Vitamin physiological functions

Coenzyme/ Cofactor Type of Binding Derived from Vitamin Physiologic Function... 

Bde salts, cholesterol, phosphoHpids, and other minor components are secreted by the Hver. Bile salts serve three significant physiological functions. The hydrophilic carboxylate group, which is attached via an alkyl chain to the hydrophobic steroid skeleton, allows the bile salts to form water-soluble micelles with cholesterol and phosphoHpids in the bile. These micelles assist in the solvation of cholesterol. By solvating cholesterol, bile salts contribute to the homeostatic regulation of the amount of cholesterol in the whole body. Bile salts are also necessary for the intestinal absorption of dietary fats and fat-soluble vitamins (24—26). 

The possibility that vitamins might have physiological functions beyond the prevention of deficiency diseases was first recognized in 1955 with the finding (8) that niacin can affect semm cholesterol levels in humans. An explosion of research (9—11) in the intervening years has been aimed at estabUshing optimal vitamin levels and anticipating the health consequences. 

Physiological functions as well as clinical symptoms that occur in humans deficient in specific vitamins are given in Table 6. It is becoming more authenticated that vitamins have additional potential health benefits when adrninistered, via the diet or by supplementation, at levels above those required for obviating deficiency. Although for most vitamins the optimal levels are not yet estabUshed, some of the potential health benefits to be derived from vitamins are indicated (16). In one case, the level of scientific proof is such that the U.S. FDA has allowed "a health cl aim that women who are capable of... 

Besides water, the diet must provide metabolic fuels (mainly carbohydrates and lipids), protein (for growth and turnover of tissue proteins), fiber (for roughage), minerals (elements with specific metabolic functions), and vitamins and essential fatty acids (organic compounds needed in small amounts for essential metabolic and physiologic functions). The polysaccharides, tri-acylglycerols, and proteins that make up the bulk of the diet must be hydrolyzed to their constituent monosaccharides, fatty acids, and amino acids, respectively, before absorption and utilization. Minerals and vitamins must be released from the complex matrix of food before they can be absorbed and utifized. 

LEKLEM j E (1998) Vitamin B6 functions in humans. In Clinical and physiological application of vitamin B6. Eds Eeklem, J E, Reynolds, New York Liss, 297-320. 

The so-called normal range of blood and serum vitamin levels is always derived from observations on healthy young subjects. How about a comparison with healthy old subjects, whose percentage in the population is steadily increasing Much may be learned about the cause of the decrease of physiological function and of the increased susceptibility to organic disease in old age, if the role of vitamins as parameter of these alterations were investigated with a view to preventive theory. 

Physiological functions, of vitamins, 25 784 Physiology, of ascorbic acid, 25 766-773 Physisorbed water, 23 71-72 Physostigmine, 2 817-818 Phytane, 18 592... 

A vitamin has been defined [399] as a biologically active organic compound that is essential for the organism s normal metabolic and physiologic functions. Vitamins are not produced by human body itself, thereby they have to be obtained from the diet. As micronutrients, in contrast to nonessential chemical substances, they present both a minimum and a maximum level of intake beyond which arise risks of deficiency conditions or adverse effects [400]. 

Vitamin D is converted in the liver and kidneys to 1,25-dihydroxyvitamin D, which is the hormone-active compounds. The principal physiological function is to maintain the serum calcium and phosphorus concentrations in a range that support cellular processes, neuromuscular function, and bone ossihcation [417], Only a few foods contain vitamin D in quantities that have an impact on the dietary intake hsh liver, hsh liver oils, fatty fish, and egg yolks. Thus, some countries practice fortihcation of certain foods with vitamin D, most often milk, margarine, and/or butter. 

Since alcohol dehydrogenase is required for the conversion of retinol to retinal, excessive and prolonged ethanol ingestion can impair the physiological function of vitamin A. The decreased conversion of retinol to retinal results from competitive use of the enzyme by ethanol. Night blindness may result, since the visual cycle is a retinol-dependent physiological process. 

The facility of metal complex formation is underscored by the fact that most porphyrin systems with any type of physiological function occur as metal complexes (e.g. Fe in hemoglobins, myoglobins, cytochromes, catalases and peroxidases Mg in chlorophylls and bacteriochlorophylls Co in vitamin B12). 

A partial list of physiological functions til at have been determined to be affected by vitamin C deficiencies includes (1) absorption of iron (2) cold tolerance, maintenance of adrenal cortex (3) antioxidant (4) metabolism of tryptophan, phenylalanine, and tyrosine (5) body growth (6) wound healing (7) synthesis of polysaccharides and collagen (8) formation of cartilage, dentine, bone, and teeth and (9) maintenance of capillaries. 

Structure and production site Principal physiological functions with vitamins... 

