HealthDay News — Soy supplements provide no beneficial effect on lung function, results of a study published in the Journal of the American Medical Association indicate.

“Soy isoflavone supplements are used to treat several chronic diseases, although the data supporting their use are limited,” wrote Lewis Smith, MD, of the Northwestern University Feinberg School of Medicine in Chicago, and colleagues. Some data suggest that supplementation with soy isoflavone may be an effective treatment for patients with poor asthma control.

To determine wether a soy isoflavone supplement improves asthma control in adolescent and adult patients with poorly controlled asthma, the investigators studied the effects of soy in 386 patients aged 12 years and older. All of the patients were taking medications to treat their asthma, but none were consuming soy regularly before enrolling in the study. Half of the participants were assigned a soy isoflavone supplement twice daily for six months; the other half was assigned placebo.

The primary outcome measure was change in forced expiratory volume in the first second (FEV1) at 24 weeks. Secondary outcome measures were symptoms, episodes of poor asthma control, Asthma Control Test score (range, 5-25; higher scores indicate better control), and systemic and airway biomarkers of inflammation.

Mean plasma genistein level increased in participants receiving the supplement, but appeared to have no impact on lung function, symptoms, or measures of inflammation. Mean changes in prebronchodilator FEV1 over 24 weeks were 0.03 L (95% CI, −0.01 to 0.08 L) in the placebo group and 0.01 L (95% CI, −0.07 to 0.07 L) in the soy isoflavone group, which were not significantly different (P=0.36).

“These findings suggest that this supplement should not be used for patients with poorly controlled asthma,” concluded the investigators.


  1. Smith L et al. JAMA. 2015; doi: 10.1001/jama.2015.5024.