Adipose tissue glucose metabolism

Choose the ONE correct answer 23.1 In which one of the following tissues is glucose transport into the cell enhanced by insulin A. Brain B. Lens C. Red blood cells D. Adipose tissue E. Liver Correct answer = D. The major tissues in which glucose transport requires insulin are muscle and adipose tissue. The metabolism of the liver responds to insulin, but hepatic glucose transport is rapid and does not require insulin. 

Adipose tissue can metabolize glucose by means of the HMP, thereby producing NADPH, which is essential for fat synthesis (see p. 184 and Figure 24.5, ). However in humans, de novo synthesis is not a major source of fatty acids in adipose tissue. 

Insulin is a peptide hormone, secreted by the pancreas, that regulates glucose metabolism in the body. Insufficient production of insulin or failure of insulin to stimulate target sites in liver, muscle, and adipose tissue leads to the serious metabolic disorder known as diabetes mellitus. Diabetes afflicts millions of people worldwide. Diabetic individuals typically exhibit high levels of glucose in the blood, but insulin injection therapy allows diabetic individuals to maintain normal levels of blood glucose. 

Insulin resistance occurs when the normal response to a given amount of insulin is reduced. Resistance of liver to the effects of insulin results in inadequate suppression of hepatic glucose production insulin resistance of skeletal muscle reduces the amount of glucose taken out of the circulation into skeletal muscle for storage and insulin resistance of adipose tissue results in impaired suppression of lipolysis and increased levels of free fatty acids. Therefore, insulin resistance is associated with a cluster of metabolic abnormalities including elevated blood glucose levels, abnormal blood lipid profile (dyslipidemia), hypertension, and increased expression of inflammatory markers (inflammation). Insulin resistance and this cluster of metabolic abnormalities is strongly associated with obesity, predominantly abdominal (visceral) obesity, and physical inactivity and increased risk for type 2 diabetes, cardiovascular and renal disease, as well as some forms of cancer. In addition to obesity, other situations in which insulin resistance occurs includes... 

Metabolism—increased use of glucose (sugar) and liberation of fatty acids from adipose tissue... 

Insulin appears to activate a process that helps glucose molecules enter the cells of striated muscle and adipose tissue Figure 49-1 depicts normal glucose metabolism. Insulin also stimulates die synthesis of glycogen by die liver. In addition, insulin promotes protein syntiiesis and helps the body store fat by preventing its breakdown for energy. 

Figure 27-1. Metabolic interrelationships between adipose tissue, the liver, and extrahepatic tissues. In extrahepatic tissues such as heart, metabolic fuels are oxidized in the following order of preference (1) ketone bodies, (2) fatty acids, (3) glucose. (LPL, lipoprotein lipase FFA, free fatty acids VLDL, very low density lipoproteins.)...

In adipose tissue, the effect of the decrease in insulin and increase in glucagon results in inhibition of lipo-genesis, inactivation of lipoprotein lipase, and activation of hormone-sensitive lipase (Chapter 25). This leads to release of increased amounts of glycerol (a substrate for gluconeogenesis in the liver) and free fatty acids, which are used by skeletal muscle and liver as their preferred metabolic fuels, so sparing glucose. 

Fat is the major way we store energy. Fat is stored principally in adipose tissue and can be stored in virtually unlimited amounts. Fat is metabolized through (1 oxidation to acetyl-CoA and then through the TCA cycle, where it s burned completely to C02 to make ATP. Fat cannot be used to make carbohydrate because acetyl-CoA cannot be converted directly to precursors of glucose without losing its carbon atoms first (you ll see what this really means later). 

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