I'd missed this journal editorial from last month on the topic of the current scientific consensus on the utility (or rather lack of utility) of dietary supplements. The evidence presently strongly favors the view that for people who have no vitamin deficiencies adding more dietary supplements does nothing or may even harm long term health - such as when dietary antioxidants block the hormetic processes necessary to benefit from exercise. The supplement industry is somewhat louder than the scientific community, however.
Reviews and guidelines that have appraised the role of vitamin and mineral supplements in primary or secondary prevention of chronic disease have consistently found null results or possible harms. Evidence involving tens of thousands of people randomly assigned in many clinical trials shows that β-carotene, vitamin E, and possibly high doses of vitamin A supplements increase mortality and that other antioxidants, folic acid and B vitamins, and multivitamin supplements have no clear benefit.
The large body of accumulated evidence has important public health and clinical implications. Evidence is sufficient to advise against routine supplementation, and we should translate null and negative findings into action. The message is simple: Most supplements do not prevent chronic disease or death, their use is not justified, and they should be avoided. This message is especially true for the general population with no clear evidence of micronutrient deficiencies, who represent most supplement users in the United States and in other countries.
The evidence also has implications for research. Antioxidants, folic acid, and B vitamins are harmful or ineffective for chronic disease prevention, and further large prevention trials are no longer justified. Vitamin D supplementation, however, is an open area of investigation, particularly in deficient persons. Clinical trials have been equivocal and sometimes contradictory.
With respect to multivitamins, [studies] and previous trials indicate no substantial health benefit. This evidence, combined with biological considerations, suggests that any effect, either beneficial or harmful, is probably small. As we learned from voluminous trial data on vitamin E, however, clinical trials are not well-suited to identify very small effects, and future trials of multivitamins for chronic disease prevention in well-nourished populations are likely to be futile. Although available evidence does not rule out small benefits or harms or large benefits or harms in a small subgroup of the population, we believe that the case is closed - supplementing the diet of well-nourished adults with (most) mineral or vitamin supplements has no clear benefit and might even be harmful. These vitamins should not be used for chronic disease prevention. Enough is enough.