BETWEEN A ROCK AND A HARD PLACE - METHODS FOR SETTING DIETARY-ALLOWANCES AND EXPOSURE LIMITS FOR ESSENTIAL MINERALS

For each essential trace element, there are two ranges of intake assoc iated with adverse health effects: intakes that are too low and can le ad to nutritional deficits and intakes that are too high and can lead to toxicity, Between these two ranges, there is a range of safe and ad equate intakes that is compatible with good health; the challenge is t o define that range quantitatively. The exposure limit for toxicity tr aditionally has been determined by dividing a ''no-observed-adverse-ef fect level,'' often from studies in experimental animals, by a series of uncertainty factors. Dietary allowances to meet nutritional require ments, on the other hand, have been based primarily on data in humans, Uncertainty, variability and limitations in the data base tend to inc rease estimates of nutritional requirements and decrease estimated tox ic exposure limits, driving the values closer together. Chromium (III) and zinc are contrasting cases. Other factors, such as bioavailabilit y, selection of the critical effect and interactions also can play imp ortant roles, Close coordination between the nutrition and toxicology communities, as envisioned in the NRC Food and Nutrition Board initiat ive on Dietary Reference Intakes, is essential in establishing accepta ble ranges of intake for trace elements.