The link between intakes of the vitamins A, C, and E and folic acid, has been reported by some studies, but not by others. The new analysis, led by Eunyoung Cho from Brigham and Women's Hospital and Harvard Medical School, sought to clarify the situation by pooling eight prospective studies with a total population size of 430,281 participants.
The analysis, published in the International Journal of Cancer (Vol. 118, pp. 970-978), concluded: "This pooled analysis does not suggest that intakes of vitamins A, C and E and folate reduce the risk of lung cancer."
Eight prospective studies from Canada, Finland, the Netherlands, and the US, were pooled together using baseline data collected by the original study using food frequency questionnaires. Follow-up of the participants ranged from six to sixteen years, but only baseline food intakes were used in the meta-analysis.
When the results were adjusted for age, the researchers calculated that the highest intake of all four nutrients from food only was associated with a 28 per cent reduction in the risk of lung cancer.
This significant association was lost after adjusting for other variables, such as education, body-mass index, alcohol consumption and smoking. However, the associations for intakes of vitamin C and E remained significant.
Further analysis led the researchers to calculate that the vitamin E association was not significant and that vitamin C was only protective in men.
Even this link was eliminated when the researchers accounted for beta-cryptoxanthin, the compound that gives the red colour to red-orange fruits and vegetables.
"The inverse association between vitamin C intake from food-only and lung cancerrisk probably represents the association with either beta-cryptoxanthin intake or some other dietary constituents that are highly correlated with beta-cryptoxanthin in fruits," said Cho.
When supplement sources of the four micronutrients was examined, not link was found between lung cancer risk and nutrient intake for the entire sample population, but was linked to a "modest increase in lung cancer risk among women." This was said to be due to chance and did not have biological implications.
This analysis has several important limitations including the use of data only from baseline, and no indication of dietary changes that may have occurred during the six to 16 years of follow-up which may have affected the lung cancer risk.
Also, the meta-analysis design had a limited power to detect associations for high intakes of vitamins and the effects of duration of vitamin supplementation.
Over 1.2m new cases of lung cancer are diagnosed every year worldwide, with the cancer being the most commong form of cancer in Europe.