magnitude and duration of the intake. Furthermore, the critical data set documents the NOAEL (or LOAEL).
A nutrient can produce more than one toxic effect (or endpoint), even within the same species or in studies using the same or different exposure durations. The NOAELs and LOAELs for these effects will ordinarily differ. The critical endpoint used to establish a UL is the adverse biological effect exhibiting the lowest NOAEL (e.g., the most sensitive indicator of a nutrient’s toxicity). Because the selection of uncertainty factors (UFs) depends in part upon the seriousness of the adverse effect, it is possible that lower ULs may result from the use of the most serious (rather than most sensitive) endpoint. Thus, it is often necessary to evaluate several endpoints independently to determine which leads to the lowest UL.
For some nutrients, such as vitamin K, arsenic, chromium, and silicon, there may be inadequate data on which to develop a UL. The lack of reports of adverse effects following excess intake of a nutrient does not mean that adverse effects do not occur. As the intake of any nutrient increases, a point (see Figure 3-2) is reached at which intake begins to pose a risk. Above this point, increased