intramyocellular Lipid Content in Human Skeletal Muscle

May 2, 2006Obesity (Silver Spring, Md.)

Fat stored inside human skeletal muscle cells

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Abstract

Fat droplets in skeletal muscle, known as intramyocellular lipids (IMCLs), can be quantified through various methods.

Different methods exist for quantifying IMCLs, each with its own advantages and drawbacks.
IMCLs may serve as an energy source during exercise, with content decreasing during prolonged submaximal exercise.
Higher IMCL content is associated with trained muscle, particularly in oxidative, type 1 muscle fibers.
Increased plasma fatty acid levels or dietary fat intake can elevate IMCL content, indicating fat storage in skeletal muscle.
Excessive IMCL accumulation, particularly with low physical activity, may negatively impact insulin sensitivity.

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Fat can be stored not only in adipose tissue but also in other tissues such as skeletal muscle. Fat droplets accumulated in skeletal muscle [intramyocellular lipids (IMCLs)] can be quantified by different methods, all with advantages and drawbacks. Here, we briefly review IMCL quantification methods that use biopsy specimens (biochemical quantification, electron microscopy, and histochemistry) and non-invasive alternatives (magnetic resonance spectroscopy, magnetic resonance imaging, and computed tomography). Regarding the physiological role, it has been suggested that IMCL serves as an intracellular source of energy during exercise. Indeed, IMCL content decreases during prolonged submaximal exercise, and analogously to glycogen, IMCL content is increased in the trained state. In addition, IMCL content is highest in oxidative, type 1 muscle fibers. Together, this, indeed, suggests that the IMCL content is increased in the trained state to optimally match fat oxidative capacity and that it serves as readily available fuel. However, elevation of plasma fatty acid levels or dietary fat content also increases IMCL content, suggesting that skeletal muscle also stores fat simply if the availability of fatty acids is high. Under these conditions, the uptake into skeletal muscle may have negative consequences on insulin sensitivity. Besides the evaluation of the various methods to quantify IMCLs, this perspective describes IMCLs as valuable energy stores during prolonged exercise, which, however, in the absence of regular physical activity and with overconsumption of fat, can have detrimental effects on muscular insulin sensitivity.

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Intramyocellular Lipid Content in Human Skeletal Muscle

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