Hilmas criticizes meta-analysis method; welcomes opportunity to review finalized manuscript
WASHINGTON, D.C. – The Natural Products Association (NPA) is the leading representative of the dietary supplement and natural products industry. NPA Senior Vice President of Scientific and Regulatory Affairs Corey Hilmas, M.D., Ph.D., former Chief of the Dietary Supplement Regulation Implementation Branch within the Division of Dietary Supplement Programs at the Food and Drug Administration (FDA), comments on research that claims to link dietary supplement intake with an increased risk of cancer:
“Just as the name implies, dietary supplements are designed to ‘supplement’ the diet because consumers do not eat enough of the critical phytochemicals and constituents found in fresh fruits, vegetables and other foods, including fish, on a daily basis. Dietary supplements should be taken as part of a healthy lifestyle and after consulting with one’s health care provider.
“While some view meta-analyses as a sort of ‘platinum standard’, meta-analyses introduce another set of research bias on top of those biases already present in the study included as part of the meta-analysis. Meta-analyses should be performed in a way that limits the influence of subjective biases and idiosyncrasies of particular researchers. In this case, Dr. Tim Byers selected 12 trials but failed to include other studies that may have evaluated negative outcomes in a long-term prospective study. Meta-analyses include only a small fraction of available evidence and it is really at the researcher’s discretion – and therefore bias – as to which studies he or she decides to include in the analysis. We question why Dr. Byers cherry-picked these specific 12 studies and whether or not there was other research available that could have resulted in a different outcome. We simply cannot draw accurate conclusions from this sample of studies without understanding which were chosen and why.
“What’s more, multiple meta-analyses of the same primary evidence often reach contradictory conclusions regarding the same hypothesis. Meta-analysis relies on a narrow range of evidential diversity and so should be used as only one type of evidence appealed to when amalgamating the entire available data set against a hypothesis. A better study to monitor such negative outcomes and eliminate bias is to follow a long-term cohort in a prospective study. Prospective studies eliminate many of the sources of bias and confounding factors that are inherent in retrospective studies. Prospective studies will follow a cohort of subjects over a long period and evaluate their risk for a particular outcome over time. Since cancer is thought to take years to develop, it would hold true that a long-term cohort prospective study following supplement intake over many years would be the gold standard to demonstrate any association between supplement intake and cancer risk.
“Finally, we must ask, why haven’t Dr. Byers’ previous studies over the years suggested that there is a positive association between dietary supplement/vitamin/mineral intake and cancer? Besides testing whether there is a beneficial effect, it would seem that his studies would be powered sufficiently to monitor for any negative associations as well, yet we don’t see any negative associations.
“The Natural Products Association welcomes the opportunity to fully review Dr. Byers’ research once it is published and made available to the public. Consumers should continue to remain confident that their dietary supplements are safe and effective and can contribute to one’s overall health and wellness.”
