The supplements industry has embraced claims suggesting a link between soy intake and decreased asthma severity,  but a randomized, double-blind study, where half of the participants took a soy isoflavone supplement twice daily for six months, and the other half took a placebo,  found that soy supplements do not improve lung function for patients with asthma. 

Supplements are an unregulated, multi-billion industry, and it got that way using bold claims about treating and preventing disease, but they do it without real data. Instead they rely on epidemiological curve matches, like that Chinese people eat more soy and have less asthma. There is evidence that supplements for soy isoflavone -- plant-based compounds in food such as tofu and edamame -- protect against hot flashes during menopause and osteoporosis, but there is no miracle food.

"You are what you eat, but that's a whole constellation of foods, not just a single food or a single component of a food," says Dr. Lewis Smith, professor of medicine at Northwestern University Feinberg School of Medicine and first author of the study in JAMA. 

While analyzing the results of a study on diet and asthma, Smith and colleagues previously noticed that asthmatics taking soy isoflavone had better lung functioning than their counterparts. They confirmed the observation in a different group of patients, and followed up in the laboratory: In cell cultures, they saw that an isoflavone called genistein reduces eosinophil inflammation, a key factor in asthma.  Asians who consume more soy products do have less asthma but that could be due to many different factors. Still, since so many people suddenly took it up as a fad, it merited investigation.

The new study explored the effects of soy in 386 adults and children aged 12 or older with poorly controlled asthma. All were taking medicine to treat their asthma -- either corticosteroids or leukotriene modifiers -- but none were consuming soy.

"We found that the supplement, though able to increase blood levels of the key soy isoflavone genistein, did not improve lung function, symptoms or measures of inflammation in these individuals," Smith said.

Why didn't the soy-asthma link claimed in epidemiology papers show up? They probably chose to ignore confounding factors such as diet and lifestyle patterns. Though genistein reduced inflammation in cell cultures, in the human body additional cells may nullify that benefit. 

"This study highlights why it is so important to perform well-designed, placebo-controlled studies when associations are reported between specific nutrients and disease outcomes," Smith said.

Supported by grants U01HL087987, U01HL0088367 and U54TR001018 from the National Heart, Lung and Blood Institute of the National Institutes of Health and by the American Lung Association.