Micronutrients play a central role in metabolism and in the maintenance of tissue function, but effects in preventing or treating disease which is not due to micronutrient deficiency cannot be expected from increasing the intake. There is a highly integrated system to control the flux of micronutrients in illness, and this demonstrates just how important the body perceives the micronutrients to be. An adequate intake therefore is necessary to sustain metabolism and tissue function, but provision of excess supplements to individuals who do not need them may be harmful. Clinical benefit is most likely in those individuals who are severely depleted and at risk of complications, and is unlikely if this is not the case. Much more research is needed to characterise better markers of micronutrient status both in terms of metabolic effects and antioxidant effects, so that at-risk patients can be identified and supplementation modified accordingly. Large-scale trials of different doses of micronutrients are required with precise outcome markers to optimise intakes in different groups of patients as well as in the general population.