Chapter 18. Dietary Reference Intakes for Vitamins

18 Dietary Reference Intakes for Vitamins Suzanne P Murphy and Susan I Barr CONTENTS Introduction 559 Estimated Average Requirement 559 Recommended Dietary Allowance 560 Adequate Intake 561 Tolerable Upper Intake Level 561 Dietary Reference Intakes for Vitamins 562 Appropriate Uses of Vitamin Dietary Reference Intakes 565 Conclusions 569 References 570 INTRODUCTION Dietary reference intakes (DRIs) are nutrient reference standards to be used for planning and assessing diets of apparently healthy individuals and groups DRIs were developed by the United States and Canada to update, expand on, and replace the former recommended nutrient intakes for Canadians and recommended dietary allowances (RDA) for Americans The process was overseen by the Standing Committee on the Scientific Evaluation of Dietary Reference Intakes of the Food and Nutrition Board, Institute of Medicine, The National Academies, in collaboration with Health Canada Instead of releasing a report that covered all nutrients in a single volume, as was done previously, a series of reports on groups of related nutrients was released, reflecting the work of nutrient panels composed of Canadian and American scientists (1–6) The first nutrient report was released in 1997 and covered calcium, phosphorus, magnesium, vitamin D, and fluoride (1); the final nutrient report was released in 2004 and covered electrolytes and water (6) Reports were also published on using a risk assessment model to establish upper levels (ULs) (7) and on the use of DRIs in dietary assessment and planning (8,9) There are several types of DRIs and each has a specific definition Figure 18.1 shows how these DRIs relate to one another ESTIMATED AVERAGE REQUIREMENT The estimated average requirement (EAR) is defined as ‘‘the daily intake value that is estimated to meet the requirement, as defined by the specified indicator of adequacy, in half the apparently healthy individuals in a life stage or gender group’’ (6) Several aspects of this definition warrant further elaboration: ß 2006 by Taylor & Francis Group, LLC EAR RDA UL 0.5 1.0 0.5 Risk of adverse effects Risk of inadequacy 1.0 Observed level of intake FIGURE 18.1 Dietary reference intakes This figure shows that the estimated average requirement (EAR) is the intake at which the risk of inadequacy is 0.5 (50%) to an individual The recommended dietary allowance (RDA) is the intake at which the risk of inadequacy is very small—only 2%–3% The adequate intake (AI) does not bear a consistent relationship to the EAR or the RDA because it is set without the ability to estimate the requirement (however, it is thought to meet the needs of most individuals) At intakes between the RDA (or AI) and the tolerable upper intake level (UL), the risks of inadequacy and of excess are both close to zero At intakes above the UL, the risk of adverse effects may increase (although the intake level at which this may occur is not known with precision) (Adapted from Food and Nutrition Board, Institute of Medicine, Dietary Reference Intakes: Applications in Dietary Assessment, National Academies Press, Washington, DC, 2000 With permission.) Daily intake value: All DRIs are expressed as amounts per day; however, they are more appropriately considered as average intakes over a longer period of time (e.g., weeks or months) Requirement: A requirement is defined as ‘‘the lowest continuing value of a nutrient that, for a specified indicator of adequacy, will maintain a defined level of nutriture in an individual’’ (6) The specified indicator of adequacy is identified for each nutrient, although in some cases it may differ among different age groups The indicators of adequacy used for the vitamins are described in the next section (Dietary Reference Intakes for Vitamins) Half the apparently healthy individuals: Although the word ‘‘average’’ is used in the EAR, its definition implies a median value rather than an average The EAR is expected to meet or exceed the requirements of 50% of healthy individuals in a life stage or gender group, and to fall below the requirements of the other 50% The median and the average remain the same when the requirement distribution is symmetrical, which is assumed to be the case for all of the vitamins RECOMMENDED DIETARY ALLOWANCE The RDA is defined as ‘‘the average daily intake level that is sufficient to meet the nutrient requirement of nearly all (97%–98%) apparently healthy individuals in a particular life stage and gender group’’ (6) The primary use of the RDA is as a daily intake goal for individuals: at this level of intake, there is a very high probability (97%–98%) that the given individuals will meet or exceed their requirements This is in contrast to the EAR, which would be expected to fall below the requirements of 50% of individuals The RDA for vitamin intakes is set based on the EAR plus twice the standard deviation (SD) of the EAR: RDA ¼ EAR þ 2SD As the requirements are thought to be normally distributed, intake at the RDA will be sufficient for 97%–98% of individuals If sufficient ß 2006 by Taylor & Francis Group, LLC data are not available to calculate the SD, a coefficient of variation (CV; SD=EAR) of 10% is frequently used in place of the SD This is based on the variability of other biological variables In this case, the RDA is set as the EAR and twice the CV of 10%: RDA ¼ EAR þ (0.1 Â EAR) ¼ 1.2 Â EAR In some cases, when there is evidence of greater variability (but still insufficient data to accurately identify the SD), a larger CV was assumed For example, for vitamin A the CV was assumed to be 20%; thus, the RDA ¼ 1.4 Â EAR For niacin, the CV was 15%, so that the RDA is 1.3 Â EAR ADEQUATE INTAKE For some nutrients, sufficient scientific evidence was not available to establish an EAR, and in these situations, adequate intakes (AIs) were set instead The AI is defined as ‘‘the recommended average daily intake value based on observed or experimentally determined approximations or estimates of nutrient intake by a group (or groups) of apparently healthy people who