Rudman and Feller (18) proposed three criteria for establishing conditional essentiality of nutrients: (a) decline in the plasma level of the nutrient into the subnormal range; (b) appearance of chemical, structural, or functional abnormalities; and (c) correction of both of these by a dietary supplement of the nutrient. All these criteria must be met to establish unequivocally that a nutrient is conditionally essential.
Conditional essentiality represents a qualitative change in requirements, i.e., the need for a nutrient that is ordinarily dispensable. Alterations in the need for an essential nutrient, from whatever cause, and health benefits from consumption of nonnutrients, dispensable nutrients, or essential nutrients in excess of amounts needed for normal physiologic function do not fit this category. Such situations should be dealt with separately.
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