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Approximately 40 nutrients are required by the human body. Nutrients are essential if they cannot be synthesized by the body and if a deficiency causes recognizable abnormalities that disappear when the deficit is corrected. Required nutrients include the essential amino acids, water-soluble vitamins, fat-soluble vitamins, minerals, and the essential fatty acids. The body also requires an adequate energy substrate, a small amount of metabolizable carbohydrate, indigestible carbohydrate (fiber), additional nitrogen, and water.

Nutritional requirements have been most commonly expressed by recommended dietary allowances (RDAs). Published and periodically reviewed by the Food and Nutrition Board of the National Academy of Sciences, the RDAs were initially designed to meet the known nutritional needs of practically all healthy persons. RDAs have been established for carbohydrate and protein; the water-soluble vitamins thiamine, riboflavin, niacin, vitamin B6, folate, vitamin B12, and vitamin C; the fat-soluble vitamins A, D, and E; and the minerals copper, phosphorus, magnesium, iron, zinc, iodine, molybdenum, and selenium.

The Food and Nutrition Board has since developed a broader approach to defining nutritional adequacy. Known as dietary reference intakes (DRIs), these guidelines go beyond the prevention of classic nutritional deficiency diseases and address the role of nutrients and other food components in long-term health and the reduction of risk of chronic diseases. The DRIs consist of four reference intakes: the RDA, the estimated average requirement (EAR), the tolerable upper intake level (UL), and the adequate intake (AI). The RDA remains the dietary intake that is sufficient to meet the nutritional requirements of nearly all individuals in an age- and gender-specific group. RDAs are intended as goals for individuals. The EAR is the intake value that is estimated to meet the requirements of 50% of individuals in an age- and gender-specific group. The UL is the maximum level of daily nutrient intake that is unlikely to pose health risks to most individuals. The AI is determined when insufficient data are available to establish the EAR and RDA for a given nutrient. It is based on fewer data and more expert opinion but is also intended as a goal for individuals. The DRIs for vitamins, elements, and macronutrients are shown in eTables 29–1, 29–2 and 29–3.

eTable 29–1.Recommended dietary reference intakes of vitamins for individuals.1

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