Dietary sources of vitamin

An alkene, sometimes caJled an olefin, is a hydrocarbon that contains a carbon-carbon double bond. Alkenes occur abundantly in nature. Ethylene, for instance, is a plant hormone that induces ripening in fruit, and o-pinene is the major component of turpentine. Life itself would be impossible without such alkenes as /3-carotene, a compound that contains 11 double bonds. An orange pigment responsible for the color of carrots, /3-carotene is a valuable dietary source of vitamin A and is thought to offer some protection against certain types of cancer. 

Carotene, a yellow food-coloring agent and dietary source of vitamin A, can be prepared by a double Wittig reaction between 2 equivalents of jS-ionvlideneacetaldehyde and a diylide. Show the structure of the /0-carotene product. 

Terpenoids are classified according to the number of five-carbon multiples they contain. Monoterpenoids contain 10 carbons and are derived from two isopentenyl diphosphates, sesquiterpenoids contain 15 carbons and are derived from three isopentenyl diphosphates, diterpenoids contain 20 carbons and are derived from four isopentenyl diphosphates, and so on, up to triterpenoids (C30) and tetraterpenoids (C40). Monoterpenoids and sesquiterpenoids are found primarily in plants, bacteria, and fungi, but the higher terpenoids occur in both plants and animals. The triterpenoid lanosterol, for example, is the precursor from which steroid hormones are made, and the tetraterpenoid /3-carotene is a dietary source of vitamin A (Figure 27.6). 

Plants contain to some extent less bioavailable forms of vitamin B6, e.g., glycosylates, or biologically inactive metabolites, e.g., e-pyridoxin-lysin-complexes. In addition, the release of vitamin B6 from foods rich in fiber is assumed to be delayed. The bioavailability of vitamin B6 from animal-derived foods is therefore overall higher than from plant-derived foods. Good dietary sources of vitamin B6 include chicken, fish, pork, beans, and pulses [1]. 

The vitamin K requirement is met from the diet (vitamin Ki) and microorganisms in the intestine (vitamin K2). The richest dietary source of vitamin K is green leafy vegetables but it is also present in meat and dairy produce. The structural formulae of vitamin Ki (phylloquinone) and vitamin K2 (menaquinone) are given in Figure 15.15. It was discovered in 1929 by Henrik Dam in Copenhagen, who discovered that it was necessary for the clotting... 

Principal dietary sources of vitamin A are milk fat (cheese and butter) and eggs. Since it is stored in the Uver, inclusion of Uver in the diet also provides vitamin A. A plant pigment, carotene, is a precursor for vitamin A and is present in highly pigmented vegetables, such as carrots, rutabaga, and red cabbage. 

Davidson and J. A. Sadowski. Tea and coffee brews are not dietary sources of vitamin K-1 (phylloquinone). J Amer Diet Ass 1995 95(1) 82-83. 

Vitamin B12 consists of a porphyrin-like ring with a central cobalt atom attached to a nucleotide. Various organic groups may be covalently bound to the cobalt atom, forming different cobalamins. Deoxyadenosylcobalamin and methylcobalamin are the active forms of the vitamin in humans. Cyanocobalamin and hydroxocobalamin (both available for therapeutic use) and other cobalamins found in food sources are converted to the active forms. The ultimate source of vitamin Bi2 is from microbial synthesis the vitamin is not synthesized by animals or plants. The chief dietary source of vitamin Bi2 is microbially derived vitamin B12 in meat (especially liver), eggs, and dairy products. Vitamin Bi2 is sometimes called extrinsic factor to differentiate it from intrinsic factor, a protein normally secreted by the stomach that is required for gastrointestinal uptake of dietary vitamin B12. 

The major form of vitamin D in both cows and human milk is 25(OH)D3. This compound is reported to be responsible for most of the vitamin D in the blood serum of exclusively breast-fed infants. Whole cows milk contains only about 0.03 pg vitamin D per 100 g and 1 litre of milk per day will supply only 10-20% of the RDA. Therefore, milk is often fortified (at the level of c. 1-10 fig 1 ) with vitamin D. Fortified milk, dairy products or margarine are important dietary sources of vitamin D. The concentration of vitamin D in unfortified dairy products is usually quite low. Vitamin D levels in milk vary with exposure to sunlight. 

