Metabolic processes must respond to the immediate needs of the body and therefore vitamin requirements are subject to continuous variation between certain limits. The Food and Nutrition Board of the Institute of Medicine in the United States defines a requirement as the lowest continuing intake level of a nutrient that, for a specific indicator of adequacy, will maintain a defined level of nutriture in an individual. A Recommended Dietary Allowance (RDA) of a nutrient is the average daily dietary intake level that is sufficient to meet the requirement of nearly all (97 to 98 per cent) apparently healthy individuals in a particular life stage and gender group. The RDA is derived from an Estimated Average Requirement (EAR), which is an estimate of the intake at which the risk of inadequacy to an individual is 50 per cent. RDAs have been published for vitamins A, D, E and K, thiamin, riboflavin, niacin, vitamin B6, folate, vitamin B and vitamin C (National Research Council, 1989). In the case of pan-tothenic acid and biotin, there is insufficient evidence to calculate an EAR and a reference intake called an Adequate Intake (AI) is provided instead of an RDA (Institute of Medicine, 1998). The AI is a value based on experimentally derived intake levels or approximations of observed mean nutrient intakes by a group (or groups) of apparently healthy people.
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