People with high levels of a vitamin found in meat, potatoes, fish and whole grains have half the risk of developing lung cancer, irrespective of whether they smoke.
The finding, from the largest study of the link between diet and cancer in the world, suggests that the risk of lung cancer might be dramatically affected by the food we eat.
Smoking causes eight out of 10 cases of lung cancer, which is the commonest cause of cancer death in the world. Many of the deaths occur among people who have stopped smoking but whose risk remains high. If this could be reduced by changes to the diet, thousands of lives might be saved.
Researchers from the International Agency for Research on Cancer (IARC) identified 900 people with lung cancer from among 500,000 European volunteers participating in the European Prospective Investigation into Cancer and Nutrition (EPIC) study, the largest data study into diet and nutrition in the world. The volunteers were recruited in 10 European countries, between 1992 and 2000.
The 900 lung cancer patients were compared with almost 1,800 people who did not have cancer. Blood tests showed people with above average levels of vitamin B6 had less than half the risk of developing lung cancer.
A lower risk was also seen among those with high levels of the amino acid methionine, found in similar foods. The results are reported in the Journal of the American Medical Association.
Paul Brennan, of IARC, said: "At the moment we don't know if this is a causal effect. It is a very strong, quite obvious effect and we have tried to eliminate all possible biases but it could be a marker for an underlying cause that is probably dietary. We see the effect in every country – it is consistent across Europe."
He rejected suggestions that people should take supplements to boost levels of the vitamin. "We have had several bad experiences in the past with supplements. Twenty years ago, beta-carotene was given to people with lung cancer and found to increase, not decrease, deaths. I doubt if trials with supplements will go ahead."
Previous research has suggested that deficiencies in B vitamins may increase the risk of DNA damage and subsequent gene mutations. Low levels of B vitamins are common in Western populations.
The authors plan to conduct further studies on populations in Asia to confirm the link between lung cancer and diet and will also examine genetic influences on blood levels of B6 which could indicate whether the link is causal.
Paolo Vineis from Imperial College, London, said: "It has always been thought that diet may be important in lung cancer risk, in addition to smoking. This study gives the strongest evidence to date of a link between diet and lung cancer."