Smokers who have higher levels of vitamin B6 and certain essential proteins in their blood have a lower risk of getting lung cancer than those deficient in these nutrients, according to study by cancer specialists.
Scientists at the International Agency for Research on Cancer (IARC) said that although they had not found a causal link, the results may be a clue to why some smokers never get lung cancer and some non-smokers or former smokers do.
Lung cancer is the most common form of the disease in the world and 90 percent of all cases are caused by cigarette smoking. It kills 1.2 million people a year.
About 10 to 15 percent of smokers develop lung cancer -- although they often die of other smoking-related causes like heart disease, stroke or emphysema. Lung cancer is also known to kill people who never smoked or who gave up years ago.
The IARC study, which looked at around 900 people with lung cancer, found a link to low levels of vitamin B6 and an amino acid called methionine, found in protein like meat, fish and nuts. B6 is also found in meat, nuts, vegetables and bananas.
"What we have found is that these two things are strong markers of lung cancer risk, but we have not shown they are causing that rise in risk," said Paul Brennan of the Lyon-based IARC, who led the study and published its findings in the Journal of the American Medical Association (JAMA) on Tuesday.
"This indicates that diet may have an important role in lung cancer development, but it's still a little premature to say simply that if you change your diet and eat more foods with these vitamins then you'll change your future lung cancer risk."
Brennan's team studied around 900 lung cancer patients, mostly smokers but also including about 100 who never smoked and 260 who had quit.
Brennan said the change in risk of lung cancer linked to B6 and methionine levels was the same for all three groups, although of course the overall risk of getting the disease was much higher in the smokers to start with.
"For the two nutrients together, the risk reduction was about 60 percent," he said. "Obviously if you had a very high risk because you smoke, then a 60 percent reduction of that is quite important, although not as important as quitting smoking."
Brennan said his findings appeared to reinforce previous research which suggested deficiencies in B vitamins may increase the probability of DNA damage and subsequent gene mutations.
A Swedish study in 2005 found that women with high levels of vitamin B6 had a lower risk of developing colorectal cancer.
"Basically, these B vitamins and nutrients are all involved in the pathway which is responsible for the creation and maintenance of DNA," Brennan said. "So obviously you would want that pathway to work as well as possible."