Newswise — Although some data have suggested that supplementation with soy isoflavone may be an effective treatment for patients with poor asthma control, a randomized trial that included nearly 400 children and adults found that use of the supplement did not result in improved lung function or clinical outcomes, including asthma symptoms and episodes of poor asthma control, according to a study in the May 26 issue of JAMA. Soy isoflavones are plant (soybean) derived chemicals that have anti-oxidant effects.

Increases in asthma prevalence and severity over the last several decades are likely due at least in part to environmental factors. Diet is one environmental factor that is associated with asthma prevalence and severity. Soy isoflavone supplements are used to treat several chronic diseases, although the data supporting their use are limited. Some data suggest that supplementation with soy isoflavone may be an effective treatment for patients with poorly controlled asthma. The soy isoflavone genistein inhibits a key pathway that may contribute to asthma severity. With the increasing cost of prescription drugs for asthma, it is important to identify effective, safe, and less expensive therapies than those currently available, according to background information in the article.

Lewis J. Smith, M.D., of Northwestern University, Chicago, and colleagues randomly assigned 386 adults and children age 12 years or older with symptomatic asthma while taking a “controller” medicine (either inhaled corticosteroids and/or a leukotriene modifier) and low dietary soy intake to receive soy isoflavone supplement containing 100 mg of total isoflavones (n=193) or matching placebo (n=193) in 2 divided doses administered daily for 24 weeks. The trial was conducted at 19 adult and pediatric pulmonary and allergy centers in the American Lung Association Asthma Clinical Research Centers network.

The researchers found that average changes in pre-bronchodilator forced expiratory volume in the first second (FEV1; a measure of lung function) over 24 weeks were not significantly different between the soy isoflavone group and the placebo group. The supplement also did not improve other aspects of asthma control, including additional measures of lung function, symptoms, quality of life, and airway and systemic inflammation.

“These findings suggest that this supplement should not be used for patients with poorly controlled asthma,” the authors conclude.(doi:10.1001/jama.2015.5024; Available pre-embargo to the media at

Editor’s Note: This study was supported by grants from the National Institutes of Health and the American Lung Association. Archer Daniels Midland (Decatur, Il.) provided the soy isoflavone supplement and the matching placebo. Please see the article for additional information, including other authors, author contributions and affiliations, financial disclosures, etc.

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