Soy Supplement Did Not Improve Asthma

Beth Skwarecki

May 27, 2015

A dietary supplement containing soy isoflavones did not improve lung function or clinical outcomes in patients with poorly controlled asthma in a multicenter, randomized, double-blind, placebo-controlled clinical trial published in the May 26 issue of JAMA. Previous population-based studies had suggested that soy intake was associated with less severe asthma symptoms.

Although the results are "disappointing," write Lewis J. Smith, MD, from the Department of Medicine at the Feinberg School of Medicine at Northwestern University in Chicago, Illinois, and colleagues, they illustrate the limitations of cross-sectional studies to predict the outcomes of carefully designed experimental trials.

In the study, 193 patients were given a 100-mg supplement twice a day of soy isoflavones, mainly genistein and daidzein. Another 193 received placebo. All patients were at least 12 years old, had poorly controlled asthma, and were taking a daily controller medication such as inhaled corticosteroids. Patients already taking soy supplements or eating soy more than once a week were excluded.

After 24 weeks, the two groups did not show significant differences in the primary outcome, forced expiratory volume in the first second (P = .36). There were also no differences in symptoms as measured by Asthma Control Test score (P = .53), Marks Asthma Quality of Life score (P = .25), numbers of episodes of poor asthma control, or any of several measures of systemic inflammation. There was a small difference in forced vital capacity (0.03 L [95% confidence interval, −0.01 to 0.08 L] in the placebo group vs −0.03 L [95% confidence interval, −0.08 to 0.02] in the soy isoflavone group), which the investigators described as "not clinically meaningful."

The investigators measured levels of genistein in patients' blood, where it rose from 4.87 ng/mL to 37.67 ng/mL over the course of the study in patients taking the supplement, suggesting patients were taking the supplement and that it was adequately absorbed. According to in vitro experiments, investigators expected this higher level to be able to inhibit airway inflammation.

The investigators, considering possible explanations for their results, speculate that the benefits of soy isoflavones may depend on their metabolism in the intestines, which has been known to vary between ethnic groups. They also write that a limitation of the study may have been the lack of current systemic inflammation in patients at baseline.

"Would I have loved this to have been positive?" asked Bradley Chipps, MD, from Capital Allergy and Respiratory Disease Center in Sacramento, California, who was not involved in the study. "You bet I would, because we would have been able to say, here is an absolutely safe dietary supplement you can use to control your asthma at a much cheaper price than expensive asthma medication. It didn't work, but it's important that this hypothesis was tested and that we have this in the peer-reviewed literature, and it's an important thing I can talk to patients about."

He called the trial "a very strong and well-done study," with strengths including the length of the trial, number of patients, and blood tests to show patients were actually taking the supplement. Unanswered questions, he said, include whether a soy supplement might improve symptoms if it were taken for a longer term, possibly years, and whether it might be effective in younger children.

Dr Smith reports receiving personal fees for Data and Safety Monitoring Board membership from Merck and cochairing an educational program from Scientific Therapeutics Information. One coauthor reports receiving personal fees from Forest Laboratories, Boehringer Ingelheim, Merck, and Quantia Communications and grants to his institution from Boehringer Ingelheim, GlaxoSmithKline, PneumRx, and Spiration. Another coauthor reports receiving personal fees for consultancy from Boehringer Ingelheim, GlaxoSmithKline, Forest Laboratories, Novartis, Sunovion, AstraZeneca, Janssen, Genentech, Bristol-Myers Squibb, Pulmonx, Merck, Spiration, and Teva, and grants/grants pending to his institution from GlaxoSmithKline/American Lung Association, AstraZeneca, BIPI, Forest Laboratories, Pearl, and Sunovion. The other authors have disclosed no relevant financial relationships. Bradley Chipps, MD, has disclosed that he has received grants for clinical research from GlaxoSmithKline, AstraZeneca, and Novartis, and grants for educational activities from GlaxoSmithKline, Sepracor, AstraZeneca, Schering-Plough, Merck, and Novartis. Dr Chipps has also disclosed that he has served as an advisor or consultant to GlaxoSmithKline, Sepracor, AstraZeneca, Schering-Plough, Merck, and Novartis.

JAMA. 2015;313:2033-2043. Abstract

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