Adipose Tissue in Fasting

A. Carbohydrate metabolism
Glucose transport by insulin-sensitive GLUT-4 into the adipocyte and its subsequent metabolism are decreased because of low levels of circulating insulin. This results in decreased TAG synthesis.
B. Fat metabolism
1. Increased fat degradation: The PKA-mediated phosphorylation and activation of HSL  and subsequent hydrolysis of stored fat (TAG) are enhanced by the elevated catecholamines norepinephrine and epinephrine. These hormones, which are secreted from the sympathetic nerve endings in adipose tissue and/or from the adrenal medulla, are physiologically important activators of HSL (Fig. 1, ).

Figure 1: Major metabolic pathways in adipose tissue during fasting. [Note: The numbers in the circles, which appear both in the figure and in the corresponding citation in the text, indicate important pathways for fat metabolism.] CoA = coenzyme A; TCA = tricarboxylic acid.
2. Increased fatty acid release: FA obtained from hydrolysis of TAG stored in adipocytes are primarily released into the blood (see Fig. 1, ). Bound to albumin, they are transported to a variety of tissues for use as fuel. The glycerol produced from TAG degradation is used as a gluconeogenic precursor by the liver, which contains glycerol kinase. [Note: FA can also be oxidized to acetyl CoA, which can enter the TCA cycle, thereby producing energy for the adipocyte. They also can be reesterified to glycerol 3-phosphate (from glyceroneogenesis), generating TAG and reducing plasma FA concentration.]
3. Decreased fatty acid uptake: In fasting, LPL activity of adipose tissue is low. Consequently, FA in circulating TAG of lipoproteins are less available to adipose tissue than to muscle.

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