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The Recommended Dietary Allowance (RDA) of vitamin B-6 for young women was recently reduced from 1.6 to 1.3 mg/d based on an adequate plasma pyridoxal phosphate (PLP) concentration of 20 nmol/L. To assess vitamin B-6 requirements and suggest recommendations for intake, seven healthy young women consumed a controlled diet providing 1.2 g protein/kg body weight for a 7-d adjustment period (1.0 mg vitamin B-6/d) and three successive 14-d experimental periods (1.5, 2.1 and 2.7 mg/d, respectively). Direct and indirect vitamin B-6 status indicators were measured in plasma, erythrocytes and urine. Indicators most strongly correlated with vitamin B-6 intake [i.e., plasma and erythrocyte PLP, urinary 4-pyridoxic acid (4-PA) and total vitamin B-6] were regressed on vitamin B-6 intake and the dietary vitamin B-6 to protein ratio. Inverse prediction using adequate and baseline values estimated vitamin B-6 requirement. Adequate values were determined for plasma PLP and urinary 4-PA from baseline values of 60 previous subjects, using the statistical method suggested by Sauberlich. The current study suggests a vitamin B-6 Estimated Average Requirement (EAR) for young women of 1.1 mg/d or 0.016 mg/g protein, and a RDA of 1.5 mg/d or 0.020 mg/g protein. When results from this study are combined with data from four other recent studies, the combined data predict an EAR of 1.2 mg/d or 0.015 mg/g protein, and a RDA of 1.7 mg/d or 0.018 mg/g protein. This study suggests that the current vitamin B-6 RDA may not be adequate.
The Recommended Dietary Allowances serve two basic uses: As guidelines for planning diets and food supplies and as a tool for evaluating nutritional adequacy of food consumed. Establishing standards to serve these functions is not simple. Differences in individuals in different population groups dictate allowances with relatively high margins of safety to prevent deficiencies. Even so, anomalies arise. Surveys have shown that intakes of vitamin A and protein are likely to exceed their respective allowances. On the other hand, calcium and thiamin intakes of older women and iron in teenage boys and women in the reproductive years are difficult to meet in terms of nutrient density ratios. In addition, insufficient data make it impossible to set allowances for some essential trace elements--and, other essential elements may yet be discovered. Thus the allowances are not guaranteed to represent the totality of nutritional needs.
Determinants of the recommended dietary allowance (RDA) for vitamin C include the relationship between vitamin C dose and steady-state plasma concentration, bioavailability, urinary excretion, cell concentration, and potential adverse effects. Because current data are inadequate, an in-hospital depletion-repletion study was conducted. Seven healthy volunteers were hospitalized for 4-6 months and consumed a diet containing <5 mg of vitamin C daily. Steady-state plasma and tissue concentrations were determined at seven daily doses of vitamin C from 30 to 2500 mg. Vitamin C steady-state plasma concentrations as a function of dose displayed sigmoid kinetics. The steep portion of the curve occurred between the 30- and 100-mg daily dose, the current RDA of 60 mg daily was on the lower third of the curve, the first dose beyond the sigmoid portion of the curve was 200 mg daily, and complete plasma saturation occurred at 1000 mg daily. Neutrophils, monocytes, and lymphocytes saturated at 100 mg daily and contained concentrations at least 14-fold higher than plasma. Bioavailability was complete for 200 mg of vitamin C as a single dose. No vitamin C was excreted in urine of six of seven volunteers until the 100-mg dose. At single doses of 500 mg and higher, bioavailability declined and the absorbed amount was excreted. Oxalate and urate excretion were elevated at 1000 mg of vitamin C daily compared to lower doses. Based on these data and Institute of Medicine criteria, the current RDA of 60 mg daily should be increased to 200 mg daily, which can be obtained from fruits and vegetables. Safe doses of vitamin C are less than 1000 mg daily, and vitamin C daily doses above 400 mg have no evident value.
