Vitamin B6 appears to be linked to a lower risk of developing lung cancer, say researchers who cautioned more study is needed before considering supplements.

The European study of nearly 520,000 adults showed those with above-average blood levels of vitamin B6 as well as an amino acid called methionine were at lower risk of lung cancer.

'It is unlikely that further intervention trials of B vitamins would be advisable.'— Study authors

In Wednesday's issue of the Journal of the American Medical Association, researchers compared 899 people in the European study who developed lung cancer to 1,770 of the same age, sex and country who hadn't developed the disease.

This type of observational study cannot determine a cause-and-effect relationship. It would take a randomized trial that assigns subjects to take supplements and compares them to similar people who don't to attribute the difference in lung cancer risk to the supplements rather than some other factor in their lives.

"If our observations regarding serum methionine, B6, or both are shown to be causal, identifying optimum levels for reducing future cancer risk would appear to be appropriate," Paul Brennan of the International Agency for Research on Cancer (IARC) in France and his colleagues concluded. IARC calculated the incidence rates.

Clarifying the role of B vitamins in lung cancer is likely to be relevant for both former smokers and for those who never smoked, the researchers said. Reducing the number of people who smoke tobacco remains essential for lung cancer prevention, they added.

Those with the highest levels of vitamin B6 in their blood had nearly half the lung cancer risk (0.44 times lower) of those with the lowest levels, after accounting for smoking. There was a similar decrease among those with the highest levels of methionine in their blood.

Dietary sources

The blood levels could reflect diet rather than supplement use, the researchers noted, and it is unclear whether self-reported use of supplements could offer the same protective effect that foods might.

Vitamin B6 is found in fortified cereal, meat, poultry, tuna, salmon, and some fruits and vegetables, such as bananas and potatoes. The main sources of methionine are red meat, fish and beans.

The latest European findings may add support to previous research suggesting deficiencies in B vitamins may increase the chance the DNA damage and gene mutations, based on studies of colorectal cancer.

The study was limited in that researchers measured vitamin levels only when participants were recruited in 1992 to 2000. There could also be other explanations for the differences in levels between the lung cancer cases and control group, such as workplace exposure to carcinogens.

The study was funded by the World Cancer Research Fund. One of the study's authors is on the steering board of the non-profit Foundation to Promote Research into Functional Vitamin B12 Deficiency.