Certain human populations depend on dietary sources of vitamin D because of insufficient biosynthesis of the vitamin due to inadequate skin exposure to sunlight. The classic symptoms of vitamin D deficiency are rickets in children and osteomalacia in adults. 25-Hydroxyvitamin D3 is the major circulating metabolite in the blood, but the hormonally active form of the vitamin is 1,25-dihydroxyvitamin D3. The latter metabolite stimulates the intestine to absorb calcium and phosphate by two independent mechanisms and acts with parathyroid hormone to mobilize calcium, accompanied by phosphate, from the bone fluid compartment into the bloodstream. 1,25-dihydroxyvitamin D 3 is also involved in the formation of osteoclasts—giant cells that are solely responsible for the resorption of bone matrix (33). Resorption is an essential process for the development, growth, maintenance, and repair of bone. 

Vitamin A is an essential nutrient for humans and other vertebrates. Dietary sources of vitamin A are provided either by retinol esters, which are present in foods of animal origin and are hydrolyzed in the intestine to form retinol, or by plant carotenoids. More than 600 carotenoids have been identified in nature, of which 50 to 60 possess provitamin A properties and 10 have nutritional relevance (De Flora et al., 1999). 

The vitamin B12 that occurs in nature is produced almost entirely by bacterial synthesis in animals but not in humans (Battersby, 1994). The richest dietary sources of vitamin B12 are organ meats, such as fiver and kidney. Lesser amounts are present in shellfish, chicken, fish, muscle meats, and dairy products (the principal source in lacto-vegetarians). Plants contain no vitamin B12 unless they are contaminated by bacteria, and foods that contain microorganisms often provide the only source of vitamin B12 for strict vegetarians, such as the vegans of southern India. 

Synthetic ergocalciferol is used for enrichment and fortification of foods its metabolic fate is the same as that of dietary cholecalciferol. Except where there are known to be differences between the two vitamers, it is assumed that aU of the following discussion applies equally to ergocalciferol and cholecalciferol. There are few rich dietary sources of vitamin D, and the major source is usually photosynthesis in the skin. Dietary vitamin D is absorbed in chylomicrons and taken up rapidly by the liver as chylomicron remnants are cleared from... 

Mono and i 084uiterpene are found primarily in plants, but the bi terpenea offcur in both planta and animals, and many have important logical role. The triterpene lanuwtercd. for example, is the precursor wrhich ad steroid h omionc are made the tetrat rpen carotene is a dietary source of vitamin A. 

Niacin (nicotinic acid and nicotinamide) is also known as vitamin B3. Both nicotinic acid and nicotinamide can serve as the dietary source of vitamin B3. 

A-38 The requirement for vitamin Be in the diet is proportional to the level of protein consumption ranging from 1.4 - 2.0 mg/day for a normal adult Following are the dietary sources of Vitamin B-6 ... 

Usual dietary sources of vitamin B12 are animal food products (meat, milk, eggs, and shellfish), but not plant food products. To evaluate whether foods contain true vitamin B12 or inactive corrinoids, vitamin B12 compounds were purified and characterized using TLC on silica gel. The Rf values of the unknown vitamin B12 compound, purified from an algal health food (Spmilina tablets) were identical to those of pseudo vitamin B12 (Rf values 0.14 and 0.42), but not to those of vitamin Bi2 (or 5,6-dimethylbenzimidazolyl cyanocobamide) (Rf values 0.23 and 0.56), benzimidazolyl cyanocobamide (Rf values 0.18 and 0.52), 5-dydroxybenzunida-zolyl cyanocobamide (Rf values 0.20 and 0.47), and p-cresolyl cyanocobamide (Rf values 0.38 and 0.62) by TLC on silica gel 60 with l-butanol-2-propanol-water (10 7 10 vol/vol) and 2-propanol-NH40H (28%)-water (7 1 2 vol/vol) as solvents, respectively. The results indicate that an inactive vitamin B12 compound (pseudovitamin B12) is predominant in Spirulina tablets. 

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