When this does occur it may be due to composition of the soil, extremely unusual food choices, or errors during food fortification.
The use of nutrient supplements is growing in the United States and Canada, with reports from the Third National Health and Nutrition Examination Survey (NHANES III) suggesting that half the population is using nutritional supplements. Although this information is not sufficiently quantitative for estimations of population intakes, it is known that in some population subgroups nutrient intakes exceed the UL. Supplements should not be treated casually even though excessive intakes appear to be harmless because they are excreted or do not incur a toxic response. It is important to remember that the ULs are based on chronic exposures. The amounts of a nutrient considered toxic upon acute exposure are generally considerably higher than the UL, but have not been established for many nutrients.
How serious is the risk of adverse effects for individuals chronically consuming nutrients at levels greater than the Tolerable Upper Intake Level (UL)?
The critical adverse effects used to set the UL are listed in Table 6-1. The dose, the seriousness of the adverse effects, and the extent to which the effects are reversible upon intake reduction should be considered in evaluating the risk of adverse effects.
If the mean intake of a population equals the UL, is there no risk?
A population mean intake at the UL suggests that a large proportion (as much as half) of the population is consuming levels above the UL. This would represent a very serious population risk of adverse effects.
How different are the ULs from doses that would confer acute toxicity?
The ULs are the maximum levels that can be consumed daily on a chronic basis without adverse effects. The ULs will generally be much lower than the levels that are necessary to produce adverse effects after a single exposure. Few evaluations of the acute toxic intake of nutrients have been made.