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The general trend toward increased consumption of polyunsaturated fatty acids is apparent in the linoleate level of adipose tissue (13.0 plus or minus 1.3%) and erythrocyte lipids (14.0 plus or minus 1.9%) in the present group of female undergraduate student volunteers compared to values reported in the early 1960's. On the basis of the level of linoleate in their diets (19.5 plus or minus 0.8%), it is also apparent that further increases in tissue lipid linoleate levels are to be anticipated, which in turn will result in an increased requirement for vitamin E. It is suggested that adipose tissue linoleate levels in the general population be used as a baseline for the periodic evaluation and revision of the recommended dietary allowance for vitamin E. The recommended dietary allowance could then be phrased in terms of the quantity of vitamin E activity to be consumed per gram linoleate in 100 g adipose tissue fatty acids. A recommendation of 0.6 IU vitamin E activity/g linoleate in 100 g adipose tissue fatty acids is tentatively suggested.
This review has approached zinc requirements, bioavailabilities and recommended dietary allowances from a historical view. For example, a requirement for zinc was first demonstrated for the microorganism Aspergillus niger more than a century ago, although zinc has been recognized as a dietary nutrient for humans only within the last decade. Dietary requirement was defined as that quantity of zinc which must be provided daily in order to meet the metabolic requirement. The degree of bioavailability determines the total dietary zinc which must be consumed by humans to remain in metabolic equilibrium. In regard to recommended dietary allowances (RDA), they are recommended average daily intakes of nutrients that population groups should consume. RDA are not synonymous with requirements. Metabolic requirements (the quantity that must be provided to the circulating blood daily) have been determined previously using radio-isotope studies and have indicated a calculated need of approximately 6 mg for an adult per day. More recently a similar amount (4-6 mg) has been found to be required daily (intravenously) in order to maintain plasma zinc concentrations and daily urinary zinc excretion within normal range in hospitalized subjects. Average zinc intakes of large segments of the U.S. population are receiving levels approaching one-half or less the RDA with no apparent deleterious effects. These subjects include pregnant and lactating women. In addition, the zinc content of breast milk is lower than previous studies indicated. Thus, it appears that the majority of breast-fed infants are receiving no more than 70% of the recommended intake, with a recent study indicating less than 50%. Factors reported to affect bioavailability of zinc from foods are discussed. These include fiber and phytate. It was concluded that no definite conclusion can be reached regarding the overall effect of food fiber on zinc balance. Many of the studies were of short duration, with the longest being 32 days. It is probable that different sources of food fiber may have different effects on zinc balance. The apparent discrepancy in the literature regarding the effect of soy protein on zinc requirement was cited. Lastly, the phytate:zinc molar ratio concept was designed to test it as a predictor of zinc bioavailability to humans. The concept must be expanded to recognize the relevance of the total daily dietary zinc intake. Specifically, impairment of absorption of zinc by phytate would be more critical if the total dietary intake was low since the metabolic requirements might not be met.(ABSTRACT TRUNCATED AT 400 WORDS)
This article analyzes the 1989 Recommended Dietary Allowance (RDAs) for folate, which are approximately 50% less than the amounts recommended previously. The RDA committee used the following types of data to estimate allowances: (a) the quantity of folate required to elicit established physiologic responses or replace daily losses corrected for bioavailability and individual variability and (b) dietary folate intake data related to prevalence of deficiency in population groups. Some studies cited by the committee evaluated the response to synthetic folate, whereas other studies used dietary folate to estimate folate requirements. Responses to these two forms of folate may differ and depend on the individual's state of folate depletion. The correction for bioavailability is an estimate based on limited data determined by various experimental approaches. Estimates of the folate content of food vary as a function of a number of factors in the method of analysis and the specific type of food consumed. Additional variables include food preparation, food losses, and nutrient interactions. For these reasons, estimates of folate intake based on computer databases may not accurately reflect folate consumed or physiologically/metabolically available folate. Assumptions that current dietary intakes of folate are sufficient to maintain status in the US population were based, in some cases, on data from population surveys not designed to assess folate status. The new RDAs for folate may not provide an adequate safety allowance for specific population groups at risk and should be reevaluated as new information evolves.
