Women with lung cancer who ate the most soy before their diagnosis might live a little longer than those who ate the least, according to a new study.
Of 444 Chinese women with lung cancer, researchers found those who consumed the most soy milk, tofu and similar products were 7 to 8 percent less likely to die over a 13-year period, compared to women who ate an average amount of soy.
"To our knowledge this is the first study to suggest this association. Although this finding is promising, it would be premature to make any recommendation based on the findings of a single study," said Dr. Gong Yang, the study's lead author from Vanderbilt University School of Medicine in Nashville, Tennessee.
Another study from Yang's group published last year found women who ate the most soy were less likely to develop lung cancer in the first place.
"Based on that study, we hypothesized people with a history of eating a lot of soy food - if they're diagnosed with lung cancer, their lung cancer would be less aggressive," he said.
Laboratory and animal research has also suggested phytoestrogens - plant-based estrogens similar to those produced by women's ovaries, which are present in soy - can prevent tumor cells from spreading.
According to the researchers, only 15 percent of U.S. women who develop lung cancer are still alive five years after their diagnosis. So any improvement in survival could be meaningful.
For the new study, the researchers used information from the Shanghai Women's Health Study, which began in 1997 and surveys 75,000 women every two to three years.
Between 1997 and 2010, 444 women in the study developed lung cancer, and 318 of them died. They were - on average - 66 years old at diagnosis, and 92 percent of them had never smoked.
Before the women were diagnosed, they had eaten an average of 16 grams of soy from food each day, according to diet questionnaires.
The researchers found women who consumed more than that - 21 to 31 g of soy per day - were about seven to 8 percent less likely to die during the study, compared to women who ate 16 g daily.
On the other hand, women who only ate about 6 g of soy per day were over 40 percent more likely to die during that time.
"This longitudinal follow-up study provides the first evidence that soy food consumption before cancer diagnosis may favorably affect clinical outcomes of lung cancer in women," the researchers wrote in the Journal of Clinical Oncology .
But Matthew Schabath, a lung cancer researcher who was not involved with the new study, stressed that it can't prove eating soy prevents people with lung cancer from dying.
For example, Schabath, from the Moffitt Cancer Center in Tampa, Florida, said that women who eat a lot of soy may be in better health to begin with, and would be expected to live longer than women in poorer health.
Researchers also said more studies on soy and lung cancer are needed to confirm the findings.
"We need to take it with a grain of salt until this data is validated in clinic trials," Schabath said.