Fatty acids, dietary metabolism

Figure 6.15 (a) Metabolic modification of endogenously synthesized fatty acids, (b) Metabolic modification of essential dietary fatty acids... 

Fig. 13.12 Polyunsaturated fatty acids required for eicosanoid synthesis. Oleic acid is the only fatty acid synthesized by mammals de novo. Linoleic (co-3) and a-linolenic acid (9 or greater fatty acids. Ingested o>3 fatty acids are metabolized to other co-3 fatty acids with o>9 double bonds. The same applies to co-6 fatty acids. The major dietary sources of polyunsaturated fatty acids are fish and plants oils...

In mammals, both n-6 and n-3 fatty acids are metabolized by the same enzyme systems. Therefore, there is a competition between these fatty acids. The n-6 fatty acids interfere with the metabolism of n-3 fatty acids and vice versa (1,2). Two enzymatic steps catalyzing A6 and, to a lesser extent, A5 desaturation, are regarded as the rate-limiting steps for this pathway (1,2). Although ALA is preferred as a substrate for A6 desaturase compared with LA, its conversion to DHA in humans is <5%. This is because of the relatively high n-6 fatty acid content in human diets (2-4). For infants, whose metabolic capacity is low, dietary intake of n-3 fatty acids. 

When fats or oils rich in unsaturated fatty acids are fed (oleic or linoleic, for example), an increase in the iodine number of body fat is observed. The iodine number is an index of the degree of unsaturation of a fat. Thus, fatty acids of dietary origin are deposited, in some measure, without extensive modification of the chain. But since some of the double bonds may be destroyed by the reversible action of fatty acid dehydrogenases, the use of the double bond as a label in the study of fatty acid intermediary metabolism is limited in scope. ... 

One of the most striking features of the common fatty adds is that they have an even number of carbon atoms (Table 27.1, p. 1062). This even number results because all fatty acids are derived biosynthelically from acetyl CoA by sequential addition of two-carbon units to a growing chain. The acetyl CoA, in turn, arises primarily from the metabolic breakdown of carbohydrates in the glycolysis pathway that weTl see in Section 29.5. Thus, dietary carbohydrates consumed in excess of immediate energy needs are turned into fats for storage. 

The pentose phosphate pathway is an alternative route for the metabolism of glucose. It does not generate ATP but has two major functions (1) The formation of NADPH for synthesis of fatty acids and steroids and (2) the synthesis of ribose for nucleotide and nucleic acid formation. Glucose, fructose, and galactose are the main hexoses absorbed from the gastrointestinal tract, derived principally from dietary starch, sucrose, and lactose, respectively. Fructose and galactose are converted to glucose, mainly in the liver. 

Unsaturated fatty acids are probably the most abundant oxidizable endogenous substrates. In the past it was erroneously believed that unsaturated fatty acids are just products of lipid peroxidation. Now, it has been shown that they have dietary origin. Family of unsaturated fatty acids includes linoleic (Ci8), arachidonic (C2o), docosahexaenoic (C22), and other fatty acids containing two, three, four, five, or six double bonds. Some acids can be in vivo converted into others for example, linoleic acid can be metabolized to linolenic and eicosa-trienoic acids [78]. 

Most of our fat intake will consist of fatty acids with an even number of carbon atoms, but not all dietary fatty acids nor all those synthesized in the liver are saturated. A variable, but probably not inconsiderable, proportion of dietary fatty acids are unsaturated, partly perhaps because a high intake of unsaturated fat is recommended to help reduce the risk for diseases of the heart and vascular system. Unsaturated and odd-numbered fatty acids pose particular chemical problems to the 3-oxidation pathway and additional enzymes are required for their metabolism. 

The physiological functions of carboxylesterases are still partly obscure but these enzymes are probably essential, since their genetic codes have been preserved throughout evolution [84] [96], There is some evidence that microsomal carboxylesterases play an important role in lipid metabolism in the endoplasmic reticulum. Indeed, they are able to hydrolyze acylcamitines, pal-mitoyl-CoA, and mono- and diacylglycerols [74a] [77] [97]. It has been speculated that these hydrolytic activities may facilitate the transfer of fatty acids across the endoplasmic reticulum and/or prevent the accumulation of mem-branolytic natural detergents such as carnitine esters and lysophospholipids. Plasma esterases are possibly also involved in fat absorption. In the rat, an increase in dietary fats was associated with a pronounced increase in the activity of ESI. In the mouse, the infusion of lipids into the duodenum decreased ESI levels in both lymph and serum, whereas an increase in ES2 levels was observed. In the lymph, the levels of ES2 paralleled triglyceride concentrations [92] [98],... 

Now here is the central nnderstanding—the role of several vitamins is to serve as coenzymes or as metabolic precursors for coenzymes that is, the vitamin itself may serve as coenzyme or it may be converted in the human body to a coenzyme. The other key point 1 suppose is obvious but 1 am going to state it anyway we need vitamins in our diet because we cannot make them ourselves. In that sense, they are like essential amino acids or essential fatty acids stuff that we need but cannot make ourselves and so must obtain from dietary sources. So let s get started in understanding these critical molecules and how they serve the needs of human beings. 

Lipoprotein metabolism. Entero-cytes release absorbed lipids in the form of triglyceride-rich chylomicrons. Bypassing the liver, these enter the circulation mainly via the lymph and are hydrolyzed by extrahepatic endothelial lipoprotein lipases to liberate fatty acids. The remnant particles move on into liver cells and supply these with cholesterol of dietary origin. 

Mechanism of Action An antioxidant that prevents oxidation of vitamins A and C, protects fatty acids from aff ack by free radicals, and protects RBCs from hemolysis by oxidizing agents. Therapeutic Effect Prevents and treats vitamin E deficiency. Pharmacokinetics Variably absorbed from the GI tract (requires bile salts, dietary fat, and normal pancreatic function). Primarily concentrated in adipose tissue. Metabolized in the liver. Primarily eliminated by biliary system. 

Burghardt, P. R., Kemmerer, E. S., Buck, B. J., Osetek, A. J., Yan, C., Koch, L. G., Britton, S. L., and Evans, S. J. (2010). Dietary n-3 n-6 fatty acid ratios differentially influence hormonal signature in a rodent model of metabolic syndrome relative to healthy controls. Nutr. Metab. (bond) 7, 53. 

Sea animals are rich in soluble dietary fibers, proteins, minerals, vitamins, antioxidants, phytochemicals, and polyunsaturated fatty acids, with low caloric value. Polysaccharides from marine animals have been reported to possess biological activities with potential medicinal values in addition to their current status as a source of dietary fibers and prebiotics. Moreover, they have a lot of dietary fiber, which lowers blood cholesterol, and iodine, which improves metabolism, vascular and cardiac action, body temperature, and perspiration regulation, and are effective in... 

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