People who eat a lot of unfermented soy products like tofu may have a smaller chance of getting lung cancer, a fresh look at past research suggests.
There is still no proof that soy itself is protective, but compounds in the soy called isoflavones have been shown to slow cancer cell growth in the lab.
Because researchers studying the link between diet and lung cancer have come to mixed conclusions on soy, Chinese and U.S. scientists decided to get an overview of the medical literature.
They found 11 observational studies, a few of which followed people for a decade or longer. Pooling all of the results, people who got the most soy in their diet had a 23 percent lower risk of lung cancer than those who got the least.
According to the American Cancer Society, about eight percent of men will develop lung cancer at some point, while six percent of women will get the disease.
The new findings, published in the American Journal of Clinical Nutrition, come with several caveats, according to Wan-Shui Yang from the Shanghai Jiaotong University School of Medicine and colleagues.
The link between soy and cancer only held for unfermented products such as tofu and soy milk, for example. What's more, it was only found in people who never smoked, in women and in Asian populations.
Matthew Schabath, a researcher at the Moffitt Cancer Center in Tampa, Florida, whose study was included in the analysis, cautioned that more research is needed to tease out the relationship between soy and lung cancer.
"It may not be soy alone," Schabath told Reuters Health. "It could be a collection of other nutrients packaged in the food."
"We've been looking at this for decades," he added. "The observational studies do consistently show that healthy diets will provide beneficial effects. We just haven't found the one magic pill that will be able to prevent this. It just tells you about the complexity of this."
Schabath said until a link is found there is one surefire way to cut lung cancer risk.
"If you want to reduce your risk of lung cancer you need to stop smoking," he said. "The next step is to be prudent of the information out there."