Soy supplements do not improve lung function in people with asthma, a randomised trial shows.

Writing in JAMA the US researchers say previous data had suggested that supplementation with soy isoflavone may be an effective treatment for patients with poorly controlled asthma.  

But with the increasing cost of prescription drugs for asthma, it was important to identify effective, safe, and less expensive therapies than those currently available.

The researchers randomly assigned 386 adults and children age 12 years or older to receive soy isoflavone supplement containing 100 mg of total isoflavones (n=193) or matching placebo (n=193) twice daily for 24 weeks.

All participants had symptomatic asthma and were using a controller medication.

Average changes in pre-bronchodilator FEV₁ over 24 weeks were not significantly different between the soy isoflavone group and the placebo group, the researchers found.

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. 

This was despite evidence that plasma genistein increased to levels that inhibit eosinophil cysteinyl leukotriene synthesis in vitro and ex vivo, the researchers said. 

While it was possible that the dose of isoflavone may have been too low, the researchers conclude that the supplement “should not be used for patients with poorly controlled asthma.”