A new study published in JAMA has found that supplementation with soy isoflavone did not improve lung function or critical outcomes in patients with poor asthma control, despite previous research suggesting that it may be an effective treatment.

Because it is believed that the soy isoflavone genistein inhibits a key pathway that may contribute to asthma severity, Lewis J. Smith, MD, of Northwestern University, Chicago, and colleagues conducted a randomized controlled trial of soy isoflavone supplement for asthma patients. A total of 386 adults and children aged ≥12 years with symptomatic asthma received soy isoflavone supplement containing 100mg of total isoflavones or a matching placebo as two divided doses daily for 24 weeks. All patients were taking inhaled corticosteroids and/or a leukotriene modifier and had low dietary soy intake.

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No significant differences were observed in average changes to pre-bronchodilator forced expiratory volume in the first second (FEV1) over 24 weeks between the two groups. Other aspects of asthma control such as additional measures of lung function, symptoms, quality of life, and airway and systemic inflammation were not improved with soy isoflavone supplementation as well.

Based on this study, the authors advise that soy isoflavone supplement should not be used for patients with poorly controlled asthma.

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