Metabolism dietary

WOLVER T M s, JENKINS D J A (1993) Effect of dietary fiber and foods on carbohydrate metabolism. Dietary Fiber in Nutrition. In Ed. Spiller G A, Boca Raton, FL CRC Press. 111-52. 

Thiamine (vitamin Bi), in the form of thiamine diphosphate (TPP), is a coenzyme of some considerable importance in carbohydrate metabolism. Dietary deficiency leads to the condition beriberi, characterized by neurological disorders, loss of appetite, fatigue, and muscular weakness. We shall study a number of... 

Acquevella JF et al Assessment of clinical, metabolic, dietary, and occupational correlations with serum polychlorinated biphenyl levels among employees at an electric capacitor manufacturing plant. J Occup Med 28 1177-1180, 1986... 

Large amounts of C28 and C29 sterols occur in asteroids and other echinoderms but it seems that they cannot be synthesized from C27 sterols by transmethylation as is the case in plants (but not in other animals). This was confirmed with the starfish Laiaster leechii, which could convert MVA into A -cholesterol but could not alkylate this at C-24 or introduce a A -bond. Similarly the starfish Asterias rubens can only synthesize C27 sterols de novo, but here there is evidencethat a A -bond can be introduced into both cholesterol and A -cholesterol. This starfish can also synthesize A -sterols and there is some indication that cholesterol itself may be synthesized even though it was previously concluded that the family could not perform the final steps in this biosynthesis. This species also rapidly metabolized dietary steroids and such A -sterols could be converted into A -products, but it was considered unlikely that the organism had the ability to dealkylate C28 and C29 sterols. 

Vitamin K Uptake and Metabolism. Dietary vitamin and the pharmaceutical form, phytonadione or vitamin K go) must be converted to the K2 series known as menoquinones. The most common of these is menoquinone-4 or K2(20)> This conversion to the K2 series occurs in the liver and possibly the intestinal flora. It involves removing the phytyl chain producing the intermediate menadione. Menadione sometimes is prescribed when there is impaired uptake of lipids from the intestine. There is little storage reserve in the liver, and a deficiency can result when dietary intake of vitamin K is restricted or absorption is impaired. 

The mucosa of the small intestine metabolizes dietary glutamine, glutamate, asparagine, and aspartate by oxidation to CO2 and H2O, or by conversion to lactate, alanine, citrulline, and NH3. These intermediates and the unmetabolized dietary amino acids are transferred to the portal blood and then to the liver for further metabolism. 

Other approaches to the prevention of traveler s diarrhea are the use of lactobacillus preparations or bismuth subsalicylate. Lactobacilli are bacteria that metabolize dietary carbohydrate to lactic acid and other organic acids, reducing the intraluminal pH and inhibiting the growth of enteropathogens. 

It should be noted that enoyl-CoA isomerase and 2,4-dienoyl-CoA reductase only help metabolize dietary unsaturated fatty acids with double bonds in the cis configuration. Dietary fatty acids with trans double bonds are not readily metabolized and may pose a health hazard. 

Fig. 1. Summary of amino acid metabolism. Dietary proteins are digested to amino acids in the stomach and intestine, which are absorbed by the intestinal epithelium, transferred to the circulation, and taken up by cells. Amino acids are used to synthesize proteins and other nitrogen-containing compounds. The carbon skeletons of amino acids are also oxidized for energy, and the nitrogen is converted to urea and other nitrogenous excretory products.

Table 41.9 Isovaleric acidaemia and disorders of leucine metabolism, dietary preparations used in treatment ...

FIG. 2. Schematic diagram of whole-body folate metabolism. Dietary forms of polyglutamyl folates are deconjugated in the jejunum (vertical rectangle on left) by mucosal and, to a lesser extent, pancreatic hydrolases. Secretion of folate in bite accounts for an enterohepatic circulation that has not been incorporated into modeling. Tissues behave, at least superficially, as a large, kinetically slow, pool. 

Liver is a central organ affecting lipid and lipoprotein metabolism. Dietary CLA has been reported to increase hepatic lipid contents in mice, concomitant with a decrease in body fat mass (6-8), although it is not clear whether this is the case for other experimental animals and humans. Hence,... 

Since insects are unable to biosynthesize the steroid nucleus, they require dietary sterols for structural and physiological (hormonal) purposes. Cholesterol will satisfy this dietary need in most cases, but since phytophagous insects ingest little or no cholesterol from dietary materials, they must convert dietary C28 and Q9 phytosterols to cholesterol or other sterols. Through evolutionary development, certain insects have acquired the ability to metabolize dietary sterols in unique ways and to produce and utilize a variety of ecdysteroids (molting hormones) for hormonal control of development and reproduction. Thus, insects are able to flourish in virtually every conceivable ecological niche. Certain comparative studies that illustrate these evolutionary processes will be discussed in this chapter. 

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