Medical Research: What is the background for this study? What are the main findings?
Dr. Smith: We previously observed in a survey of more than 1,000 patients with asthma that those consuming soy isoflavones in their diet had better lung functioning than their counterparts who consumed little or none. Using a more detailed soy questionnaire, we confirmed the observation in a different group of patients with asthma, and followed that up with laboratory studies. In cell culture studies, we saw that genistein, the major soy isoflavone, at levels that are achieved in individuals consuming a high soy diet, reduces eosinophilic inflammation, a key feature in asthma. In addition, people who consume more soy products, mostly in Japan and parts of China, generally have less asthma than in western countries. Although these data indicate a potential beneficial effect of soy isoflavones in patients with asthma and nutritional supplements are commonly used by people to treat and prevent disease and improve their health, there was little direct data to prove that the supplement is actually effective. As a result, we explored the effects of a soy isoflavone supplement in 386 adults and children aged 12 or older with poorly controlled asthma. All were taking medicine to treat their asthma – either corticosteroids or leukotriene modifiers – but none consumed soy more than once a week. In the randomized, double-blind study, half of the participants took a soy isoflavone supplement twice daily for six months, and the other half took a placebo. We found that the supplement, though able to increase blood levels of genistein, did not improve lung function, symptoms or measures of inflammation in these individuals.
Medical Research: What should clinicians and patients take away from your report?
Dr. Smith: This study highlights that other factors may have been at play, such as diet and lifestyle patterns like eating less meat or exercising frequently, to explain the observed association between soy isoflavone consumption and asthma severity. It also demonstrates why it is so important to perform well-designed, placebo-controlled studies when epidemiologic associations are reported between specific nutrients and disease outcomes.
Medical Research: What recommendations do you have for future research as a result of this study?
Dr. Smith: We will be looking at whether there is a subset of patients with asthma who are likely to respond to soy isoflavones, if there are genetic factors that influence the response or lack of response, and whether the soy isoflavone supplement has other potential beneficial effects that were not measured in this study.
MedicalResearch.com Interview with: Lewis J. Smith, MD (2015). Soy Supplement Did Not Improve Asthma Symptoms