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Because they dissolve in fat, vitamins A, D, E, and K are called fat-soluble vitamins. They are absorbed from the small intestines, along with dietary fat, which is why fat malabsorption resulting from various diseases (e.g., cystic fibrosis, ulcerative colitis, Crohn's disease) is associated with poor absorption of these vitamins. Fat-soluble vitamins are primarily stored in the liver and adipose tissues. With the exception of vitamin K, fat-soluble vitamins are generally excreted more slowly than water-soluble vitamins, and vitamins A and D can accumulate and cause toxic effects in the body.
Vitamin A was the first fat-soluble vitamin identified (in 1913). Vitamin A comprises the preformed retinoids, plus the precursor forms, the provitamin A carotenoids. Preformed retinoids is a collective term for retinol, retinal, and retinoic acid, all of which are biologically active. The provitamin A carotenoids include beta-carotene and others, which are converted to retinoids with varying degrees of efficiency. Retinoids are sensitive to heat, light, and oxidation by air. Beta-carotene is relatively more stable. Vitamin E helps protect vitamin A from oxidation. There is some loss of vitamin A with cooking, but only after boiling for a comparatively long period.
Retinoids are converted to retinol in the intestines and transported with dietary fat to the liver, where it is stored. A special transport protein, retinolbinding protein (RBP), transports vitamin A from the liver to other tissues. Carotenoids are absorbed intact at a much lower absorption rate than retinol. Of all the carotenoids, beta-carotene has the highest potential vitamin-A activity. The active forms of vitamin A have three basic functions: vision, growth and development of tissues, and immunity.
|Vitamin||Functions||Deficiency symptoms||People at risk||Sources||Daily recommended intakes||Toxicity|
|SOURCE Wardlaw, Gordon M.; Hampl, Jeffrey S.; and Disilvestro, Robert A. (2004). Perspectives in Nutrition, 6th edition. New York: McGraw-Hill.|
|Vitamin A Preformed retinoids and provitamin A carotinoids||Vision in dim light and color vision, cell differentiation and growth, immunity||Poor growth, night blindness, blindness, dry skin, Xerophthalmia||Rare in United States but common in preschool children living in poverty in developing countries, alcoholics||Preformed vitamin A: liver, fortified milk, fish liver oils Provitamin A: red, orange, dark green, and yellow vegetables, orange fruits||Infants: 400-500 mg RAE Children: 300-400 mg RAE Adolescents: 600-900 mg RAE Adult men & women: 700-900 mg RAE Pregnant women: 750-770 mg RAE Lactating women:1200-1300 mg RAE||Headache, vomiting, double vision, hair loss, dry mucous membranes, bone and joint pain, fractures, liver damage, hemorrhage, coma, teratogenic effects: spontaneous abortions, birth defects. Upper level is 3000 mg of preformed vitamin A based on risk of birth defects and liver toxicity.|
|Vitamin D Cholecalciferol Ergocalciferol||Maintainence of intracellular and extracellular calcium concentrations||Rickets in children, osteomalacia in older adults||Dark skinned individuals, older adults, breastfed infants from vitamin D deficient mother||Vitamin D fortified milk, fish oils||0-50 years: 5 mg 51-70 years: 10 mg, >70 years: 15 mg||Calcification of soft tissues, growth restriction, excess calcium excretion via the kidney. Upper level is 50 mg based on the risk elevated blood calcium.|
|Vitamin E Tocopherols Tocotrienols||Antioxidant, prevention of propagation of free radicals||Hemolysis of red blood cells, degeneration of sensory neurons||Patients with fat malabsorption syndromes, smokers [overt deficiency is rare]||Plant oils, seeds, nuts, products made from oils||Infants: 4-5 mg Children: 6-7 mg Adolescents:11-15 mg Adult men & women: 15 mg Pregnant women: 15 mg Lactating women: 19 mg||Inhibition of vitamin K metabolism. Upper level is 1000 mg based on the risk of hemorrhage.|
|Vitamin K Phylloquinone Menaquinone||Synthesis of blood clotting factors and bone proteins||Hemorrhage, fractures||Those taking antibiotics for a long period of time; older adults with scant green vegetable intake||Green vegetables, liver synthesis by intestinal microorganisms||Infants: 2-2.5 mg Children: 30-55 mg Adolescents: 60-75 mg Adult men: 90 mg Adult women: 120 mg Pregnant/lactating women: 75-90 mg||No upper level has been set|
It has been suggested that beta-carotene and other carotenoids (also called phytochemicals) may function as antioxidants by neutralizing free radicals. Free radicals are unstable, highly reactive molecules that damage DNA, cause cell injury, and increase the risk of chronic disease. Beta-carotene has also been associated with reducing the risk of lung cancer. Lutein and zeaxanthin, yellow carotenoid pigments in corn and dark green leafy vegetables, may reduce the risk of macular degeneration and agerelated cataracts. Lycopene, a red carotenoid pigment in tomatoes, may help reduce the risk of prostrate cancer, cardiovascular disease, and skin damage from sunlight.
Author Info: Kiran B. Misra, The Gale Group Inc., Macmillan Reference USA, New York, Gale Nutrition and Well-Being A to Z, 2004
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