"In the population we studied, we found no beneficial effects of either B vitamins or omega-3 fatty acids taken over five years on cancer occurrence or cancer-related death," noted study author Valentina Andreeva, who is with the nutritional epidemiology research unit at the University of Paris XIII in Bobigny, France.
Andreeva and her colleagues report their findings in the Feb. 13 online edition of the Archives of Internal Medicine.
To explore the protective potential of B vitamins and fatty acid supplements, the authors did a secondary analysis of data that had been collected in a previous study involving almost 2,000 French men and 500 women.
All were between 45 and 80 years of age, and all had experienced cardiac trouble (heart attack, unstable angina or ischemic stroke) in the year leading up to the start of the study.
In turn, the participants were divided into one of four different groups that consumed a daily supplement regimen involving various types of vitamin B and omega-3 fatty acids at "relatively low supplementation doses."
By the end of the original five-year study, 7 percent of the participants had gone on to develop some form of cancer, and just over 2 percent ultimately died of cancer. The vast majority of cancer cases (including prostate, lung, bladder and colorectal cancer) and deaths occurred among men (81 percent and 83 percent, respectively).
The team unearthed no evidence that any form of vitamin B or omega-3 fatty acid supplement improved cancer outcomes in any way.
The investigators noted that there were some indications that cancer risk might have actually gone up, specifically among women taking vitamin B and/or omega-3 fatty acid supplementation. However, the authors stressed that this observation was based on too few cases to substantiate a firm conclusion, and called for further research involving a larger pool of participants.
"The results of our study suggest that individuals should exercise caution when deciding to take dietary supplements, especially over a long period of time and without a physician's advice," advised Andreeva. "Such supplements constitute active substances and might have adverse effects in some populations. To be on the safe side, individuals should strive to achieve dietary recommendations via healthy, balanced diets."
Joseph Su, the Washington, D.C.-based program director of the division of cancer control and population science within the U.S. National Cancer Institute's epidemiology and genomics research program, said that nothing about the findings struck him as surprising.
"So far, study findings have been very inconsistent," he noted. "But most supplement studies, if anything, have shown no beneficial effect whatsoever. Just like this one. So, I don't think there's anything that can really back up the idea that these supplements can prevent cancer."
However, Vicky Stevens, strategic director of laboratory services at the American Cancer Society in Atlanta, expressed some reservations about the French analysis.
"Compared with other trials, they used much lower levels of supplements," she noted. "From the B-vitamin point of view, dramatically lower. So, it could be argued that they just weren't using high enough levels of supplements to see any effects," Stevens suggested.
"And they used a natural form of folate [vitamin B supplement], whereas other trials use a synthetic form," Stevens added. "But the real problem in being able to evaluate the effects they do see is that they don't have enough people. And it's not really a long enough follow-up period to really see an effect of these supplements on cancer onset. Five years isn't really enough. It can take 10 or 20 years in most cases. So, what they may be seeing is an effect on preexisting abnormalities, but not the impact on cancer onset itself."
Duffy MacKay, a naturopathic doctor and vice president of scientific and regulatory affairs for the Council for Responsible Nutrition in Washington, D.C., agreed.
"When you look at an intervention like this, you're definitely not looking at the role of the supplements at preventing tumors, because the tumors likely started well before the trial," he noted. "So really what the trial is about is giving vitamin B and omega 3 and seeing if they altered the outcome, the progression, of these cancers," MacKay explained.
"And with that you have to realize that cancer is a very complex multi-factorial disease," MacKay stressed. "And two supplements would never be expected to be a successful treatment on their own. I would say, however, that proper nutrition is one of your best allies in terms of wellness, period. And while no one ever claimed these were cancer drugs, if you will, supplements make sense, cancer or no cancer."
More information
For more on vitamins and cancer, visit the American Cancer Society.
