Men who took high doses of vitamin B6 and B12 supplements had a higher risk of lung cancer, and the association was highest among current smokers, according to a study published Tuesday.
The study found a 30% to 40% increased risk of lung cancer among men taking these vitamins from individual supplements -- not from multivitamins or diet alone. But the effect seemed to be driven by current smokers who far exceeded the recommended daily amounts of the vitamins, according to study author Theodore Brasky, an epidemiologist in the division of cancer prevention and control at the Ohio State University College of Medicine.
"I think these results point to a synergism" between high-dose B vitamins, smoking and lung cancer risk among men, Brasky said.
Current male smokers taking the highest levels of vitamin B6 had triple the risk of lung cancer over six years, compared with those who didn't take supplements. For vitamin B12, that risk nearly quadrupled. These levels were more than 11 times the recommended daily amount of B6 and 23 times that of B12.
"If you look at B-vitamin supplement bottles ... they are anywhere between 50-fold the US recommended dietary allowance (to) upward of 2,100-fold," Brasky said. B12 injections have also become "in vogue" in recent years, he said.
In smaller quantities, these vitamins are involved in several vital processes in the body, including DNA replication. But many high-dose supplements, he said, claim to boost energy and provide other unproven benefits.
"That's marketing. That's not science," he said.
The study was limited to roughly 77,000 Washington state adults, ages 50 to 76. This included 139 cases of lung cancer among more than 3,200 current male smokers. Over 93% of participants were white.
There were too few cases of lung cancer among nonsmokers to include them in the full analysis. An increased risk of lung cancer was not seen among women or with the vitamin B9, also known as folate.
Other researchers have found different results. Some studies linked vitamin B6 with lower lung cancer risk, and another found that B12 had no impact on risk. The authors of the new study said that the discrepancy could be because some of these studies measure B vitamins in the blood and not through dietary surveys, like they did. Or it may be that lung cancer itself raises levels of these vitamins in the body.
"I think it's hard to say" why these studies contradict each other, said Elizabeth Kantor, an epidemiologist at Memorial Sloan Kettering Cancer Center who has studied dietary supplements and cancer risk. She was not involved in the latest research. "Is it the disease process that affects the blood levels? I think that the door remains open on that."
A focus on B vitamins may not be the most effective way to protect against lung cancer, experts warn.
"Combustible tobacco smoke is the No. 1 most important factor, not just only in lung cancer but in many cancers," Brasky said.
Cigarette smoking is a factor in 80% to 90% of lung cancers in the United States, according to the US Centers for Disease Control and Prevention. Smokers are 15 to 30 times more likely to get lung cancer or die from it than nonsmokers. Lung cancer kills more Americans than any other kind of cancer.