If you think popping a handful of vitamins each day is the key to staying healthy, well, you might be doing it wrong.
In fact, your supplements might actually be putting your health at risk: Taking high doses of vitamin B6 and B12 supplements may increase your risk of lung cancer, especially if you smoke or have smoked in the past, a new study in the Journal for Clinical Oncology suggests.
Researchers recruited over 77,000 people over age 50, and asked them about how often, how much, and for how long they took vitamin B6, B9 (folic acid), and B12 supplements over the 10-year period leading up to the study. They also quizzed them on whether they were a current smoker, a former smoker who quit within 10 years, a former smoker who quit more than 10 years ago, or a never-smoker. Then, they followed up the participants for an average of about 6 years to see how many developed lung cancer.
The researchers determined that men who currently 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. Guys who quit smoking less than 10 years ago and took more than 20 mg of B6 daily were at greater risk, too: They were 54 percent more likely to develop lung cancer than men who never used B6 and quit smoking within that same time frame.
There was an even greater link between B12 supplementation and lung cancer, too. Male smokers who took more than 55 micrograms (µg) of B12 each day for 10 years were nearly four times as likely to develop lung cancer as smokers who never took B12. As for those who quit smoking more than 10 years prior? They were 89 percent more likely to get lung cancer than those who didn't take B12.
The link between B vitamins and lung cancer was only evident in those taken from individual supplementation, not those which were included as part of a multivitamin. And that's likely just because multivitamins generally don't contain high enough dosages of B6 and B12 to pose a risk.
“All of the risk was in men taking very high doses of individual supplements—20 mg a day of B6 and 55 µg a day of B12, which is [about] 10 to 20 times the recommended U.S. recommended dietary allowance,” Theodore M. Brasky, Ph.D., lead author and research assistant professor in the department of internal medicine at Ohio State University says.
You also don’t have to worry if you regularly eat foods high in vitamin B, like chickpeas, clams, tuna, salmon, or red meat. These food sources weren't shown to increase your chances of lung cancer at all.
There were too few cases of lung cancer among never-smokers to crunch numbers to see if supplement use was linked to increased risk for them, too. And taking B vitamins wasn't linked to lung cancer in women at all, either.
So why might high dosages of vitamin B6 and B12 supplements in former or current male smokers leave them more vulnerable to lung cancer? The researchers aren't exactly sure, and prior research on the topic has been limited and conflicting. But the paper does mention that B vitamins may interact with certain amino acids in something called the one-carbon metabolic pathway—and the disruption of that process may promote the development of cancer.
It's also possible that this action from the B vitamins simply hastens the development of an underlying lung tumor already caused by smoking, Brasky says. But on the flip side, it also may be that smoking can quicken the progression of a tumor sparked by the B vitamins.
Important note: Because this was an observational study, it can't prove for sure that these vitamin B supplements actually caused lung cancer. In fact, that's one of the big beefs the Council For Responsible Nutrition—an organization that represents supplement manufacturers—has with the study, which they explained in a statement they provided to Men's Health. Plus, the study relies on people's recall over 10 years to remember their supplement consumption, which may not be accurate, they say.
Bottom line: While this study doesn't definitively prove that B supplements increase your lung cancer risk, it does provide enough evidence that guys who smoke or who have smoked in the past should reconsider taking high doses of it. (Plus, we listed B vitamins as one of the supplements you should skip, too.)
“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.
Sticking with the daily recommended doses—2.4 µg for B12 and 1.7 mg of B6 for guys 51+—should be fine.
While the jury is still out on how exactly B vitamins affect lung cancer risk, there are some things you can do now to protect yourself. Your best bet? Quit smoking now, says Brasky.
It's also important to familiarize yourself with lung cancer symptoms—catching lung cancer early can help boost your chances of beating it. Persistent symptoms that last for at least a few weeks or tend to recur even with treatment, like chronic cough, chest pain that’s worse with deep breathing, coughing up blood, frequent infections like bronchitis, or shortness of breath can all warrant further evaluation, Helen Chen, M.P.H, former field manager for the Cancer Prevention Institute of California told us in the past.
If you notice those symptoms—especially if you smoke or have smoked in the past—may warrant further testing, like CT scans, to look for cancer.
This article first appeared on Men's Health.