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Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc
UNA, 1985). If data are not available for estimation of a standard deviation, then a CV of 10 percent is assumed depending on the information that is available.
Method for Setting the RDA when Nutrient Requirements Are Not Normally Distributed
When factorial modeling is used to estimate the distribution of requirements from the distributions of the individual components of requirement (e.g., losses, accretion), it is necessary to add the individual distributions. For normal component distributions, this is straightforward since the resultant distribution is also normal, with a mean that is the sum of component means and a variance (the square of the SD) that is the sum of the individual variances. The ninety-seven and one-half percentile is then estimated as the mean value plus two SDs.
If the requirement of a nutrient is not normally distributed but can be transformed to normality, its EAR and RDA can be estimated by transforming the data, calculating a fiftieth and a ninety-seventh and one-half percentile, and transforming these percentiles back into the original units. In this case, the difference between the EAR and the RDA cannot be used to obtain an estimate of the CV because skewing is usually present.
If normality cannot be assumed for all of the components of requirement, then Monte Carlo simulation is used for the summation of the components. This approach involves simulation of a large population of individuals (e.g., 100,000) each with his or her own requirement for a particular nutrient. To accomplish this, the component parts of nutrient needs (the factorial components) are treated as coming from independent random distributions.
Using iron as an example (see Chapter 9), for basal iron loss, a distribution of expected losses was generated. For each individual in the simulated population, a randomly selected iron loss value was drawn from that distribution of iron losses. This is done for each component of iron need and then these components were summed for each individual yielding the simulated iron needs. The total requirement is then calculated for each individual and the median and the ninety-seven and one-half percentile calculated directly.
Information about the distribution of values for the requirement components is modeled on the basis of known physiology. Monte Carlo approaches may be used in the simulation of the distribution of components; or, where large data sets exist for similar populations (such as growth rates in infants), estimates of relative variability