Safety limits for nutrient intakes: concepts and data requirements

The possible quantitative methods calculating safety limits for nutrient intakes are related conceptually to those used to calculate safe limits for exposure to environmental chemicals and to the therapeutic index used to assess the relative safety of drugs. The impact of using a fixed SF has been compared with the use of variable SFs. Of the methods identified, the SRM gives lower limits than does the MPM. However, neither of these methods calculates safety limits below the RDA, even for nutrients with narrow margins of safety. The acceptability criteria for toxicity data for use in identifying safety limits are an issue of major importance and must be resolved before calculated limits may be used to support policy or regulatory decisions. An advantage of adopting a standard formula involving systematically varying SFs to calculate safety limits is that the margin of safety below the expected range of toxicity for each nutrient would be systematic, without having the safety limit for any nutrient regress below its RDA. Once the data acceptability criteria were met, the safety limit would be identified objectively. The confidence in and reasonableness of safety limits, regardless of the method used to define them, will be enhanced if the objectives, data criteria, and the quantitative method have been agreed upon ahead of time by groups responsible for nutrition and health policy. Even with such agreement, the confidence in using such procedures to support policy decisions will be improved by the extent and quality of the data base on toxicity and adverse reactions associated with consumption of excessive levels of the nutrients under consideration.