High doses of B vitamins can dramatically increase the risk of lung cancer in men who smoke, US researchers claim.
Compared with non-users, male smokers taking 55 microgram daily supplements of vitamin B12 for 10 years were four times more likely to develop the disease, a new study has found.
Those taking more than 20mg per day of vitamin B6 over the same time period tripled their chances of being diagnosed with lung cancer.
Scientists looked at data on more than 77,000 participants in the Vital (Vitamins and Lifestyle) study, a major US investigation of the impact of vitamin and mineral supplements on cancer risk.
The patients, aged 50 to 76, provided detailed information about their vitamin B usage over 10 years, including supplement dosages.
"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," said lead researcher Dr Theodore Brasky, from Ohio State University Comprehensive Cancer Centre.
"This is certainly a concern worthy of further evaluation.
"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."
B complex vitamins are vital for the metabolism of proteins, fats and carbohydrates. Vitamins B6 and B12 are also important for healthy nerve function.
The new research, published in the Journal of Clinical Oncology, is the latest to challenge the alleged health benefits of vitamin supplements.
Excessive doses of vitamin E, beta-carotene and folic acid (vitamin B9) have all been linked to increased cancer risk.
Dr Brasky's team accounted for the influence of numerous factors including personal smoking history, age, race, education, body size, alcohol consumption, cancer history, chronic lung disease and use of anti-inflammatory drugs.
"This sets all of these other influencing factors as equal, so we are left with a less confounded effect of long-term B6 and B12 super-supplementation," he said.
An additional study under way at the Ohio cancer centre will look at the effects of long-term B6 and B12 supplementation on lung cancer risk in post-menopausal women.
