Medical brain notes 47

  DIETARY REFERENCE STANDARDS   The Food and Nutrition Board of the Institute of Medicine (IOM) has been developing reference stan-dards for vitamins and other nutrients called Dietary. Reference Intakes (DRIs). In the past, the recommended dietary allowances (RDAs), which are the levels of in-take of essential nutrients that are considered to be ade-quate to meet the known nutritional needs of practically all healthy persons, were the primary reference value for vitamins and other nutrients. The DRIs also include other reference values, such as the estimated average re-quirement (EAR) and the adequate intake (AI). The RDA, EAR, and AI reference standards define nutri-tional intake adequacy. Since these recommendations are given for healthy populations in general and not for indi-viduals, special problems, such as premature birth, inher-ited metabolic disorders, infections, chronic disease, and use of medications, are not covered by the requirements. Separate RDAs have been de...

  DEFICIENCY DISEASES   Medical personnel who work in affluent areas are un-likely to see large numbers of people with vitamin defi-ciency diseases. However, certain groups of the popula-tion are particularly at risk, such as low-income families and chronically ill patients. The classic symptoms of any vitamin deficiency disease as observed in laboratory an-imals are often blurred in humans. The clinical picture is often complicated by deficiencies of other vitamins, min-erals, calories, and protein and by infections and parasite infestations, which usually accompany longstanding mal-nutrition. Biochemical, physiological, and behavioral changes can occur in the marginal deficiency state with-out or before the appearance of more specific symptoms. Since the nonspecificity of these changes makes them difficult to detail, this section focuses on the symptoms associated with individual vitamin deficiency diseases.

  VITAMIN TOXICITY     Toxic effects have been observed when large dosages of some vitamins are ingested. Generally the water-soluble vitamins are less toxic, since excess quantities are usu-ally excreted in the urine. Excessive amounts of fat-soluble vitamins, however, are stored in the body, which makes toxic levels of these vitamins easier to obtain.

  PHYSIOLOGICAL FUNCTION AND DIETARY SOURCES   Vitamins are usually classified as either fat soluble (vi-tamins A, D, E, and K) or water soluble (vitamins B and C). The fat-soluble vitamins are generally metabolized slowly and are stored in the liver. In contrast, the water-soluble vitamins are rapidly metabolized and are read-ily excreted in the urine.   Fat-Soluble Vitamins Vitamin A   Vitamin A, or retinol, is essential for the proper main-tenance of the functional and structural integrity of ep-ithelial cells, and it plays a major role in epithelial differentiation. Bone development and growth in children have also been linked to adequate vitamin A intake. Vitamin A, when reduced to the aldehyde 11- cis- retinal, combines with opsin to produce the visual pigment rhodopsin. This pigment is present in the rods of the retina and is partly responsible for the process of dark adaptation.   Principal dietary sources of vitamin A are milk fat (cheese and butter) an...

  THERAPEUTIC USES   All of the vitamins are used as specific treatments for their respective deficiency diseases. The dosages re-quired will vary depending on the severity of the dis-ease and the vitamin. Vitamins have also been used like drugs to “treat” diseases. However, unlike drug prod-ucts, vitamins are not reviewed by the U. S. Food and Drug Administration before formulations appear on the market. Vitamins are considered to be dietary sup-plements under the Dietary Supplement Health and Education Act (DSHEA). Vitamins and other dietary supplements are not permitted to be marketed as a treatment or cure for a specific disease or condition un-less the vitamin is approved as a drug for that purpose. However, under DSHEA, supplement manufacturers may make health claims, such as the link between a food substance and a disease or health-related condition. This may make it difficult for patients to assess the need for vitamin supplementation.   Legitimate clinical resear...

  VITAMIN–DRUG INTERACTIONS   Drug interactions and the adverse effects that can re-sult are a special concern. Although vitamins are not al-ways thought of as being drugs, these nutrients can in-teract with drugs and result in a variety of effects. Vitamin–drug interactions can produce either a de-crease or an increase in the effectiveness of the drug; conversely, the intake of drugs can affect the disposi-tion of vitamins in the body. Many drugs, such as some laxatives and cholestyramine, can produce vitamin mal-absorption or fecal nutritional loss, resulting in drug-in-duced nutrient depletion and hypovitaminosis. Both fat-soluble and water-soluble vitamins can be affected by drug intake.     Vitamin A   Vitamin A absorption from the small intestine requires dietary fat and pancreatic lipase to break down retinyl esters and bile salts to promote the uptake of retinol and carotene. Drugs, such as mineral oil, neomycin and cholestyramine, that can modify lipid ...

ANEMIA   Anemia occurs when the hemoglobin concentration of blood is reduced below normal levels. This condition may result from chronic blood loss, abnormal hemolysis, or nu-tritional deficiency. Many therapeutic agents can induce this change in hemoglobin as an unwanted side effect. Different classifications of anemia are based in part on the pathophysiological factor inducing the decreased hemoglobin concentration. Anemias due to cell hy-poproliferation include aplastic anemia and iron defi-ciency anemia. Hemolytic anemia results from exces-sive destruction of red blood cells. Megaloblastic anemia, sideroblastic anemia, and iron deficiency ane-mia result from an abnormality in the maturation of red blood cells.     Iron Deficiency Anemia   Iron is a constituent of hemoglobin, and iron deficiency will lead to a decrease in hemoglobin synthesis. Since iron is conserved by the body, deficiency usually results from acute or chronic loss of blood or insufficient iron i...