Effect of a soy isoflavone supplement on lung function and clinical outcomes in patients with poorly controlled asthma: a randomized clinical trial.

This was a multicenter randomized, placebo-controlled trial conducted at 19 adult and pediatric pulmonary and allergy centers in the United States from May 2010 through August 2012. The active soy isoflavone arm included 193 randomized participants who were adults and children aged 12 years or older with symptomatic, poorly controlled asthma and low dietary soy intake.

Soy isoflavone tablets were given orally twice daily for 24 weeks, for a total of 100 mg/day. Each tablet contained 49 mg of soy isoflavones, including genistein, daidzein, and glycitein, with approximately 32 mg as aglycone form; the regimen was compared with placebo.

Soy isoflavone supplementation did not improve lung function or clinical outcomes compared with placebo, and it should not be used for patients with poorly controlled asthma. The primary endpoint, change in prebronchodilator FEV1 at 24 weeks, was 0.01 L in the soy isoflavone group versus 0.03 L in the placebo group (P = .36). Plasma genistein increased markedly with soy isoflavone treatment, from 4.87 ng/mL at baseline to 37.67 ng/mL at week 24 (P < .001), while placebo changed from 5.10 to 7.11 ng/mL (P = .22).

The study was limited to participants aged 12 years or older with poorly controlled asthma and low baseline soy intake, which narrows generalizability. The intervention period was 24 weeks, and the packet does not provide active-arm completer counts or full adverse-event detail here. No clinical benefit was observed despite a clear rise in plasma genistein, suggesting limited efficacy rather than inadequate exposure.