In a trial comparing 1,674 patients treated for lung cancer and 1,735 healthy 'control' volunteers, researchers from the University of Texas MD Anderson Cancer Center found that those who reported eating the highest amount of foods containing phytoestrogens were much less likely to have developed lung cancer.
They assessed the risk of the disease to be 46 per cent lower for those with the highest phytoestrogen intake from all foods, or 21 per cent if they had a high intake of phytosterols, the components added to products to lower cholesterol.
The study's lead author, postdoctoral researcher Matthew Schabath, said it supports "a small but growing body of evidence that suggests oestrogenic-like compounds in food may help protect against development of lung and other cancers".
The same team demonstrated last year that use of hormone therapy was a significant protective factor for lung cancer in women. This led them to investigate whether natural products that acted like hormones could also have this benefit.
In the new trial, published in the Journal of the American Medical Association (vol 294, pp1493-1504), they found that women who used hormone replacement therapy enhanced the protective effect already gained from eating fruits and vegetables.
In general, women who ate the most fruit and vegetables lowered their risk of lung cancer by 41 per cent.
But women did not see the same benefits from each phytoestrogen class as men. For example, isoflavones reduced lung cancer risk in men by 44 per cent, and lignans reduced the risk by 27 per cent.
Yet in women, only intake of total phytoestrogens from food sources was statistically significantly higher in controls than in cases. High consumption of these foods reduced risk by 34 per cent, but no effect was seen when individual classes of phytoestrogens were evaluated.
The researchers could not explain this gender difference, nor why former smokers benefited less than non-smokers.
But they did underline the potential importance of phytoestrogens for non-smokers.
"About 15 per cent of lung cancers occur in lifetime never smokers, and besides exposures to second-hand smoke, other risk factors for these cancers are yet to be determined," noted Schabath.
The scientists believe that phytoestrogens may help protect against lung cancer development by latching on to oestrogen receptors that are present in both normal and malignant lung tissue.
This binding could play a role in the regulation or deregulation of cancer growth.
The investigators caution that much more research is needed to prove a definitive chemoprevention effect. The current trial relied on subjects' memories to remember the food they ate months before and is therefore limited.
The participants were asked detailed questions about their diet for the year prior to their enrollment or to their cancer diagnosis, with the assumption that what they ate that year reflected their general eating pattern over a number of years.
But the opportunities it presents are "enormous", according to the study's principal investigator Margaret Spitz, given that lung cancer is the cancer that causes more deaths in the United States and Europe than any other.
Several epidemiological studies have already suggested that foods containing plant oestrogens might protect against other cancers, including breast, endometrial and prostate cancers.
Furthermore, lung cancer rates are substantially lower in Asian populations that typically eat larger amounts of phytoestrogens than is consumed in America.