Vitamin human requirements

The recommended daily allowance for vitamin E ranges from 10 international units (1 lU = 1 mg all-rac-prevent vitamin E deficiency in humans. High levels enhance immune responses in both animals and humans. Requirements for animals vary from 3 USP units /kg diet for hamsters to 70 lU /kg diet for cats (13). The complete metaboHsm of vitamin E in animals or humans is not known. The primary excreted breakdown products of a-tocopherol in the body are gluconurides of tocopheronic acid (27) (Eig. 6). These are derived from the primary metaboUte a-tocopheryl quinone (9) (see Eig. 2) (44,45). 

Riboflavin was first isolated from whey in 1879 by Blyth, and the structure was determined by Kuhn and coworkers in 1933. For the structure determination, this group isolated 30 mg of pure riboflavin from the whites of about 10,000 eggs. The discovery of the actions of riboflavin in biological systems arose from the work of Otto Warburg in Germany and Hugo Theorell in Sweden, both of whom identified yellow substances bound to a yeast enzyme involved in the oxidation of pyridine nucleotides. Theorell showed that riboflavin 5 -phosphate was the source of the yellow color in this old yellow enzyme. By 1938, Warburg had identified FAD, the second common form of riboflavin, as the coenzyme in D-amino acid oxidase, another yellow protein. Riboflavin deficiencies are not at all common. Humans require only about 2 mg per day, and the vitamin is prevalent in many foods. This vitamin... 

The nutritional requirement for vitamin Bjg is low. Adult humans require only about 3 micrograms per day, an amount easily acquired with normal eating habits. However, because plants do not synthesize vitamin Bjg, pernicious anemia symptoms are sometimes observed in strict vegetarians. 

BARTLEY w, KREBS H A and o brien J p (1953) Vitamin C Requirements of Human Adults, London, HMSO. 

The award will be made to the investigator (or group of investigators) who (1) determines the clinical value of vitamin A (if any) in human medicine, or (2) determines the vitamin A requirements of human beings, or (3) determines whether vitamin A in amounts more than obtained in a well-balanced diet is of benefit in human physiology. 

The amount of vitamin B6 required by humans is not well established,73 and only recently has evidence been obtained that the needs are variable. Hansen and Bessey74 have found that in some babies 3 or 4 times as much vitamin B6 is needed to prevent the excretion of xanthurenic acid after a test dose of tryptophane than in others. It is these particular babies who develop clinical vitamin B6 deficiency when the intake is low. These findings seem to indicate strongly that some babies have vitamin B6 requirements 3 or 4 times as high as others. 

The human requirement for vitamin K appears to be very small but mild clotting disorders may result from digestive disturbances with consequent insufficient absorption of vitamin K. 

Two essential enzymatic reactions in humans require vitamin B12 (Figure 33-2). In one, methylcobalamin serves as an intermediate in the transfer of a methyl group from /V5-methyltetrahydrofolate to homocysteine, forming methionine (Figure 33-2A Figure 33-3, section 1). Without vitamin B12, conversion of the major dietary and storage folate, N5-... 

The structure of vitamin K is characterized by methylnaphthoquinone rings with a side chain at position 3. It exists naturally in two forms phylloquinone (vitamin Kt 6.13) occurs only in plants, while menaquinones (vitamin K2 6.14) are a family of compounds with a side chain consisting of between 1 and 14 isoprene units. Menaquinones are synthesized only by bacteria (which inhabit the human gastrointestinal tract and thus provide some of the vitamin K required by the body). Menadione (vitamin K3 6.15) is a synthetic compound with vitamin K activity. Unlike Kj and K2, menadione is water soluble and is not active until it is alkylated in vivo. 

Vitamins are chemically unrelated organic compounds that cannot be synthesized by humans and, therefore, must must be supplied by the diet. Nine vitamins (folic acid, cobalamin, ascorbic acid, pyridoxine, thiamine, niacin, riboflavin, biotin, and pantothenic acid) are classified as water-soluble, whereas four vitamins (vitamins A, D, K, and E) are termed fat-soluble (Figure 28.1). Vitamins are required to perform specific cellular functions, for example, many of the water-soluble vitamins are precursors of coenzymes for the enzymes of intermediary metabolism. In contrast to the water-soluble vitamins, only one fat soluble vitamin (vitamin K) has a coenzyme function. These vitamins are released, absorbed, and transported with the fat of the diet. They are not readily excreted in the urine, and significant quantities are stored in Die liver and adipose tissue. In fact, consumption of vitamins A and D in exoess of the recommended dietary allowances can lead to accumulation of toxic quantities of these compounds. 

The antiscurvy (antiscorbutic) activity was called vitamin C, and when its structure became known it was called ascorbic acid. The fat-soluble factor preventing rickets was designated vitamin D. By 1922, it was recognized that another fat-soluble factor, vitamin E, is essential for full-term pregnancy in the rat. In the early 1930s vitamin K and the essential fatty acids were added to the list of fat-soluble vitamins. Study of the human blood disorders "tropical macrocytic anemia" and "pernicious anemia" led to recognition of two more water-soluble vitamins, folic acid and vitamin B12. The latter is required in minute amounts and was not isolated until 1948. Have all the vitamins been discovered Rats can be reared on an almost completely synthetic diet. However, there is the possibility that for good health humans require some as yet undiscovered compounds in our diet. Furthermore, it is quite likely that we receive some essential nutrients that we cannot synthesize from bacteria in our intestinal tracts. An example may be the pyrroloquinoline quinone (PQQ).e... 

