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, and another found that B12 had 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 in the body.
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.
"When we're talking about what to be concerned about most: If you're a male smoker and you want to take B vitamins, you can stop smoking," Brasky said.
"Smoking is the most important thing here, and that's preventable."
To B or not to B?
"In the average person in this country, it's tough to be deficient" in B vitamins, Brasky said.
Those who are -- those with anemia or celiac disease, for example -- will feel tired and run down. For them, supplements might help.
But taking "megadoses" of these supplements doesn't do much for the average healthy person, Brasky said, nor does it cause immediate harm. The body tends to get rid of excess vitamin, he said.
Stomach acid and digestion, Bailey said, are able to "rip out" B12 from food so that the body can absorb it. Some synthetic supplements, however, may be more easily absorbed.
In high concentrations, however, the exact relationship between the vitamins and lung cancer is unclear. If the vitamins are indeed responsible for increasing the lung cancer risk, Brasky said, another question would be whether B vitamins are hastening the development of a lung cancer that's already there or leading to new cancers.
Bailey warned that we are nowhere close to claiming that these high-dose supplements cause cancer. She added that the dietary survey the researchers used -- which calculated the average daily intake over the prior 10 years -- can be imprecise. But Brasky said that adults generally recall which supplements they've taken, allowing researchers to get a good idea of their average doses.
"In my mind, people take supplements because they're sick and trying to get better or because they're healthy and want to stay that way," she said.
"There might be one reason why somebody takes something, but it can have other effects on our bodies," Kantor said. "We don't know the whole host of effects."
The good news, Bailey said, is that most people aren't taking the single-vitamin, high-dose supplements that go far beyond recommended levels.
"Most people are taking multivitamins," she said, "and for that, there's really been no (cancer) association, which I think is a success story."