Is there a metabolic basis for dietary supplementation?

To be efficacious, dietary supplements must either provide a nutrient that is normally undersupplied to cells or exert a pharmacologic effect on cellu lar processes. In the first case, optimal function is achieved when a nutri ent required by the organism reaches a specific concentration within the ce ll. A supplement has benefit only when the normal intake of a bioavailable form of a nutrient is lower than the amount that would provide maximum bene fit as judged from all biological perspectives. Metabolic, environmental, a nd genetic factors can make individual nutrient requirements differ from th e estimated needs calculated from population-based data. For example, under certain circumstances intracellular antioxidants may be depleted and a die tary supplement might restore optimal antioxidant protection. In the second case, the dietary supplement contains a constituent that is normally not r equired by the cell, but this substance is capable of altering normal cell function. For example, herbal preparations may contain ephedrine (a drug), which might alter heart rate so that the amount of blood pumped by the hear t is enhanced. An understanding of how the variation in nutrient requiremen ts comes about and of the pharmacologic actions of nutrient supplements can help to identify which individuals are most likely to benefit from dietary supplements.