CHICAGO (American Medical Association) -- A diet higher in plant-derived compounds known as phytoestrogens is linked with a lower lung cancer risk, according to a study in the September 28 issue of JAMA.

Phytoestrogens are plant-derived nonsteroidal compounds found in soy products, grains, carrots, spinach, broccoli, and other fruits and vegetables, according to background information in the article. They have weak estrogen-like activity.

The three main classes of phytoestrogens are isoflavones, lignans, and cumestrans. A fourth group of plant-derived steroidal compounds believed to have estrogenic properties are the phytosterols.

Phytoestrogens have been shown to have a protective effect against some solid tumors, but there has been little epidemiologic research focused on dietary intake of phytoestrogens and lung cancer risk.

Matthew B. Schabath, Ph.D., and colleagues at the University of Texas M.D. Anderson Cancer Center, Houston, analyzed data from an ongoing case-control study to examine the relationship between dietary intake of phytoestrogens and the risk of lung cancer.

The study included 1,674 patients with lung cancer (cases) and 1,735 matched healthy controls.

From July 1995 through October 2003, study participants were personally interviewed to obtain information on demographics, socioeconomics, and smoking history. Women were asked whether they had taken hormone therapy in the previous six months. A food frequency questionnaire was used to collect dietary data on intake of 12 individual phytoestrogens.

"Our main findings were that patients with lung cancer tended to consume lower amounts of phytoestrogens than controls, that there were sex-specific differences both in intake and in protective effects, and that the apparent benefits were evident in both never and current smokers but less so in former smokers," the authors report.

Reduction in lung cancer risk tended to increase with increasing phytoestrogen intake.

"The highest quartiles of total phytosterols, isoflavones, lignans, and phytoestrogens were each associated with reductions in risk of lung cancer ranging from 21 percent for phytosterols to 46 percent for total phytoestrogens from food sources only," the authors write.

Sex-specific effects were also apparent.

"For men, statistically significant trends for decreasing risk with increasing intake were noted for each phytoestrogen group, with protective effects for the highest quartile of intake ranging from 24 percent for phytosterols to 44 percent for isoflavones, while in women, significant trends were only present for intake of total phytoestrogens from food sources only, with a 34 percent protective effect for the highest quartile of intake," the authors report.

The apparent benefits of high phytoestrogen intake were evident in both current smokers and those who had never smoked, but less apparent in former smokers.

In women, statistically significant joint effects were evident between hormone therapy use and phytoestrogen intake.

"Specifically, high intake of the lignans [metabolites] enterolactone and enterodiol and use of hormone therapy were associated with a 50 percent reduction in risk of lung cancer," the authors report.

"In summary, these data provide further support for the limited but growing epidemiologic evidence that estrogens and phytoestrogens are associated with a decrease in risk of lung cancer, especially in never and current smokers," they conclude.

"However, confirmation of these findings is still required in large-scale longitudinal studies."