MD, PhD, FMedSci, FRSB, FRCP, FRCPEd

Soy isoflavone supplements are used to treat several chronic diseases, although the data supporting their use are limited. Some data suggest that supplementation with soy isoflavone may be an effective treatment for patients with poor asthma control. In fact, genistein, one of the isoflavones in soy, is a powerful anti-inflammatory agent and as such it might help control asthma symptoms. But speculation and wishful thinking does not lead anywhere. What we need is a good study to test the efficacy of soy in asthma patients.

Yes, you guessed it: such an investigation has just been published, The objective of this trial was to determine whether a soy isoflavone supplement improves asthma control in adolescent and adult patients with poorly controlled disease.

The study was designed as a multi-centre, randomized, double-blind, placebo-controlled trial conducted between May 2010 and August 2012 at 19 pulmonary and allergy centres in the American Lung Association Asthma Clinical Research Centers network. Three hundred eighty-six patients aged 12 years or older with symptomatic asthma while taking a controller medicine and low dietary soy intake were randomized, and 345 (89%) completed spirometry at week 24.

Participants were randomly assigned to receive soy isoflavone supplement containing 100 mg of total isoflavones (n=193) or matching placebo (n=193) in 2 divided doses administered daily for 24 weeks.

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 changes in pre-bronchodilator 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. The difference was not significantly different. Mean changes in symptom scores on the Asthma Control Test, number of episodes of poor asthma control, and changes in exhaled nitric oxide did not significantly improve more with the soy isoflavone supplement than with placebo. Mean plasma genistein level increased from 4.87 ng/mL to 37.67 ng/mL (P < .001) in participants receiving the supplement.

The authors concluded that among adults and children aged 12 years or older with poorly controlled asthma while taking a controller medication, use of a soy isoflavone supplement, compared with placebo, did not result in improved lung function or clinical outcomes. These findings suggest that this supplement should not be used for patients with poorly controlled asthma.

This is a good study and a squarely negative finding. As far as I can see, there is no dietary supplement that is of proven effectiveness for asthma. Yet, the hype and bogus claims in this realm continue unabated. This trial provides solid evidence, but will it change much? I doubt it. Supplement manufacturers will simply say: THEY USED THE WRONG PRODUCT/DOSE/TREATMENT SCHEDULE !!! And the exploitation of asthma patients will continue just as before.

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