Men who took high doses of vitamin B for years had a significantly higher chance of getting lung cancer, according to a new study.
Vitamin B6 and B12 has been previously found to have a protective effect against the disease but researchers who studied more than 77,000 people in the US concluded it appeared to be a “double-edged sword”.
Taking supplements containing thousands of times the recommended daily dose over a period of 10 years was associated with a 30 to 40 per cent increased risk of lung cancer, but only among men, not women, they found.
Smokers appeared to be more strongly affected by regularly taking vitamins B6 and B12 in high doses, with a three and four times higher risk of getting lung cancer for the two supplement respectively.
However other experts commenting on the research suggested the findings had been "over-dramatised" and should be taken with a "pinch of salt".
The researchers, from Ohio State University and the National Taiwan University, pointed out that most Americans already received enough vitamin B in their diet and so did not need to take more.
The people in the survey may have been taking extra vitamins in the belief that it would help stave off cancer. But the researchers wrote in the Journal of Clinical Oncology: “These B-vitamins may have a ‘double-edged sword’ effect on lung cancer in possessing dual … effects that [are] time and dose-dependent.
“Our study found that consuming high-dose individual B6 and B12 vitamin supplements over a 10-year period is associated with increased lung cancer risk, especially in male smokers.
“Consistent with prior evidence of harm for other vitamin supplements on lung cancer risk in male smokers, the associations we observed provides evidence that high-dose B6 and B12 supplements should not be taken for lung cancer prevention and may, in fact, increase the risk of this disease in men.”
They noted that half the US adult population uses at least one form of dietary supplement.
Dr Theodore Brasky, one of the researchers, stressed their results related to people who were taking unusually large amounts of the two vitamins.
“Our data shows that taking high doses of B6 and B12 over a very long period of time could contribute to lung cancer incidence rates in male smokers. This is certainly a concern worthy of further evaluation,” he said. “These are doses that can only be obtained from taking high-dose B vitamin supplements, and these supplements are many times the US Recommended Dietary Allowance.”
They are now carrying out two further studies: one to see whether the results among men are repeated in another similarly large study and another to examine whether there is no elevated risk among post-menopausal women.
However the other experts expressed caution about drawing any firm conclusions from the study.
Dr Kourosh Ahmadi, of Surrey University, described it as “a credible and powerful prospective observational study” but added it was likely that “very, very few people” would take such high doses of the vitamins.
And he added that: “First and foremost, I think the take-home message would be that the study shows that ‘normal’ supplemental use is not observationally associated with lung cancer per se.
“This is not emphasised enough in the paper and I feel the over-dramatization of the conclusion may have a negative influence on the lay public.”
Dr Ahmadi also raised methodological concerns, saying that “there are only two or three nominally significant” results for the “highest dose categories which suggest very few subjects contributed to the highlighted associations”.
And Professor Paul Pharoah, of Cambridge University, said the study’s “headline results” should be taken with a “pinch of salt”.
“The most likely explanation for those findings is statistical chance,” he said.
“The authors have investigated many sub-groups … and they have focused on the results that are most significant.
“Previous randomised controlled trials have found little evidence of association for supplementation of these vitamins on lung cancer risk.
“In summary, this in an intriguing finding, but it cannot be considered definitive. While it is possible that the findings are real, the most likely explanation is statistical chance.”
He recommended people should get “the essential vitamins required for good health” from a balanced diet with plenty of fruit and vegetables, rather than from taking supplements.
The Department of Health pointed to the "clear guidance" provided on the NHS Choices website about vitamins. For vitamin B6, it recommends about 1.4mg a day for men and 1.2mg a day for women. "You should be able to get all the vitamin B6 you need from your daily diet," it adds.
But the website warns: "When taking a supplement, it's important not to take too much. Taking more than 200mg a day of vitamin B6 for a long time can lead to a loss of feeling in the arms and legs known as peripheral neuropathy.
"This will usually improve once you stop taking the supplements. But in a few cases when people have taken large amounts of vitamin B6 – particularly for more than a few months – the effect can be permanent.
"Taking doses of 10-200mg a day for short periods may not cause any harm. But there's not enough evidence to say how long these doses could be taken for safely."
For B12, it recommended adults need about 1.5mcg a day and that people who eat meat, fish or dairy should get enough from their diet. Vegans, however, might not get enough this way and should get specialist advice.
NHS Choices said there was "not enough evidence to show what the effects may be of taking high doses of vitamin B12 supplements each day".
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