Data from Lexi-Comp Inc., Hudson, Ohio, 2004; table from Kronel S, Mascarenhas: Vitamin deficiencies and excesses. In Burg FD, Ingelfinger JR, Polin RA, Gershon AA, editors: Current Pediatric Therapy, Philadelphia, 2006, Elsevier, Table 3, pp 104–105. bid, Two times per day; CF, cystic fibrosis; IM, intramuscular; IV, intravenous; LBW, low birth weight; PO, by mouth; q, every; SC, subcutaneous; tid, three times per day; WHO, World Health Organization.
A variety of chemical forms of folate are nutritionally active. Folate high requirement of the vitamin to support erythropoiesis. functions in transport of single-carbon fragments in synthesis of Other conditions with risk of deficiency include pregnancy, nucleic acids and for normal metabolism of certain amino acids alcoholism, and treatment with anticonvulsants (phenytand in conversion of homocysteine to methionine. Food sources oin) or antimetabolites (methotrexate). First occurrence and include green leafy vegetables, oranges, and whole grains; folate recurrence of neural tube defects are reduced significantly fortification of grains is now routine in the United States. by maternal supplementation during embryogenesis. Because
Folate deficiency, characterized by hypersegmented neu-closure of the neural tube occurs before usual recognition of trophils, macrocytic anemia, and glossitis, may result from pregnancy, all women of reproductive age are recommended a low dietary intake, malabsorption, or vitamin-drug inter-to have a folate intake of at least 400 μg/day as prophylaxis. actions. Deficiency can develop within a few weeks of birth because infants require 10 times as much folate as adults rela-Vitamin B12 tive to body weight but have scant stores of folate in the newborn period. Folate is particularly heat labile. Heat-sterilizing
Decision-Making Algorithms home-prepared formula can decrease the folate content by Available @ StudentConsult.com half. Evaporated milk and goat’s milk are low in folate. Patients with chronic hemolysis (sickle cell anemia, thalassemia) may require extra folate to avoid deficiency because of the relatively Vitamin B12 is one of the most complex of the vitaminmolecules, containing an atom of cobalt held in a corrin ring (similar to that of iron in hemoglobin). The cobalt ionis at the active center of the ring and serves as the site forattachment of alkyl groups during their transfer. The vitamin functions in single-carbon transfers and is intimatelyrelated to folate function and interconversions. Vitamin B12 is essential for normal lipid and carbohydrate metabolism inenergy production and in protein biosynthesis and nucleicacid synthesis.
In contrast to other water-soluble vitamins, absorption of vitamin B12 is complex, involving cleavage of the vitamin from dietary protein and binding to a glycoprotein called intrinsic factor, which is secreted by the gastric mucosa (parietal cells). The cobalamin–intrinsic factor complex is efficiently absorbed from the distal ileum.
As vitamin B12 is absorbed into the portal circulation, it is transported bound to a specific protein, transcobalamin II. Its large stores in the liver also are unusual for a water-soluble vitamin. Efficient enterohepatic circulation normally protects from deficiency for months to years. Dietary sources of the vitamin are animal products only. Strict vegetarians should take a vitamin B12 supplement.