Vitamin actions/effects

The effect of vitamin D deficiency upon plasma calcium is not primarily due to malabsorption of calcium from the gastrointestinal tract. In addition to its well-known effect on calcium absorption, vitamin D contributes directly to the maintenance of plasma calcium by the skeleton by a direct action on bone similar to that of parathyroid hormone (C2, C16). The vitamin D effect may be mediated by an action on the Krebs glycolytic cycle, resulting in inhibition of citrate oxidation (S6). 

Steenbock et al. consider it possible that the vitamin D effects observed are secondary to the effect on calcium absorption, and that these effects would then be a secondary result of the primary action of vitamin D. It would be very interesting indeed if it could be established that vitamin D has an effect on acid-base balance, over and above what must follow from a shift of so much calcium from the intestine to the blood stream. 

With this one exception (the action of vitamin B12. in cyanide poisoning) the only certain chemotherapeutic action of vitamin B12 seems to be to correct the metabolic disorder which results from an existing deficiency of the vitamin. The effects, in terms of clinical improvement in pernicious anemia, for example, are remarkable, and much has been learned about the chemotherapeutic action of vitamin B12 by studying this disease. In other blood diseases—even in disorders so closely allied that the marrow is megaloblastic—vitamin B12 is without effect unless the disorder is due to lack of the vitamin. The same holds for diseases of the nervous system. The striking effect of vitamin B12 in subacute combined degeneration should not arouse false hopes that Bi2 will prove effective in cerebral, spinal, or neural disorders not due to deficiency of vitamin Bi2. 

The nutrient sparing effect of antibiotics may result from reduction or elimination of bacteria competing for consumed and available nutrients. It is also recognized that certain bacteria synthesize vitamins (qv), amino acids (qv), or proteins that may be utilized by the host animal. Support of this mode of action is found in the observed nutritional interactions with subtherapeutic use of antibiotics in animal feeds. Protein concentration and digestibiHty, and amino acid composition of consumed proteins may all influence the magnitude of response to feeding antibiotics. Positive effects appear to be largest... 

Lead is toxic to the kidney, cardiovascular system, developiag red blood cells, and the nervous system. The toxicity of lead to the kidney is manifested by chronic nephropathy and appears to result from long-term, relatively high dose exposure to lead. It appears that the toxicity of lead to the kidney results from effects on the cells lining the proximal tubules. Lead inhibits the metaboHc activation of vitamin D in these cells, and induces the formation of dense lead—protein complexes, causing a progressive destmction of the proximal tubules (13). Lead has been impHcated in causing hypertension as a result of a direct action on vascular smooth muscle as well as the toxic effects on the kidneys (12,13). 

FIGURE 18.30 The physiological effects of ascorbic acid (vitamin C) are the result of its action as a reducing agent. A two-electron oxidation of ascorbic acid yields dehy-droascorbic acid. 

Levodopa interacts with many different drugs. When levodopa is used with phenytoin, reserpine, and papaverine, there is a decrease in response to levodopa The risk of a hypertensive crisis increases when levodopa is used with the monoamine oxidase inhibitors (see Chap. 31). Foods high in pyridoxine (vitamin B6) or vitamin B6 preparations reverse the effect of levodopa However, when carbidopa is used with levodopa, pyridoxine has no effect on the action of levodopa hi fact, when levodopa and carbidopa are given together, pyridoxine may be prescribed to decrease the adverse effects associated with levodopa... 

Blasiak J, Kowalik J. 1999. Effect of paraoxon-methyl and parathion-methyl on DNA in human lymphocytes and protective action of vitamin C. Pestic Sci 55 1182-1186. 

Ascorbic acid and alpha-tocopherol are effective blocking agents against N-nitroso compound formation. Ascorbic acid is effective particularly in aqueous media, and tocopherol effective particularly in lipid phases. They should be used in conjunction due to the mutually complementary actions of the two vitamins in blocking nitrosamine formation in both aqueous and lipid media. 

In the Unites States, the daily intake of 3-carotene is around 2 mg/day Several epidemiological studies have reported that consumption of carotenoid-rich foods is associated with reduced risks of certain chronic diseases such as cancers, cardiovascular disease, and age-related macular degeneration. These preventive effects of carotenoids may be related to their major function as vitamin A precursors and/or their actions as antioxidants, modulators of the immune response, and inducers of gap-junction communications. Not all carotenoids exert similar protective effects against specific diseases. By reason of the potential use of carotenoids as natural food colorants and/or for their health-promoting effects, research has focused on better understanding how they are absorbed by and metabolized in the human body. 

Results obtained in in vivo and ex vivo experiments are of various types. Some studies have found positive effects of the consumption of carotenoids or foods containing carotenoids on the markers of in vivo oxidative stress, even in smokers. Other studies demonstrated no effects of carotenoid ingestion on oxidative stress biomarkers of lipid peroxidation. " It should be noted that for studies using food, the activity observed may also be partly due to other antioxidant molecules in the food (phenols, antioxidant vitamins) or to the combination of actions of all the antioxidants in the food. 

Possibly the most significant discovery in the metabolism of aromatic azo compounds had implications that heralded the age of modem chemotherapy. It was shown that the bactericidal effect of the azo dye Prontosil in vivo was in fact due to the action of its transformation product, sulfanilamide, which is an antagonist of 4-aminobenzoate that is required for the synthesis of the vitamin folic acid. Indeed, this reduction is the typical reaction involved in the first stage of the biodegradation of aromatic azo compounds. 

Mueller-Limmroth, W., D. Berges et al. (1958). The effects of helenien and vitamin A on the primary visual processes. I. An electroretinographic study on the problem of the sites of helenien and vitamin A actions (German). Z. Biol. 110(5-6) 457 -75. 

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