The relationship between soy and asthma has attracted attention for a long time, given the perceived nutritional benefits of soy with its antioxidant (isoflavones) and ALA omega-3 fatty acid effects, and the inflammatory nature of asthma. Recently a larger multicenter study was done in Northwestern University. Results were mixed, but did not appear to show that those who received isoflavone supplements in their food had improved lung function.
One of the criticisms of the study is that by selecting those with poorly controlled asthma, the study’s goal to see soy as a therapeutic agent rather than a preventive one as it is known for, missed the mark. The other criticism is that the endpoints did not include measuring inflammatory markers, and that would make more sense as a measure of soy’s dietary impact on progression of asthma.
How is inflammation related to asthma?
Inflammation is a reaction of tissue to injury whether caused by infection or trauma. This reaction is promulgated by substances that are known as inflammatory mediators, or cytokines. Sometimes, this inflammatory reaction in the body is desirable for defense and repair. Sometimes it is the source of diseases, such as arthritis, gastrointestinal diseases, and even cancer, especially when those pro-inflammatory mediators circulate without a cause. In the case of asthma, it is inflammation that is responsible for the persistent thickening of the inner mucosal lining of the lungs, and narrowing of the airways, impeding air flow. This is actually the source of the shortness of breath in asthma.
What is soy?
Soy is a legume that surprisingly contains many elements that offer anti-inflammatory properties. It is a complete protein with the full nine essential amino acids (those are the amino acids that the body needs from outside sources to make the rest of the amino acids needed to make the proteins in the body ), it has omega-3 polyunsaturated fatty acids (PUFA) and it also contains isoflavones, which are substances that have hormone-like modulating activities.
The omega-3 PUFAs are thought to have the most potent anti-inflammatory properties, with evidence that it acts against interleukins IL1 responsible for diseases such as Lupus and Crohn’s disease. It is known, for example, that foods containing soy inhibit the known inflammatory “Nuclear Factor Kappa B” implicated in prostate cancer. The growing popularity of the “anti-inflammatory diet,” is built around the notion of replacing foods that aggravate the inflammatory process with those that decrease inflammation. Foods in this diet include green leafy and bright colored vegetables, nuts and seeds, whole grains, and soy derivatives.
It’s important to note that the isoflavones in soy act on specific estrogen receptors that down regulate the effects of the other estrogen receptors known to stimulate certain cancers, like breast cancer. The American Cancer Society therefore supports consuming two to three servings of soy daily.
In all fairness, there are ongoing arguments against soy as well. As a food that has so many components, there is the risk that some individuals are allergic to soy. Soybean oil, which does not contain the proteins, is less likely to cause allergies. It is the proteins in nuts (and in this case a legume) that may cause allergic reactions. Every individual should be carefully assessed for possible untoward reactions.
What are the implications of this discussion?
Now that we are aware of the importance of persistent inflammatory substances and their effect on diseases, it would make sense to take all measures to avoid exposure to these substances and to reduce their activity and progression. This may include supplemental products that contain antioxidants but more simply, dietary modification like replacing animal-based proteins with a plant-based protein like soy, may accomplish both reduction of exposure and improve a person’s inflammatory defenses.
Eli Hendel, M.D., is a board-certified internist/pulmonary specialist with board certification in Sleep Medicine. An Assistant Clinical Professor of Medicine at Keck-University of Southern California School of Medicine, and Qualified Medical Examiner for the State of California Department of Industrial Relations, his areas include asthma, COPD, sleep disorders, obstructive sleep apnea, and occupational lung diseases. Favorite hobby? Playing jazz music. