What is known about the safety of multivitamin-multimineral supplements for the generally healthy population? Theoretical basis for harm.

Assessment of the safety of nutrients presents a challenge different from that posed by the assessment of other chemicals in food such as additives or contaminants. Because nutrients are essential, a dose-response relation exists at both ends of the intake range, separated by a safe range of intake that reflects normal homeostatic processes. The safe intake may not be the same for all population groups and life stages. The size of the safe intake range for each nutrient will vary and in a few cases may be very small. Certain nutrients such as vitamin A and manganese have known and potentially serious adverse effects at high intakes, whereas others such as iron or vitamin C may have more minor adverse effects that are readily reversible and may only be associated with supplement intake. The risk of harm occurring from taking dietary supplements will depend on the safe intake range of the nutrient concerned, the susceptibility of the individual, and the likely intake of the same nutrient from other supplements or the rest of the diet. In many cases, the available database for the safety of nutrients is very limited because the studies, where available, were not designed to assess adverse effects but may have detected problems when they occurred. Further information on the safety of nutrients could be obtained through careful experimental design.