Men who take energy-boosting vitamin B supplements could be at double the risk of lung cancer.
A study found men who take high doses of vitamins B6 or B12 over a long period were much more likely to develop the disease.
If they also smoked, their risk of cancer was up to four times higher if they took the supplements.
A study found men who take high doses of vitamins B6 or B12 over a long period were much more likely to develop the disease. File picture
The body needs vitamins B6 and B12 to help ensure red blood cells are healthy and to process proteins, fat and carbohydrate.
They are naturally found in meat, fish, cheese, eggs and milk, as well as fortified breakfast cereals, but millions of people in the UK take supplements as well.
Vitamin B6 and B12 pills are often marketed as energy or metabolism-boosting products.
The NHS-recommended dose for vitamin B6 is around 1.4mg (milligrams) a day for men and 1.2mg a day for women, while for B12 it is 1.5mcg (micrograms) a day for adults.
But tablets available on the high street in the UK can contain as much as 100mg of B6 and 500mcg of B12.
For the study, researchers in the U.S. followed 77,000 people aged between 50 and 76 as part of the Vital (Vitamins and Lifestyle) study – a major US investigation into the impact of vitamins and minerals on cancer risk.
Participants provided detailed information about their B-vitamin use over the past 10 years, including what dose they took.
Previously, doctors had thought supplements could help reduce cancer risk.
But they found men who took high doses of vitamin B6 and B12 for ten years were at a higher risk of cancer – even when factors like age, smoking history, family history of cancer and alcohol consumption were taken into account.
Previously, doctors had thought supplements could help reduce cancer risk. File pic of a patient with lung cancer
Male smokers who took more than 20mg of B6 a day for 10 years were three times more likely to develop lung cancer than people who did not take supplements.
The risk was four times greater for male smokers taking at least 55 micrograms of B12 a day for 10 years.
But there was no similar risk in women who took the supplements.
Study lead Dr Theodore Brasky, of The Ohio State University Comprehensive Cancer Centre, said: ‘What we found was that men who had used dietary supplements, in particular B6 and vitamin B12, at high doses for 10 years, were at significant increased risk of developing lung cancer.
‘In fact, all men who used these supplements in high doses for a decade had approximately double the risk developing lung cancer, and in men who smoked, the risk was three to four times as great.’
He warned many vitamin B supplements contained doses which were ‘much, much higher than the daily recommended amount’.
‘It’s very easy to get all the vitamin B you need from eating meats, chickpeas and foods like cereal that are fortified with them, so there really is no reason to supplement your vitamin B intake at these levels, and certainly not for years on end,’ he added.
But he said most multivitamin tablets did not contain such high doses so taking those daily should not have the same effect.
Further studies will now examine whether the same effect is seen in post-menopausal women or whether it only affects men.
The 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 previously been linked to increased cancer risk.
The Health Food Manufacturers’ Association (HFMA) said the observational study was ‘unreliable’ as it relied on people’s memories.
A spokesman said: ‘The association was only found with very high intakes when taken over a period of 10 years, and especially in smokers, and therefore does not apply to more typical vitamin users.
‘No link at all was found in women. Current safety assessments of micronutrients in both the UK and the EU do not identify data linking B12 or B6 with cancer development.’