are assumed to be adequate—used when an RDA cannot be determined’’ (6) In the case of vitamins, AIs were established for vitamin D, vitamin K, pantothenic acid, biotin, and choline As a recommended intake, the AI is expected to meet or exceed the amount needed to maintain a defined nutritional state or criterion of adequacy in almost all members of an apparently healthy population In other words, it is likely that the AI would be at or above the RDA if it had been possible to determine the requirement distribution and set a value for the RDA This is particularly likely to be the case if an AI was based on average intakes of free-living individuals For example, AIs for infants aged 0–6 months were set for all nutrients (except vitamin D) as the average intake by full-term infants born to presumably healthy, well-nourished mothers and exclusively fed human milk Under these conditions, infants grow well and it is therefore assumed that their intake from human milk meets or exceeds their requirements The AI is similar to the RDA in that both are recommended intake levels for individuals, expected to meet or exceed amounts needed to maintain a specified indicator of adequacy in almost all individuals However, there is much less certainty about AIs than RDAs, and the presence of an AI is an indication that additional research is required Eventually, it is hoped that additional knowledge of nutrient requirements will allow AIs to be replaced by EARs and RDAs TOLERABLE UPPER INTAKE LEVEL The UL is the ‘‘highest level of daily nutrient intake that is likely to pose no risk of adverse health effects in almost all apparently healthy individuals in the specified life stage group As intake increases above the UL, the potential risk of adverse effects increases’’ (6) Although the UL is thought to represent an intake that the body can biologically tolerate, it is not a recommended intake There are no established benefits to healthy individuals of intakes that exceed the RDA or AI It is important to note that the UL is intended to apply to chronic consumption rather than to intakes on any given day, and that it does not apply to individuals who are treated while under medical supervision For example, the UL for niacin for adults is 35 mg=day, an amount which may be exceeded by individuals who are treated for hypercholesterolemia (2) However, in this situation the individual can be monitored by their physician for adverse effects The ULs for nutrients are based on evaluations conducted using a risk assessment framework An important feature of this process is the concept that adverse effects of nutrients are not expected until intake exceeds a threshold Just as requirements for nutrients vary among individuals, it appears that the thresholds for adverse effects also vary An intake that might be tolerated by one individual could result in adverse effects in another The intent is to set the UL so that it is below the threshold of even the most sensitive members of a group ß 2006 by Taylor & Francis Group, LLC At present, ULs have not been set for all vitamins: specifically, vitamin K, thiamin, riboflavin, vitamin B12 , biotin, and pantothenic acid not have ULs This does not mean that these vitamins are safe in unlimited quantities In most cases, the data are not sufficient to identify the potential risk of adverse effects DIETARY REFERENCE INTAKES FOR VITAMINS The first DRIs were released in 1997 (1), and included those for vitamin D Additional vitamin DRIs were published through 2001 (2–4) Table 18.1 shows the DRIs that apply to vitamins, and gives the values for nonpregnant, nonlactating adults The complete sets of DRI values, including those for infants, children, and adolescents, may be found at http:==www.iom TABLE 18.1 Vitamin DRIs for Males and Females 19 Years of Age and Older EAR Vitamin A (mg RAE=day)a Vitamin D (mg=day) Vitamin K (mg=day) Vitamin E (mg=day) Vitamin C (mg=day) Thiamin (mg=day) Riboflavin (mg=day) Niacin (mg NE=day)f Vitamin B6 (mg=day) Vitamin B12 (mg=day) Folate (mg DFE=day)i Pantothenic acid (mg=day) Biotin (mg=day) Choline (mg=day) j 500=625b RDA AI 700=900b 5=10=15d 90=120b 12 60=75b 0.9=1.0b 0.9=1.1b 11=12b 1.1=1.3=1.4g 2.0 320 15 75=90b 1.1=1.2b 1.1=1.3b 14=16b 1.3=1.5=1.7g 2.4h 400 30 425=550b UL 3000c 50 — 1000e 2000 — — 35e 100 — 1000e — — 3500 Sources: Food and Nutrition Board, Institute of Medicine, Dietary Reference Intakes for Calcium, Phosphorus, Magnesium, Vitamin D and Fluoride, National Academies Press, Washington, DC, 1997; Food and Nutrition Board, Institute of Medicine, Dietary Reference Intakes for Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin, and Choline, National Academies Press, Washington, DC, 1998; Food and Nutrition Board, Institute of Medicine, Dietary Reference Intakes for Vitamin C, Vitamin E, Selenium and Carotenoids, National Academies Press, Washington, DC, 2000; Food and Nutrition Board, Institute of Medicine, Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc, National Academies Press, Washington, DC, 2001 a RAE ¼ Retinol activity equivalent For women=men c Applies to intake of preformed vitamin A (retinol) only d For ages 19–50=51–70=>70 years; amount needed in the absence of adequate exposure to sunlight e Applies to synthetic forms in supplements and fortified foods f NE ¼ Niacin equivalent g For ages 19–50=women >50=men >50 h Adults over 50 are advised to meet the RDA mainly by consuming foods fortified with B12 or a supplement containing B12 i DFE ¼ Dietary folate equivalent j Although AIs have been set for choline, there are few data to assess whether a dietary supply of choline is needed at all stages of the life cycle, and it may be that the choline requirement can be met by endogenous synthesis at some of these stages b ß 2006 by Taylor & Francis Group, LLC edu=file.asp?id ¼ 21372 Note that most nutrients have several DRIs (e.g., vitamin C has an EAR, an RDA, and a UL); thus it is inappropriate to refer to ‘‘the DRI’’ for a nutrient For many of the vitamins, values for children and adolescents are extrapolated from those for adults For infants