case of nutrients, it is exceedingly important to consider the possibility that the intake of one nutrient may alter in detrimental ways the health benefits conferred by another nutrient. Any such alteration (referred to as an adverse nutrient-nutrient interaction) is considered an adverse health effect. When evidence for such adverse interactions is available, it is considered in establishing a nutrient’s UL.
As is true for all chemical agents, adverse health effects can result if the intake of nutrients from a combination of food, water, nutrient supplements, and pharmacological agents is excessive. Some lower level of nutrient intake will ordinarily pose no likelihood (or risk) of adverse health effects in normal individuals even if the level is above that associated with any benefit. It is not possible to identify a single risk-free intake level for a nutrient that can be applied with certainty to all members of a population. However, it is possible to develop intake levels that are likely to pose no risk of adverse health effects to most members of the general population, including sensitive individuals. For some nutrients these intake levels may still pose a risk to subpopulations with extreme or distinct vulnerabilities.
The development of a mathematical model for deriving the Tolerable Upper Intake Level (UL) was rejected for reasons described elsewhere (IOM, 1997). Instead, the model for the derivation of ULs consists of a set of scientific factors that always should be considered explicitly. The framework under which these factors are organized is called risk assessment. Risk assessment (NRC, 1983, 1994) is a systematic means of evaluating the probability of occurrence of adverse health effects in humans from excess exposure to an environmental agent (in this case, a nutrient) (FAO/WHO, 1995; Health Canada, 1993). The hallmark of risk assessment is the requirement to be explicit in all the evaluations and judgments that must be made to document conclusions.
Risk assessment is a scientific undertaking having as its objective a characterization of the nature and likelihood of harm resulting from