For some nutrients (e.g., fluoride, phosphorus, vitamin C) the distribution of usual intake would need to include intake from all sources, while for others (e.g., magnesium, folate, niacin, vitamin E) only the distribution of usual supplement intake would be needed.

Figure 6-1 provides a hypothetical example of the relationship between population median intakes and the risk function for intakes at all levels. It can be seen that the percentage of the population at risk would differ depending on the steepness of the risk function. As noted above, however, the risk function (the dose-response curve) for all nutrients is unknown.

Figure 6-2 illustrates a distribution of usual nutrient intakes in a population; the proportion of the population with usual intakes above the UL represents the potential at-risk group. An evaluation of the public health significance of the risk to the population consuming a nutrient in excess of the UL would be required to determine if action was needed.

If no discernible portion of the population consumes the nutrient in excess of the UL, no public health risk should exist. However, if

FIGURE 6-1 Hypothetical example of risk of adverse effects compared to population intake. The fraction of the population having usual nutrient intakes above the Tolerable Upper Intake Level (UL) is potentially at risk; the probability of adverse effects increases as nutrient intakes increase above the UL, although the true risk function is not known for most nutrients. NOAEL = no-observed-adverse-effect level, LOAEL = lowest-observed-adverse-effect level.