Evaluation of the adequacy of an individual's usual intake of a nutrient has been discussed. Since food fortification is now commonplace and supplement intake is also on the rise, it is important to evaluate whether an individual's usual intake of a nutrient might be excessive. To decide whether an individual has chronic consumption of a nutrient at levels that may increase the risk of adverse effects, the usual nutrient intake is compared to the Tolerable Upper Intake Level (UL) established for the nutrient.
Because usual intakes are unobservable, the uncertainty of how well observed mean intake estimates usual intake must be accounted for, similar to comparing intake to the Adequate Intake (AI) as discussed in the previous section. In this case, however, the z-statistic is constructed by subtracting the UL from the observed mean intake, and dividing the difference by the SD of daily intake over the square root of the number of days of intake available for the individual.
z = (observed mean intake − UL) / (SD of daily intake/ )
The resulting z-statistic is compared to tabulated values (Table B-6), and the confidence level associated with the conclusion that the usual intake is below the UL is obtained. If the resulting confidence level is at least as high as the desired level, then it can be concluded that the individual's usual intake of the nutrient is below the UL and thus a tolerable level of intake for the individual. If the resulting confidence level is not as high as the desired level, then it cannot be conclusively stated that intake is risk free.
Caution also applies in this case. The z-test performs well when daily intakes are approximately normally distributed, but may give incorrect confidence levels when the distribution of daily intakes departs from the normal. The SD of daily intake should accurately reflect the day-to-day variability in intakes for the individual. In the absence of better information about individual SD of daily intake, it is recommended that the pooled estimate of the SD of intake computed from a large nationwide food consumption survey be used. Use of this pooled estimate of the SD of daily intakes is not ideal for the individual, but a reliable alternative is not available at this time.
In the case of regular supplement users, an overestimate of the individual day-to-day variability of intakes may result. If the day-to-day variability for a supplement user were smaller, then the z-statistic obtained from the assessment would be an underestimate.