The recently released Recommended Dietary Allowance of vitamin C for women, 75 mg daily, was based on data for men. We now report results of a depletion-repletion study with healthy young women hospitalized for 186 +/- 28 days, using vitamin C doses of 30-2,500 mg daily. The relationship between dose and steady-state plasma concentration was sigmoidal. Only doses above 100 mg were beyond the linear portion of the curve. Plasma and circulating cells saturated at 400 mg daily, with urinary elimination of higher doses. Biomarkers of endogenous oxidant stress, plasma and urine F(2)-isoprostanes, and urine levels of a major metabolite of F(2)-isoprostanes were unchanged by vitamin C at all doses, suggesting this vitamin does not alter endogenous lipid peroxidation in healthy young women. By using Food and Nutrition Board guidelines, the data indicate that the Recommended Dietary Allowance for young women should be increased to 90 mg daily.
In Japan the recommended dietary allowances (RDAs) of vitamin A for adult male is calculated at 600 micrograms RE, on the data in a human vitamin A depletion-repletion study that minimum requirement for maintenance of a adequate retinol storage at liver is 6.6 micrograms/kg body weight/day. By contrast, it is estimated at 1,000 micrograms RE in U.S., on the basis of a study that minimum requirement for prevention of follicular hyperkeratosis, not impaired dark adaptation, is 910 micrograms RE/day in most adult men. On fourth edition of Japanese RDAs in 1989, the estimated safe and adequate daily dietary intake of vitamin E for adult male was established at 8 mg alpha TE, on the data that the average of unsaturated fatty acid intake was 14.3 g/day and a desirable [vitamin E (mg) to PUFA (g)] ratio of 0.4, whereas the 8-10th editions of U.S. RDAs of vitamin E for adult male had been estimated at 10 mg alpha TE. And the RDAs of other fat-soluble vitamins were discussed on the differences between Japan and other countries.
This is the first in a series of articles reviewing the recent revisions of the Recommended Dietary Allowances (RDA) and the resulting Dietary Reference Intakes (DRI). In the United States our nutrient guidelines have had far reaching applications to human health and agricultural practices. The broad use of the previous edition of the Recommended Dietary Allowances has necessitated a complete reevaluation of the criteria, uses and reporting of nutrient guidelines. The resulting documents, a series of reports collectively referred to as the Dietary Reference Intakes, are being released over a number of years, beginning in 1997 and expected to be completed by the year 2003.
The current recommended dietary allowance (RDA) for vitamin C, as proposed by the Food and Nutrition Board/National Research Council in 1980 and reconfirmed in 1989, is 60 mg daily for nonsmoking adult males. Levine et al. [Levine, M., Conry-Cantilena, C., Wang, Y., Welch, R. W., Washko, P. W., et al. (1996) Proc. Natl. Acad. Sci. USA 93, 3704-3709], based on a study of vitamin C pharmacokinetics in seven healthy men, have now proposed that the RDA should be increased to 200 mg daily. I have examined, in brief, the experimental and conceptual bases for this new recommendation and its implications for public health and nutrition policy and programs. Using, for illustrative purposes only, data extracted from each of two recent dietary surveys of noninstitutionalized adult males living in households in the Netherlands and the United States, it is predicted that the prevalence of intakes inadequate to meet the individual's own requirement would be about 96% or 84%, respectively, if the criteria of adequacy used for derivation of the 200 mg RDA are accepted. Depending upon the particular average requirement value for ascorbic acid that might be derived from their data, the proposal by Levine et al. would mean a desirable increase in mean intakes in these two populations by as much about 2-to 3-fold. Hence, before an action of this kind is to be recommended, an answer must be sought to the question whether current experimental data including the criteria selected (saturation kinetics) are adequate to establish a new set of requirements for vitamin C, which then carry such profound policy implications. This will require critical assessment of all of the available evidence emerging from laboratory, clinical, and epidemiological studies to determine whether it provides a sufficient rationale for accepting criteria of vitamin C adequacy such as those proposed by Levine et al. and the requirement estimates so derived.
