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7.3A: Muscle Macronutrient Metabolism

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  • Compared to extrahepatic tissues as a whole, in the muscle the following pathways are not performed or are not important:

    • Fatty acid synthesis
    • Ketone body breakdown

    These pathways are crossed out in the figure below.

    Figure 7.311 The metabolic pathways that are not performed or important in the muscle, compared to extrahepatic tissues as a whole1

    Removing those pathways, the following metabolic pathways make up the muscle metabolic capability:

    • Glycogen synthesis and breakdown
    • Glycolysis
    • Protein synthesis and breakdown
    • Triglyceride synthesis and breakdown
    • Fatty acid breakdown
    • Lactate synthesis

    Figure 7.312 Muscle metabolic capability1

    Muscle is a major extrahepatic metabolic tissue. It is the only extrahepatic tissue with significant glycogen stores. However, unlike the liver, the muscle cannot secrete glucose after it is taken up (no glucose-6-phosphatase). Thus, you can think of the muscle as being selfish with glucose. It either uses it for itself initially or stores it for its later use.