Among men, taking B6 and B12 from only individual supplement sources was associated with a 30% to 40% increase in risk for lung cancer.
The risk almost tripled (HR, 2.93; 95% CI, 1.50-5.72; P =.04) and quadrupled (HR, 3.71; 95% CI, 1.77-7.74; P .01) in men who were current smokers who took vitamin B6 and B12 at the highest levels, respectively (P .01), compared to male smokers who did not take B6 or B12 supplements.
Current smokers who reported 10-year use of vitamin B6 at doses greater than 20 mg per day appeared almost three times as likely to develop lung cancer as nonusers (HR = 2.93; 95% CI, 1.5-5.72). Taking B6 at this dose increased the lung cancer risk by three times and taking B12 at this dose increased it by four times.
"That's marketing. That's not science", he said.
The researchers determined that men who now smoked and reported taking more than 20 milligrams (mg) of vitamin B6 daily during the 10-year period leading up to the study were about three times as likely to develop lung cancer than current smokers who didn't use B6 supplements at all. They were 89 percent more likely to get lung cancer than those who didn't take B12.
While the study seemed to show a relationship between high doses of vitamin B and lung cancer, some health experts remain skeptical. It found all men who took high-dose vitamin B supplements for a decade had about double the risk of developing lung cancer. And that's likely just because multivitamins generally don't contain high enough dosages of B6 and B12 to pose a risk. Long-Term, Supplemental, One-Carbon Metabolism-Related Vitamin B Use in Relation to Lung Cancer Risk in the Vitamins and Lifestyle (VITAL) Cohort.
The participants were recruited from the state of Washington between 2000 and 2002 and were made to answer questions about their vitamin use over the previous 10 years. An average healthy body will flush out any excess, unneeded vitamin B, so taking a higher dose probably doesn't provide much benefit.
Brasky pointed out that taking a high dose of vitamin B won't prevent lung cancer if you smoke.
"Now if you're a male and you're a smoker, and these findings were proven to be replicated over time, then there would be some concern that you probably don't want to be taking megadoses [of B 6 and B12]", Brasky says. These supplements have been broadly thought to reduce cancer risk.
"As far as the magnitude of the association, I think you could characterize our reaction as concerned; especially if you consider how common these supplements are".
Although the associations weakened among recent and former smokers who reported the highest usage of both vitamins, the HRs for lung cancer risk in both groups remained greater than 1.