Glucose from fat

The energy content of foodstuffs varies. The approximate caloric content of generic carbohydrates, fats, and proteins are listed in Table 23-1. These values are based on a varied diet, i.e., all components are present. Recall that humans cannot synthesize glucose from fat. In the absence of carbohydrates, therefore, fat metabolism becomes inefficient, and the caloric value of fat decreases. 

We cannot get the net conversion of fats into glucose, because the only means to get the carbons from fats into oxaloacetate, the precursor to glucose, is through the citric acid cycle. However, although two carbon atoms enter the cycle as acetyl CoA, two carbon atoms are lost as CO2 before the oxaloacetate is formed. Thus, although some carbon atoms from fats may end up as carbon atoms in glucose, we cannot obtain a net synthesis of glucose from fats. 

Gluconeogenesis Formation of glucose from precursors other than carbohydrates (especially by the liver and kidney) using amino acids from proteins, glycerol from fats, or lactate produced by muscle during anaerobic glycolysis. 

To burn the acetyl-CoA made from fat, glucose, or protein in order to make ATP in cooperation with oxidative phosphorylation. 

Fat is only an energy storage form (Fig. 17-4). Fat cannot be converted to carbohydrate equivalents. This is a very important point. Remember it The reason for this is a bit subtle. The carbon skeleton of fatty acids is metabolized to acetyl-CoA only. Glucose precursors such as oxaloacetate can be synthesized from acetyl-CoA by going around the TCA cycle. However, acetyl-CoA has 2 carbon atoms. Going around the TCA cycle burns off 2 carbon atoms (as C02). The net number of carbon atoms that ends up in oxaloacetate is then zero. No carbohydrate can be made from fat.5... 

Glucose (or more correctly its metabolites) is essential for the functioning of the TCA cycle. For the TCA cycle to keep turning, the intermediates of the cycle must be maintained at a reasonable level. Since these intermediates are used for things other than the TCA cycle, they must be replaced constantly. The trouble is that the intermediates of the TCA cycle cannot be synthesized from fat (our most abundant storage form of energy). So we re got to have glucose or its equivalent to bum fat. 

From these six intermediates, only three end-products are produced carbon dioxide, glucose or fat, and the first two are quantitatively the most important. A simple overview of these processes is given in Figure 8.7. (The details of the reactions that convert the individual amino acids to one of the intermediates are given in Appendix 8.3.)... 

Figure 8.9 The ATP produced from oxidation of half of the amino acids is used to synthesise glucose from the other half of the amino acids. This is a general metaboiic point, it does not appiy in aii conditions the ATP can be used in other processes (e.g. urea cycie). In starvation, ATP is generated from fat oxidation, since oxoacids are not oxidised but are converted to giucose.

Insulin, which is formed in the B cells of the pancreas, has both endocrine and paracrine effects. As a hormone with endocrine effects, it regulates glucose and fat metabolism. Via a paracrine mechanism, it inhibits the synthesis and release of glucagon from the neighboring A cells. 

Starch<—The inversion method described for flour (p. 63) is employed. From 5 to 10 grams of the substance are freed from fat by extraction with petroleum ether or other suitable solvent and from sugars (and dextrins) by treatment with 25% alcohol. The glucose found, multiplied by 0-9, gives the quantity of starch. 

The different mechanisms of action of the various classes of hypoglycemic drugs makes combined therapy feasible the sulfonylureas and meglitinides stimulate insulin production by different mechanisms, the biguanides reduce glucose production by the liver and excretion from the liver, acarbose reduces the absorption of glucose from the gut, and the thiazolidinediones reduce insulin resistance in fat. It is not necessary to wait until the maximal dose of... 

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