The EAR cut-point method works when intakes and requirements are independent (see Figure 4-8) and the number of points in triangles A and B are virtually identical. Thus we have suggested dividing the total amount of B12 in those foods by five (Jones et al., 2007; Heyssel et al., 1966) as a more realistic estimate of how much the diet would actually contribute to the amount of the vitamin absorbed. This is to be expected because one of the features of the method (and also of the method proposed by NRC) is that it partially removes the day-to-day variability in intakes. Carriquiry and colleagues (1997) successfully applied the ISU method to adjust intake distributions and distributions of blood biochemical measurements using data collected in the Third National Health and Nutrition Examination Survey (NHANES III), even though sample sizes for some life stage and gender groups were moderately small (fewer than 70 to 80 individuals) and the proportion of replicate observations was low (approximately 6 percent). Frequently, it is assumed that the distribution of requirements is normal; however for some nutrients, such as iron for menstruating women, this assumption is not warranted (some women have very large menstrual losses of iron, which leads to a distribution that is positively skewed—i.e., more women have higher requirements than indicated by a normal distribution). Figure 4-3 shows an example of a risk curve. Elderly, who are at greater risk of vitamin B12 deficiency because of their difficulty in releasing and absorbing the vitamin from food, are recommended to consume a higher proportion from fortified foods or supplements, from which the vitamin is better absorbed as it is in the free form (Institute of Medicine, 2000). However, because one-half of the group has usual intakes less than the average requirement and one-half has usual intakes exceeding the average requirement, the cut-point method would estimate that 50 percent of the group is at risk of inadequate intakes when, in fact, the prevalence of inadequacy is zero. At the EAR, 50% of the individuals in a group are below their requirement, and 50% are above it. Smokers have lower levels of ascorbic acid in their serum and amniotic fluid. This was one of the few UL that was set using data in rats, because sufficient and appropriate quantitative data assessing long-term adverse effects of vitamin E supplements in humans was not available (Traber, 2007). The variation coefficients for copper and molybdenum requirements were set at 15%. Each approach has specific limitations, and in most large-scale evaluations, speciation, although important, is still overlooked. FIGURE 4-2 Plot of usual intakes and requirements of 3,000 hypothetical individuals in a population. FIGURE 4-1 Plot of usual intakes and requirements of 24 hypothetical individuals in a group. Accordingly, when usual intake and requirement are correlated, using the EAR cut-point method (i.e., determining the number of individuals to the left of the intake = EAR line) would overestimate the number of people with inadequate intakes (those in the shaded area above the 45° line where intake = requirement). estimated average requirement: (EAR) ( es'ti-mā'ted av'răj rĕ-kwīr'mĕnt ) The daily intake of a specific nutrient estimated to meet the requirement in 50% of healthy people in an age- and gender-specific group. This is because it includes both the within-person (day-to-day) variation and the individual-to-individual variation, thus leading to estimates of prevalence of inadequacy or excess that are likely to be higher than the true prevalence. Recall that the variability of intakes usually exceeds the variability in requirements and that the RDA is equal to the EAR plus two standard deviations of requirement.
Regardless of the method chosen to assess prevalence of inadequate nutrient intakes in a group of individuals, information is required about the distribution of intakes of the nutrient in the group. The precise amount of a nutrient that will be adequate for any given individual is therefore unknown. At least in North America, the situation where variation in individual requirements is greater than variation in individual usual intakes is most likely to arise for institutionalized subpopulations—for example, prison inmates or residents of a long-term care facility —who are all fed similar diets. The Food and Nutrition Board has established a basic framework for tackling this question. However, symptoms such as fingernail changes have also been reported for lower Se intakes, so that a threshold of 1260 μg day− 1 is considered as the reference at which clinical selenosis appears, whereas the ULs (corresponding to no adverse effect) usually range between 350 and 450 μg day− 1. FIGURE 4-8 Joint distribution of intakes and requirements from a hypothetical population of 3,000 individuals.
It is generally accepted that there is no health risk from high doses of vitamin B12. Controversies have arisen about some specific nutrients. Women who smoke more than 20 cigarettes per day and regular aspirin users may require twice as much, as may heavy users of alcohol and street drugs. The Recommended Dietary Allowance (RDA) is the average daily dietary intake that is sufficient to meet the nutrient requirement of 97.5% of individuals and is set at 20% above the EAR+2 CVs where the CV is 10%.
These univariate distributions can be combined and the prevalence of inadequate nutrient intakes can be estimated statistically by using the probability approach (NRC, 1986).
It is far preferable to use the EAR cut-point method and the adjusted distribution of usual intakes to estimate the proportion of a group with inadequate intakes. Because of the low abundance of soil Se in some areas around the globe, a wide variety of Se-enriched materials have been produced in order to supply populations with RDA adequate levels of Se. It is with respect to these chronic degenerative diseases that genetic variation of nutrient metabolism is most significant today. The Institute of Medicine concluded that there appear to be essentially no risks of adverse effects to the general population even at high intakes (Institute of Medicine, 2000). For example, at the EAR, 50% of the individuals in a group are below their requirement and 50% are above it. The current intake guidelines take little note of such differences. 1 RAE=1 μg retinol, 12 μg β-carotene, 24 μg α-carotene, or 24 μg β-cryptoxanthin. Human dietary intake is normally in the range of 5–20 μg V/day.28,29 An upper tolerable level (UL) for vanadium has been established at 1.8 mg/day, about 100 times the average intake.4 The amount of vanadium used in diabetes studies is far in excess of the UL, which is allowable for clinical studies with careful safety monitoring.4 Vanadium is less toxic to humans than rodents. Some regions of northern Europe, the United States, and other countries exhibit relatively low levels of soil Se, but only a few extremely deficient regions were documented, mainly located in China, Australia, and Malawi.
A significant weakness of the current recommendations relates to the extremely narrow basis of supporting data for several nutrients. c Data to determine the shape of requirement distributions are lacking for most nutrients; therefore, symmetry is assumed unless there are data adequate to indicate otherwise. Figure 6 represents the range of theoretical dose–response curves used by various institutions to calculate such reference intakes, their hypothetical correspondence to plasma Se level, and possible endpoints for the evaluation of Se status. What proportion of individuals in a group has a usual intake of a nutrient that is less than the requirement for that nutrient? For dietary assessment applications, however, estimates of nutrient requirements and nutrient intakes must be at the same level of aggregation: individual, household, or population. Vitamin C intake data from NHANES III for men and women; using the EAR to determine expected prevalence of inadequacy.
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