The requirements of animals for D vitamins in terms of actual weight are extremely small, It is estimated that human beings need about 400 international units of vitamin D per day. Since an international unit of vitamin D corresponds to 0.025 microgram of crystalline vitamin D, this means that the daily human requirement is about 0.01 milligram. Foods can contain as little as 0.02 parts per million of vitamin D and yet furnish an ample supply on the basis of the foregoing estimate,... 

M Levine, KR Dhariwal, RW Welch, Y Wang, JB Park. Determination of optimal vitamin C requirements in humans. Am J Clin Nutr 62 1347S-1356S, 1995. 

P Weber, A Bendich, W Schalch. Vitamin C and human health—a review of recent data relevant to human requirements. Int J Vit Nutr Res 66 19-30, 1996. 

Dietary fat is also important as a source of the fat-soluble vitamins, A, D, E, and K. Requirements for the fat-soluble vitamins are several orders of magnitude lower than that of the essential fatty acids, being measured in micrograms rather than grams per day. Since the fat-soluble vitamins are required for growth and development, much interest has been focused on the concentration and availability of these vitamins in human milk. 

Human milk contains about 2 xg/L of vitamin K. Vitamin K denotes a group of compounds containing the 2-methyl-1,4-naphthoquinone chemical moiety. Phylloquinone is the plant form of the vitamin and is the most prevalent homologue in milk. The vitamin is required for the biosynthesis of prothrombin and many other essential blood-clotting factors (Jensen, 1992). 

Cobalt is an integral part of the only metal containing vitamin B12. The level of cobalt in foods varies widely, from as little as 0.01 ppm in com and cereals to 1 ppm in some legumes. The human requirement is very small and deficiencies do not occur. 

Vitamin A levels are frequently expressed in International Units (IU), although this unit is officially no longer accepted. One IU equals 0.344 pg of crystalline vitamin A acetate, or 0.300 pg vitamin A alcohol, or 0.600 pg P-carotene. The recommended daily allowance (RDA) of vitamin A of the National Research Council Food and Nutrition Board is 5000 IU for an adult. Other sources quote the human requirement at about 1 pg/day. Conditions of rapid growth, pregnancy, or lactation increase the need for vitamin A. 

The unit of activity of vitamin D is the IU, which is equivalent to the activity of 1 mg of a standard preparation issued by the WHO. One IU is also equivalent to the activity of 0.025 fig of pure crystalline vitamin D2 or D3. The human requirement amounts to 400... 

Humans and guinea pigs are the only primates unable to synthesize vitamin C. The human requirement of vitamin C is not well defined. Figures ranging from 45 to 75 mg/ day have been listed as daily needs. Continued stress and drug therapy may increase the need for this vitamin. 

Very few direct studies have been performed to determine human vitamin A requirements. In the Sheffield study (Hume and Krebs, 1949), 16 subjects were depleted of vitamin A for 2 years only three subjects showed clear signs of impaired dark adaptation. One of these subjects was repleted with 390 /rg of retinol per day, which resulted in a gradual restoration of dark adaptation the other two subjects received /3-carotene. On this basis, the minimum requirement was presumed to be 390 /rg, and the reference intake was set at 750 /rg. 

There is a school of thought that human requirements for vitamin C are considerably higher than the reference intakes discussed previously. Pauling (1970) measured the vitamin C intake of gorillas in captivity, assumed that this was the same as their intake in the wild (where they eat considerably less fruit than under zoo conditions), and then assumed that because they had this intake, it was their requirement - an unjustified assumption. Scaling this to humans, he suggested a requirement of 1 to 2 g per day. He also quoted the rate of... 

Hume E and Krehs H (1949) Vitamin A Requirements of Human Adults. Report of the Vitamin A Sub-committee of the Accessory Foods Factors Committee. London Medical Research Council, His Majesty s Stationery Office. 

Medical Research Council Vitamin C Sub-committee ofthe Accessory Food Factor Committee (1948) Vitamin C requirements of human adults. Lancet 1, 835-58. 

Sauberlich HE (1975) Human requirements and needs. Vitamin C status methods and findings. Annals of the New York Academy of Sciences 258,438-50. 

Whereas tetrahydrobiopterin is biosynthesized from GTP via just three enzyme-catalyzed steps (2), some coenzyme biosynthetic pathways are characterized by enormous complexity. Thus, the biosynthesis of vitamin B12 requires five enzymes for the biosynthesis of the precursor uroporhyrinogen III (16) from succinyl-CoA (10) and glycine (11) that is then converted into vitamin B12 via the sequential action of about 20 enzymes (3). Additional enzymes are involved in the synthesis of the building blocks aminopropanol and dimethylbenzimidazole (4, 5). Vitamin B12 from nutritional sources must then be converted to coenzyme B12 by mammalian enzymes. Ultimately, however, coenzyme B12 is used in humans by only two enzymes, albeit of vital importance, which are involved in fatty acid and amino acid metabolism (6). Notably, because plants do not generate corrinoids, animals depend on bacteria for their supply of vitamin B12 (which may be obtained in recycled form via nutrients such as milk and meat) (7). 

Human requirements for vitamin parallel protein metaboHsm, with increased intake of protein requiring higher intake of the vitamin. As Htde as 0.010—0.015 mg of vitamin per gram of protein prevents deficiency signs with adequate protein intake. Slightly higher intake maintains acceptable... 

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