Vitamins functions

Table 6. Vitamin Functions, Clinical Deficiency Symptoms, and Potential Health Benefits...

The establishment of quantitative methods for the determination of vitamins in body fluids and tissues by microbiological assay techniques should stimulate the search for the significance of vitamins in disease, not only in nutritional deficiency, but in the much wider field of all metabolic disturbances. Functional vitamin deficiencies are produced by malabsorption, by inhibitors of the vitamin function through products of the body, and particularly through drugs and other toxic substances. Vitamin deficiencies may be relative deficiencies whenever an individual s metabolism is deranged so as to require enhanced quantities of a given vitamin to cure or to counteract certain symptoms as, e.g., in Darier s disease (keratosis follicularis) (P2a). 

Although we will not discuss further the question of requirements and deficiencies involving vitamin B6, much of what has been said, both with respect to thiamine and pantothenic acid, applies in principle to vitamin B6 as well. It, like the other B vitamins, functions in every cell of the body. 

There is no firm evidence that very high doses of any vitamin produce beneficial effects unrelated to their normal vitamin function. 

Several of the B vitamins function as coenzymes or as precursors of coenzymes some of these have been mentioned previously. Nicotinamide adenine dinucleotide (NAD) which, in conjunction with the enzyme alcohol dehydrogenase, oxidizes ethanol to ethanal (Section 15-6C), also is the oxidant in the citric acid cycle (Section 20-10B). The precursor to NAD is the B vitamin, niacin or nicotinic acid (Section 23-2). Riboflavin (vitamin B2) is a precursor of flavin adenine nucleotide FAD, a coenzyme in redox processes rather like NAD (Section 15-6C). Another example of a coenzyme is pyri-doxal (vitamin B6), mentioned in connection with the deamination and decarboxylation of amino acids (Section 25-5C). Yet another is coenzyme A (CoASH), which is essential for metabolism and biosynthesis (Sections 18-8F, 20-10B, and 30-5A). 

Vitamins are minor components of foods that play an essential role in human nutrition. Many vitamins are unstable under certain conditions of processing and storage (Table 9-1), and their levels in processed foods, therefore, may be considerably reduced. Synthetic vitamins are used extensively to compensate for these losses and to restore vitamin levels in foods. The vitamins are usually divided into two main groups, the water-soluble and the fat-soluble vitamins. The occurrence of the vitamins in the various food groups is related to their water-or fat-solubility. The relative importance of certain types of foods in supplying some of the important vitamins is shown in Table 9-2. Some vitamins function as part of a coenzyme, without which the enzyme would be ineffective as a biocatalyst. Frequently, such coenzymes are phosphorylated forms of vitamins and play a role in the metabolism of fats, proteins, and carbohydrates. Some vitamins occur in foods as provitamins—compounds that are not vitamins but can be changed by the body into vitamins. Vitamers are members of the same vitamin family. 

Table 1.1 [continued] Vitamin Functions Deficiency Disease... 

Vitamin C appears to be more promising. Theoretically, this vitamin functions as an antioxidant that would enhance the synthesis or prevent the breakdown of nitric oxide, a naturally occurring gas produced in the lining of the arteries that keeps those vessels flexible and more capable of vasodilation. Studies have shown reductions in systolic blood pressure though not in diastolic pressure, when subjects were treated with vitamin C. A randomized, placebo-controlled trial with thirty-nine patients yielded nice results. Subjects took a 2 gram loading dose and then 500 mg daily for thirty days. Systolic blood pressure was reduced by 13 mm Hg, but vitamin C had no effect on diastolic pressure. 

Vitamin Function Clinical deficiency symptoms Potential health benefits... 

Vitamin Functions Some common dietary sources Deficiency s3onptoms... 

Metallic elements (and sometimes vitamins) function as coenzymes in certain chemical reactions or else they may be required to activate an enzyme that they are not actually a part of. 

Most vitamins function either as a hormone/ chemical messenger (cholecalciferol), structural component in some metabolic process (pantothenic acid), or a coenzyme (phytonadi-one, thiamine, riboflavin, niacin, pyridoxine, biotin, folic acid, cyanocobalamin). At least one vitamin has more than one biochemical role. Vitamin A as an aldehyde (retinal) is a structural component of the visual pigment rhodopsin and, in its acid form (retinoic acid), is a regulator of cell differentiation. The precise biochemical functions of ascorbic acid and a-tocopherol still are not well defined. 

Vitamins discussed in other chapters include vitamin D (Chapter 37) and vitamin K (Chapter 36). All the B vitamins function as cofactors or precursors for cofactors in enzyme-catalyzed reactions and are discussed in appropriate chapters. Less well-defined actions are reviewed here. 

The vitamins cannot be synthesised in the organism in certain cases the organism is capable of carrying out the last step from provitamin to vitamin (y -carotene to retinol). Some vitamins function as redox-catalysts in the metabolism (vitamins A, C, E and K), whereas others function as coenzyme or activators of enzymes (vitamin Bl, B2, B6, B12, folk acid, biotin and vitamin D). 

Vitamins are complex organic molecules necessary in small amounts for good health. Many vitamins function as coenzymes. 

The possible existence of bound forms of ascorbic add in animal and plant tissues forms one of the most contradictory subjects in the biochemical literature. The very high concentrations of ascorbic acid within both plant and animal cells provide reasons to look for such combined forms. The realization that most of the water-soluble vitamins function... 

Nicotinic acid (niacin) was first shown to lower serum cholesterol levels over 40 years ago (1955). This activity is not related to its vitamin functions (nicotinamide does not share its action). At daily doses of 3 g niacin s hypocholesterolemic effects are about equivalent to clofibrate. In addition to lowering triglycerides, it can also raise HDL levels. Unfortunately, at these high doses nicotinic acid can produce intense flushes and itching, resulting in poor patient compliance. Circumventing the problem with potential pro-drugs such as nicotinyl alcohol, fructose tetranicotinate, and others were not very successful. 

Two examples of radical inhibitors that are present in biological systems are vitamin C and vitamin E. Like hydroquinone, they form relatively stable radicals. Vitamin C (also called ascorbic acid) is a water-soluble compound that traps radicals formed in the aqueous environment of the cell and in blood plasma. Vitamin E (also called a-tocopherol) is a water-insoluble (hence fat-soluble) compound that traps radicals formed in nonpolar membranes. Why one vitamin functions in aqueous environments and the other in nonaqueous environments should be apparent from their structures and electrostatic potential maps, which show that vitamin C is a relatively polar compound, whereas vitamin E is nonpolar. 

Because most vitamins function as coenzymes, the symptoms of vitamin deficiencies reflect the loss of specific enzyme activities dependent on the coenzyme form of the vitamin. Thus, drugs and toxins that inhibit proteins required for coenzyme synthesis (e.g., vitamin transport proteins or biosynthetic enzymes) can cause the symptoms of a vitamin deficiency. This type of deficiency is called a functional deficiency, whereas an inadequate intake is called a dietary deficiency. 

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