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tions. For the B vitamins the EAR is some what higher than the amount needed to prevent deficiency disease (allowing a moderate safety margin) and there is laboratory evidence of sufficiency, but there is no observable health benefit beyond the prevention of signs and symptoms of deficiency. However, a special recommendation is included to address reduction of the risk of neural tube defects. The indicators used in deriving the RDAs and AIs are described below.

Nutrient Functions and the Indicators Used to Estimate the Requirements for the B Vitamins

Thiamin functions as a coenzyme in the metabolism of carbohydrates and branched-chain amino acids. Estimations of the requirement are based on the amount of thiamin needed to achieve and maintain normal erythrocyte transketolase activity while avoiding excessive thiamin excretion.

Riboflavin functions as a coenzyme in numerous oxidation-reduction reactions. Intake in relation to a combination of indicators is used to estimate the requirement for riboflavin. These indicators include the excretion of riboflavin and its metabolites, blood values for riboflavin, and the erythrocyte glutathione reductase activity coefficient.

Niacin functions as a cosubstrate or coenzyme with numerous dehydrogenases for the transfer of the hydride ion. The primary method used to estimate the requirement for niacin relates intake to the urinary excretion of niacin metabolites. The requirement is expressed in niacin equivalents, allowing for some conversion of the amino acid tryptophan to niacin.

Vitamin B6 functions as a coenzyme in the metabolism of amino acids, glycogen, and sphingoid bases. To estimate the requirement, many types of biochemical data were examined; however, when possible, priority was given to the amount of B6 consistent with maintenance of an adequate plasma pyridoxal phosphate concentration.

Folate functions as a coenzyme in single-carbon transfers in the metabolism of nucleic and amino acids. Folate is a generic term used to cover both the naturally occurring form of the vitamin (food folate) and the monoglutamate form (folic acid), which is used in fortified foods and supplements. To estimate the requirement the primary focus for all adults was on the amount of Dietary Folate Equivalents (DFEs) (values adjusted for differences in the absorption of food folate and folic acid) needed to maintain erythrocyte

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