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BOX 3-1 Development of Tolerable Upper Intake Levels (ULs)


  • Evidence of adverse effects in humans

  • Causality

  • Relevance of experimental data

  • Pharmacokinetic and metabolic data

  • Mechanisms of toxic action

  • Quality and completeness of the database

  • Identification of distinct and highly sensitive subpopulations


  • Data selection and identification of critical endpoints

  • Identification of no-observed-adverse-effect level (NOAEL) (or lowest-observed-adverse-effect level [LOAEL]) and critical endpoint

  • Assessment of uncertainty and data on variability in response

  • Derivation of a UL

  • Characterization of the estimate and special considerations

Key issues that are addressed in the data evaluation of human and animal studies are described below (see Box 3-1).

Evidence of Adverse Effects in Humans

The hazard identification step involves the examination of human, animal, and in vitro published evidence addressing the likelihood of a nutrient’s eliciting an adverse effect in humans. Decisions about which observed effects are adverse are based on scientific judgments. Although toxicologists generally regard any demonstrable structural or functional alteration as representing an adverse effect, some alterations may be considered to be of little or self-limiting biological importance. As noted earlier, adverse nutrient-nutrient interactions are considered in the definition of an adverse effect.


The identification of a hazard is strengthened by evidence of causality. As explained in Chapter 2, the criteria of Hill (1971) are considered in judging the causal significance of an exposure-effect association indicated by epidemiological studies.

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