"Although we cannot rule out benefits of supplements, such as improved quality of life, our study raises a concern regarding their long-term safety ... it is not advisable to make a causal statement of excess risk based on these observational data; however, it is noteworthy that dietary supplements, unlike drugs, do not require rigorous (randomized controlled trial) testing, and observational studies are often the best available method for assessing the safety of long-term use," they wrote.
Therefore, based on existing evidence, they argue that there is little justification for the general and widespread use of dietary supplements.
"We recommend that they be used with strong medically based cause, such as symptomatic nutrient deficiency disease," they concluded.
"Until recently, the available data regarding the adverse effects of dietary supplements has been limited and grossly underreported."
This "large, well-designed, and well-conducted" trial adds to the existing evidence that certain supplements can be harmful, and the findings concur with those of other recent observational studies, Dr. Goran Bjelakovic and Dr. Christian Gluud said in an editorial accompanying the report (Arch. Intern. Med. 2011;171:1633-4).
"The belief that antioxidant supplements are beneficial seems likely to have resulted from a collective error. Perhaps oxidative stress is one of the keys to extension of our life span," they suggested.
Indeed, the shift in the use of dietary supplementation for prevention of deficiency to use for promoting wellness and prevention of diseases – with many consumers believing that supplements are safe for use without the supervision of their physician – is problematic.
"Until recently, the available data regarding the adverse effects of dietary supplements has been limited and grossly underreported. We think the paradigm ‘the more the better’ is wrong," they said.
Rather, there are likely risks with both insufficient and too-large intake, added Dr. Bjelakovic and Dr. Gluud, both of whom are with the Copenhagen trial unit, center for clinical intervention research, Cochrane Hepato-Biliary Group, Rigshospitalet, Copenhagen, University Hospital.
"Therefore, we believe that politicians and regulatory authorities should wake up to their responsibility to allow only safe products on the market," they said, also noting that in a well-nourished population, the use of vitamin and mineral supplements as a preventive measure should not be recommended.
A possible exception based on these and other findings is with vitamin D3 supplementation, particularly when there is insufficient vitamin D supply from the sun and diet; this may apply to older women, and perhaps older men. The matter of calcium supplementation may require further study, they said.
This study was partially supported by a grant from the National Cancer Institute and by grants from the Academy of Finland, the Finnish Cultural Foundation, and the Fulbright program. One of the study authors (David R. Jacobs Jr., Ph.D.) reported being an unpaid member of the scientific advisory board of the California Walnut Commission. None of the other authors had relevant financial disclosures to report. Neither Dr. Bjelakovic nor Dr. Gluud had relevant financial disclosures to report.