Vitamin B12 Requirements

The toted body pool of viteunin B12 is of the order of 1.8 Mmol (2.5 mg), with a minimum desirable body pool of about 0.3 Mmol (1 mg). 

European Union FAO, Food and Agriculture Organization WHO, World Health Organization. 

Sources Department of Health, 1991 Scientific Committee for Food, 1993 Institute of Medicine 1998  

The daily loss is about 0.1% of the body pool in subjects with normal intrinsic factor secretion turd enterohepatic circulation of the vitamin (Section 

On this basis, the requirement is 0.3 to 1.8 nmol (1 to 2.5 /zg) per day (Herbert, 1987b). This is probably a considerable overestimate of requirements, because ptirenteral administration of less than 0.3 nmol per day is adequate to mcuntcdn normal hematology in patients with pernicious cmemia, in whom the enterohepatic recycling of the vitamin is grossly imptured. 

Requirements tue probably between 0.1 to 1 fig per day as shown in Table 10.4, reference intakes remge between 1 to 2.4 fig per day, which is considerably lower than the average inteike of 5 fig per day by nonvegetarians in most countries. 

---

Odd-chain fatty acids are an exception. While they are relatively rare in the diet, odd-chain-length fatty acids end up at propionyl-CoA (C3). Propionyl-CoA is carboxylated by propionyl-CoA carboxylase to give methylmalonyl-CoA. Methylmalonyl-CoA is rearranged to succinyl-CoA by the enzyme methylmalonyl-CoA mutase, a vitamin-B12-requiring enzyme. 

Ascorbic acid, thiamine, riboflavin, and vitamin B12 requirements increase in hyperthyroidism (issue concentrations reduced Vitamin A massive doses of vitamin A inhibit secretion of TSH thyroid hormones required for carotene and retimene conversions Vitamins A, D, E. and K requirements increased in hyperthyroidism tissue concentrations reduced in Vitamin B, . niacin conversion to phosphorylated reactive forms impaired in hyperthyroidism... 

Figure 28-5. The reaction catalyzed by methionine synthase, a vitamin B12-requiring enzyme. In this reaction, homocystine is converted to methionine, with the simultaneous production of tetrahydrofolate (THF) from 5-methyltetrahydrofolate.Methionine can then be converted to 5-adenosyhnethionine (SAM), the universal methyl-group donor.

Vitamin B12 (cyanocobalamin) is produced by bacteria vitamin B12 generated in the colon, however, is unavailable for absorption. Liver, meat, fish, and milk products are rich sources of the vitamin. The minimal requirement is about 1 pg/day. Enteral absorption of vitamin B12 requires the so-called intrinsic factor from parietal cells of the... 

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). 

A-43 Vitamin B12 is not found in plant foods. The main source of B12 in human diet is through animal products like milk, eggs and liver. Vitamin B12 requires the presence of intrinsic factor from the stomach in order to be absorbed in the small intestines. The liver can store up to six years worth of vitamin B-12, hence deficiencies in this vitamin are rare. 

Cobalt (Co) is essential for humans only as an integral part of vitamin (cobalamin). No other function for cobalt in the human body is loiown. Details of vitamin biochemistry and function are discussed above. Microflora of the human intestine cannot use cobalt to synthesize physiologically active cobalamin. The human vitamin B12 requirement must be supplied by the diet. Free (nonvitamin B12) cobalt does not interact with the body vitamin B12 pool. 

It is believed that the vitamin B12 requirement for choline synthesis contributes to the neurologic symptoms of vitamin B12 deficiency. The methyl groups for choline synthesis are donated by SAM, which is converted to S-adenosylho-mocysteine in the reaction. Recall that formation of SAM through recycling of homocysteine requires both tetrahydrofolate and vitamin B12 (unless extraordinary amounts of methionine are available to bypass the B12-dependent methionine synthase step). 

Every aspea of vitamin B12 amazes It is the active component in the liver that reverses pernicious anemia and it has multiple fimctions in DNA synthesis and intermediary metabolism. Vitamin B12 is synthesized by bacteria (and fungi), is present in soil, and is commonly obtained from meat, milk, eggs, and fish calf liver is a particularly rich source. Bacteria in the human colon synthesize this substance but, ironically, humans cannot absorb their own B12. Ingested vitamin B12 requires adequate intrinsic factor (IF) in gastric juice in order to be absorbed and IF levels tend to decrease with aging. 

Vitamin B12 is synthesized in large quantities by the intestinal flora, particularly in ruminants. The exact amount of vitamin B12 required by the normal human is not known. The absorption of vitamin B12 from the gastrointestinal tract is dependent on the presence of a gastric mucoprotein called intrinsic factor. Calcium ions seem to be necessary for the interaction of vitamin B12 with this intrinsic factor. Vitamin B12, which is absorbed only in the ileum, is stored in the liver. There are two transport proteins for vitamin Bj2 transcobalamin I and II, the latter being physiologically more important. Vitamin B12 plays an important role in the metabolism of functional groups with one carbon atom such as the methyl group... 

In the presence of the antibiotic the amount of vitamin B12 required is often reduced, the antibiotic appearing to exert a sparing action. This does not obviate the need of a dietary source of this factor for normal growth in young animals or for reproduction. Nor does the feeding of vitamin Bi2 and antibiotics at optimal levels appear to alter the need for other members of the B complex. 

---