Effect of a Soy Isoflavone Supplement on Lung Function and Clinical Outcomes in Patients With Poorly Controlled Asthma: A Randomized Clinical Trial

Abstract

A number of studies have suggested a potential role for dietary factors as modifiers of asthma severity. This study assessed whether a soy isoflavone supplement improves asthma control in patients with poorly controlled asthma.Three hundred eighty-six subjects ≥12 years old with symptomatic, poorly controlled asthma were enrolled from 19 adult and pediatric pulmonary and allergy centers in the American Lung Association Asthma Clinical Research Centers network.In this multicenter, randomized, double-blind, placebo-controlled trial, subjects were randomized (1:1) to receive either a soy isoflavone supplement or placebo twice daily for 6 months. The primary outcome measure was forced expiratory volume in 1 second (FEV1). Secondary outcomes included the Asthma Control Test score, the Asthma Symptoms Utility Index, and the Marx Asthma Quality of Life Questionnaire or Children’s Health Survey for Asthma for participants aged 12 to 16 years. Other outcomes included peak expiratory flow, symptom-free days, and rates of episodes of poor asthma control. Exhaled nitric oxide, peripheral blood eosinophil count, serum interleukin-6, serum C-reactive protein, urinary leukotriene E4, and quantification of total blood genistein level were also assessed.Mean changes in prebronchodilator FEV1 over 6 months were not significantly different between the placebo group (0.03 L, 95% confidence interval: –0.01 to 0.08 L) and the soy isoflavone group (0.01 L; 95% confidence interval: –0.07 to 0.07 L). No differences were observed for the other parameters measures when the placebo group was compared with the soy isoflavone group. Mean plasma genistein level increased from 4.87 to 37.67 ng/mL (P < .001) in participants receiving the supplement.In this study of adolescent and adult patients with poorly controlled asthma, use of a soy isoflavone supplement did not result in improved lung function or clinical outcomes compared with placebo.The authors had previously shown a relationship between dietary intake of the soy isoflavone genistein and improvement in FEV1 in a cross-sectional analysis of participants with varying asthma severity enrolled in a multicenter clinical trial. However, the results of this study do not support the use of soy isoflavone supplementation for patients with poorly controlled asthma. This highlights the limitations of cross-sectional, population-based studies of dietary nutrient intake and the use of surrogate markers of disease to predict clinically relevant outcomes in interventional studies for asthma.