Hormone Common names, (synonyms). Structure and production. site Principal physiological functions Interrelationships with vitamins... 

The principal physiological functions of tins vitamin include growth, production of visual purple, maintenance of skin and epithelial cells, resistance to infection, gluconeogenesis. mucopolysaccharide synthesis, bone development, maintenance of myelin and membranes, maintenance of color and peripheral vision, maintenance of adrenal cortex and steroid hormone synthesis, Specific vitamin A deficiency diseases include xerophthalmia, nyctalopia, hemeralopia, keratomalacia, and hyperkeratosis. 

As a brief introductory summary, vitamin D substances perform the following fundamental physiological functions (1) promote normal growth (via bone growth) (2) enhance calcium and phosphorus absorption from the intestine (3) serve to prevent rickets (4) increase tubular phosphorus reabsorpiion (5) increase citrate blood levels (6) maintain and activate alkaline phosphatase m bone (7) maintain serum calcium and phosphorus levels. A deficiency of D substances may be manifested in the form of rickets, osteomalacia, and hypoparathyroidism. Vitamin D substances are required by vertebrates, who synthesize these substances in the skin when under ultraviolet radiation, Animals requiring exogenous sources include infant vertebrates and deficient adult vertebrates, Included there are vitamin D (calciferol ergocalciferol) and vitamin D< (activated 7-dehydrocholesterol cholecalciferol). 

The nutritional value of a food commodity or diet with respect to a particular vitamin may be expressed in terms of the vitamin s bioavailability, which refers to the proportion of the quantity of vitamin ingested that undergoes intestinal absorption and utilization by the body. Utilization encompasses transport of the absorbed vitamin to the tissues, cellular uptake, and conversion to a form that can fulfill some biochemical or physiological function, either immediately or after storage. 

Vitamins and minerals are not considered medications, but these substances are essential for maintaining physiologic function and homeostasis throughout the body. Many individuals consume these substances to compliment other medications and to help promote optimal health. It is beyond the scope of this chapter to address all the pertinent issues related to vitamin and mineral metabolism. Nonetheless, a brief overview of these substances and their use as dietary supplements is provided here and summarized in Tables 38-2 and 38-3. Readers are also referred to other sources for a more detailed discussion of vitamins and minerals.4,55 76... 

Vitamin Biotin RDA/AI Men women 30 MQ/d Physiological Function Coenzyme in the synthesis of fat, glycogen, and amino acids Adverse Effects of Excessive Consumption No adverse effects have been reported ... 

Vitamin Vitamin A [retinol] RDA/AI Men 900 pg/d Women 700 pg/d Physiological function Required for normal vision, gene expression, reproduction, embryonic development, and immune function Adverse Effects of Excessive Consumption Teratological effects liver toxicity... 

Vitamin D is also known to have other physiological functions, including a role in immune suppression, hormone secretion, and the differentiation of both normal and malignant cells. 

Equally, demonstrating that a compound has a physiological function as a coenzyme or hormone does not classify that compound as a vitamin. It is necessary to demonstrate that endogenous synthesis of the compound is inadequate to meet physiological requirements in the absence of a dietary source of the compound. Table 1.3 lists compounds that have clearly defined functions, but are not considered vitamins because they are not dietary essentials endogenous synthesis normally meets requirements. However, there is some evidence that premature infants and patients maintained on long-term total parenteral nutrition may be unable to meet their requirements for carnitine (Section 14.1.2), choline (Section 14.2.2), and taurine (Section 14.5.3) unless they are provided in the diet, and these are sometimes regarded as... 

The main physiological function of vitamin D is in the control of calcium homeostasis, and vitamin D metabolism is regulated largely by the state of... 

The Methyl Folate Trap Hypothesis The reduction of meth-ylene-tetrahydrofolate to methyl-tetrahydrofolate is irreversible (Section 10.3.2.1), and the major source of folate for tissues is methyl-tetrahydrofolate. The only metabolic role of methyl-tetrahydrofolate is the methylation of homocysteine to methionine, and this is the only way in which methyl-tetrahydrofolate can be demethylated to yield free tetrahydrofolate in tissues. Methionine synthetase thus provides the link between the physiological functions of folate and vitamin B12. 

There is overwhelming epidemiological evidence that diets rich in fruit and vegetables are associated with a lower incidence of cancer, cardiovascular, and other degenerative diseases. To some extent, this may he because such diets provide less fat, and especially saturated fat, than diets that are richer in meat. The relatively high content of vitamins C and E and carotenoids in plant foods may also be important. In addition, fruits and vegetables contain a wide variety of compounds that have (potential) protective actions. These compounds are not strictly nutrients, in that they are not dietary essentials and have no physiological function. 

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