The 10th edition of Recommended Dietary Allowances (RDA) did not include an RDA for copper; rather a safe and adequate daily intake was suggested. Criteria, history and uses of RDAs were summarized along with data on dietary intakes, balance and depletion experiments, low (fats and oils, skim milk and yogurt) and high (legumes, mushrooms, nuts and seeds) copper foods and hazards of zinc supplements. Bone disease and cardiovascular disease from diets-low in copper have been studied in animals for decades. Men and women fed diets close to 1 mg of copper per day, amounts quite frequent in the US, responded similarly to deficient animals with reversible, potentially harmful changes in blood pressure control, cholesterol and glucose metabolism, and electrocardiograms. Women supplemented with trace elements including copper experienced beneficial effects on bone density. These data exceed similar data on magnesium, selenium and zinc and are sufficient for establishing an RDA. Ischemic heart disease and osteoporosis are likely consequences of diets low in copper. Numerous anatomical, chemical and physiological similarities between animals deficient in copper and people with ischemic heart disease have been noticed. Association between osteoporosis and low copper status deserves further inquiry. Augmenting low copper diets with high copper foods may be beneficial. Committees that establish RDAs should return to the traditions of the first nine editions and make recommendations that promote health and nutritional welfare, meet functional needs, prevent disease and promote public welfare.
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The current recommended dietary allowance (RDA) for vitamin C for adult nonsmoking men and women is 60 mg/d, which is based on a mean requirement of 46 mg/d to prevent the deficiency disease scurvy. However, recent scientific evidence indicates that an increased intake of vitamin C is associated with a reduced risk of chronic diseases such as cancer, cardiovascular disease, and cataract, probably through antioxidant mechanisms. It is likely that the amount of vitamin C required to prevent scurvy is not sufficient to optimally protect against these diseases. Because the RDA is defined as "the average daily dietary intake level that is sufficient to meet the nutrient requirement of nearly all healthy individuals in a group," it is appropriate to reevaluate the RDA for vitamin C. Therefore, we reviewed the biochemical, clinical, and epidemiologic evidence to date for a role of vitamin C in chronic disease prevention. The totality of the reviewed data suggests that an intake of 90-100 mg vitamin C/d is required for optimum reduction of chronic disease risk in nonsmoking men and women. This amount is about twice the amount on which the current RDA for vitamin C is based, suggesting a new RDA of 120 mg vitamin C/d.
In view of the important role that nutrient intake assessments play in establishing the Recommended Dietary Allowances (RDAs), the quality of food composition data must be assured for accuracy and representativeness. Assurance of data quality requires the definition of critical parameters in the data generation process and the evaluation of specific data for foods and components according to these parameters. An expert systems approach for evaluating the quality of analytical data has been developed by scientists at the Beltsville Human Nutrition Research Center to determine the quality of food composition data for five parameters: sampling plan, sample handling, number of samples, analytical method and analytical quality control. A rating scale for each parameter was developed with 0 representing poor or inadequately documented data and 3 representing optimal data. Specific criteria for each parameter and rating have been developed and incorporated into expert systems software to facilitate the objective assignment of ratings for each data source by the reviewer. After all ratings for a specific food-nutrient combination are assigned, the system calculates a composite score called the "confidence code" which indicates to the user the relative level of confidence in the data. By identifying and rating the important steps in the data generation process, one can begin to partition the possible sources of error or variability in the process. Limitation of the data set relative to specific purposes (e.g., setting the RDAs) can be identified. The evaluation process can provide the basis for focussed research to improve the most critical areas of the data generation process. A similar process could be established to evaluate the quality of analytical data for clinical measurements used to establish the RDAs.