Is There a Metabolic Basis for Dietary Supplements?

Dietary supplements that are efficacious must either provide a nutrient that is normally undersupplied to cells or exert a pharmacologic effect on cellular processes. In the first case, a nutrient is required by the organism, and a specific concentration of this nutrient results in optimal cell function. For a dietary supplement to exert a positive effect, normal availability of the supplied nutrient must be suboptimal. For example, a vitamin supplement increases low intracellular vitamin E levels so that optimal antioxidant protection is present in the cell. In the second case, the dietary supplement contains a constituent which is normally not required by the cell, but is capable of altering normal cell function. For example, foxglove contains the drug digitalis, which alters cardiac muscle function so that the force generated by a muscle contraction is enhanced. This talk focuses on the first example, in which a nutrient in a supplement corrects suboptimal nutrient concentrations.

The recommended daily intake of a nutrient is calculated based on the needs of the entire population. My colleagues and I recommend an amount that should permit optimal cellular function for most normal people. Because genetic, developmental, environmental, and pathological conditions differ, the actual nutrient requirements for individuals vary greatly from the recommended dietary intake. In most cases the recommendation exceeds needs, but it some cases it falls short of requirements. For example, exercise results in the production of oxygen derivatives that damage cells, and thereby create an increased demand for dietary antioxidants. Aged individuals produce less vitamin D in their skin and may benefit from supplemental vitamin D. Individuals with a genetic predisposition to overproduce free radicals might benefit from an antioxidant supplement. Complex interactions between nutrients in the diet also alter the requirement for a nutrient in a given individual. For example, a component of an earlier meal might activate metabolic pathways (such as p450 enzymes) in liver that destroy certain nutrients, thereby increasing the amount of the needed nutrient that must be ingested. An understanding of how the variation in nutrient requirements arises can help to identify which individuals are most likely to benefit from dietary supplements.