The following is an excerpt from an article in Annals of Internal Medicine summarizing the findings of many clinical trials on vitamin supplementation that followed, in total, hundreds of thousands of people over many years.  Science indicates that the large majority of people should eat healthy food and not waste their money on useless or potentially harmful vitamin supplements:

First . . . [a]after reviewing 3 trials of mutivitamin supplements and 24 trials of single or paired vitamins that randomly assigned more than 400,000 participants, the authors concluded that there was no clear evidence of a beneficial effect of supplements on all-cause mortality, cardiovascular disease, or cancer. . . . Second . . . [concerning] evaluat[ion] [of] the efficacy of a daily multivitamin to prevent cognitive decline among 5,947 men aged 65 years or older . . . [a]fter 12 years of followup, there were no differences between the multivitamin and placebo groups in overall cognitive performance or verbal memory. . . . [T]he large sample size resulted in precise estimates showing that use of a multivitamin supplement in a well-nourished elderly population did not prevent cognitive decline. . . . [The] findings are compatible with a recent review of 12 fair-to good-quality trials that evaluated dietary supplements, including multivitamins, B vitamins, vitamins E and C, and omega-3 fatty acids, in persons with mild cognitive impairment or mild to moderate dementia. None of the supplements improved cognitive function. . . .

Other 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 show that ß-carotene, vitamin E, and possibly high dose vitamin A supplements increase mortality and that other antioxidents, folic acid, and B vitamins, and multivitamin supplements have no clear benefit.

Despite sobering evidence of no benefit or possible harm, use of multivitamin supplements increased among U. S. adults from 30% between 1988 and 1994 to 39% between 2003 and 2006, while overall use of dietary supplements increased from 42% to 53%. Longitudinal and secular trends show a steady increase in multivitamin supplement use and a decline in use of some individual supplements, such as ß-carotene and vitamin E. The decline in use of  ß-carotene and vitamin E supplements followed reports of adverse outcomes in lung cancer and all-cause mortality, respectively. In contrast, sales of multivitamins and other supplements have not been affected by major studies with null results, and the U. S. supplements industry continues to grow, reaching $28 billion in annual sales in 2010. Similar trends have been observed in the United Kingdom and in other European countries.

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 other countries.

The evidence also has implications for research. Antioxidents, folic acid, and B vitamins are harmful or ineffective for chronic disease prevention, and further large prevention trials are no longer justified. . . . [C]urrent widespread use of . . . vitamin D supplementation . . . is not based on solid evidence that benefits outweigh harms.

With respect to multivitamins, the studies published in this issue and previous trials indicate no substantial health benefit. The 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.

In conclusion,  ß-carotene, vitamin E, and possibly high doses of vitamin A supplements are harmful. Other antioxidants, folic acid and B vitamins, and multivitamin and mineral suppments are ineffective for preventing mortality due to major chronic diseases. (Excerpted from Eliseo Guallar, Saverio Stranges, Cynthia Mulrow, Lawrence J. Appel, Edgar R. Miller, III; “Enough Is Enough: Stop Wasting Money on Vitamin and Mineral Supplements.” Annals of Internal Medicine. 2013 Dec;159(12):850-851.)

Watch out for quacks and vitamin pushers.