Plant-based estrogens, or phytoestrogens, come in three main classes: isoflavones, lignans and cumestrans, with isoflavones and lignans the most widely seen in nature. All act as weak estrogens with varying capacities to influence the life and death of cells.

Isoflavones, the most common, are found in a range of foods, especially soybeans, chickpeas, yams and red clover. Lignan sources include spinach, broccoli, tea, carrots and rye grains. Cumestrans are found in beans, peas, spinach and sprouts.

"Basically, we found that people with lung cancer were less likely to consume these foods," said Matthew Schabath, a postdoctoral fellow specializing in cancer prevention.

"What we saw was quite interesting in terms of protective effects in 'never smokers' and former smokers," Schabath added. However, for reasons that could not be easily explained, benefits were not apparent in current smokers, according to the findings, reported in the Journal of the American Medical Association.

Schabath and colleagues studied 1,674 lung cancer patients and 1,735 people without the disease from July 1995 to October 2003. Participants answered questionnaires on food frequency. Researchers were particularly interested in quantifying dietary intake of specific phytoestrogens.

"Phytoestrogens have a variety of protective effects that have been shown in experimental models [lab animals] and lab models [Petri dishes] to have various chemo-effects," Schabath said.

Some of those effects have been as antioxidants, which means they can inhibit the prevalence of rogue oxygen molecules called free radicals. Schabath defined other effects of weak estrogens as limiting angiogenesis, the growth of blood vessels required by tumors and inducing programmed cell death.

Schabath pointed to Asians who consume robust quantities of phytoestrogens, especially in the form of soy-based foods, as having lower rates of cancer.

However, while that may be true for many types of malignancies, it is not so for lung cancer, especially in China, where the disease has reached epidemic proportions. The World Health Organization estimates the number of lung cancer patients in China will increase by 1 million a year through 2025.

A second study in the same issue by researchers in Denmark found that cutting the number of cigarettes smoked daily could reduce lung cancer risk for those who cannot completely break the habit.

Doctors Lawrence Dacey and David Johnstone of Dartmouth-Hitchcock Medical Center in New Hampshire wrote in a critique of both analyses that while the phytoestrogen findings are good news, the old "stop smoking" message is even better.