. "14 A Research Agenda." Dietary Reference Intakes for Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin, and Choline. Washington, DC: The National Academies Press, 1998.
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DRI Dietary Reference Intakes: For Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin, and Choline
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IMPORTANT FEATURES OF STUDIES TO ESTIMATE REQUIREMENTS
Derivation of an Estimated Average Requirement (EAR) involves identification of the criterion for a particular status indicator or combination of indicators that is consistent with impaired status as defined by some clinical consequence. For many of the B vitamins, there is a dearth of information on the biochemical values that reflect abnormal function. One priority should be determination of the relationship of existing status indicators to clinical endpoints to allow their use for setting EARs. For some nutrients new clinical endpoints of impaired function need to be identified and related to status indicators.
The depletion-repletion research paradigms that are often used in studies of requirements, although not ideal, are still probably the best approach to determining vitamin requirements. However, these studies should be designed to meet three important criteria:
An indicator of vitamin status is needed for which a cutoff point has been identified, below (or above) which vitamin status is documented to be impaired. (In the case of folate, an erythrocyte level of 300 nmol/L [140 ng/mL] fits this criterion because lower levels are associated with megaloblastic changes in blood cells. In the case of vitamin B6 and several other B vitamins, however, there is little information relating levels of status indicators to functional sufficiency or insufficiency. Instead, the levels of indicators normally used to assess requirements are those exhibited by subjects on a baseline adequate diet—even though there is no information regarding whether this level of intake is greatly in excess of adequate, barely adequate, or deficient.) The amount needed for restoration of biochemical status indicators to baseline values is not necessarily equivalent to the requirement for the nutrient.
The depletion and repletion periods should be sufficiently long to allow a new steady state to be reached. This can be very problematic because turnover rates of total body content for B vitamins range from less than 1 to about 3 percent per day, which suggests