Soy improves lung cancer survival in women

The consumption of soy food before a lung cancer diagnosis might favorably affect disease outcomes in women, according to a longitudinal follow-up study from China.

In the Shanghai Women's Health Study, women with lung cancer who ate low levels of soy daily were at greater risk of dying from lung cancer than those who ate average or higher levels of the Asian food staple over their lifetimes.

Specifically, in 301 women with lung cancer, there was an 81% increased risk for death from lung cancer in those in the 10th percentile of soy intake, compared with those in those in the 50th percentile (the median). In contrast, there was an 11% decreased risk for death in women in the 90th percentile of intake, compared with those in the 50th percentile (P for overall significance = .004).

These effect sizes were found after adjustment for a wide variety of factors, including tumor stage and treatment.

In short, soy might have a disease-modulating effect, say the authors, led by Gong Yang, MD, from Vanderbilt University Medical Center in Nashville, Tennessee.

The study results were published online today in the Journal of Clinical Oncology.

"Plant-derived estrogens, such as isoflavones found mainly in soy food, appear to act as natural selective estrogen-receptor modulators," they write.

Thus, soy could compete with a woman's endogenous estrogens in binding to estrogen receptors, the authors explain. Soy might occupy the receptors so they cannot be stimulated by the body's estrogens.

Dr. Yang and his coauthors point out that emerging evidence suggests that female sex hormones negatively affect lung cancer survival. The evidence includes well-publicized data from the Women's Health Initiative, which showed that estrogen-plus-progestin therapy significantly increased the risk for death from lung cancer.

"Soy may have a mechanism of action similar to drugs like tamoxifen," Jyoti Patel, MD, from Northwestern University in Chicago, Illinois, told Medscape Medical News.

"This is the first scientific evidence that soy has a favorable effect on lung cancer survival," said Dr. Patel, who is a spokesperson for the American Society of Clinical Oncology, and provided independent comment on the study.

"It's really exciting," she summarized.

This study is part of a varied body of research that is seeking "hormonal clues" to lung cancer, she explained.

In 2012, Dr. Yang and colleagues found, in the same 74,000-patient cohort of Shanghai women, an approximately 40% reduction in the risk for incident lung cancer associated with the high intake of soy food. Thus, eating soy could both prevent and alter the disease course of lung cancer.

In the current study, the amount of soy consumed by the 301 women with lung cancer that was most protective (>20 g/day) was "not astronomical," said Dr. Patel. "But you have to make a concerted effort [to eat that much]," she added, referring especially to Westerners, who are less likely to include soy in their diet.

Americans typically consume what the study deemed to be a low level of soy (≤12 g/day), said Dr. Patel, who explained that the study results are "likely more applicable" in Asian countries.

More epidemiologic studies are needed to confirm these results, the authors note.

Almost all were non-smokers
There were actually 440 cases of lung cancer in the 74,000-women Shanghai cohort, but only 301 had information about both tumor stage and treatment. The hazard ratios for the 440 women were less dramatic than those for the 301 women reported above.

For example, in the 440 women with lung cancer, there was a 42% increased risk for death from lung cancer in the women who were in the 10th percentile of soy intake, compared with those in the 50th percentile (the median). In the 301 women, that increase was much larger (81%).

Nevertheless, although the effect sizes varied, the evidence in support of a possible protective effect of soy was constant.

As expected in an Asian population, most of the women with lung cancer (about 92%) were never smokers. Mean age at cancer diagnosis was 66.3 years.

The researchers assessed soy dietary intake with a food-frequency questionnaire, which covered soy milk, tofu, fried tofu, dried or pressed tofu, fresh green soy beans, dry soy beans, soy sprouts, and other soy products.

Mean intakes, on a dry weight basis, were 18.0 g/day for soy food and 8.8 g/day for soy protein.

The authors report that soy food intake was not related to patient characteristics such as age at diagnosis, smoking, obesity, family history of lung cancer, tumor stage, treatment regimens, or time between baseline dietary assessment and disease diagnosis.

In this study, the median follow-up time after cancer diagnosis was 36 months. During the follow-up period, 318 of the 440 women died; in 301 (94.7%), the primary cause of death was lung cancer, and in 17 (5.3%), it was another cause.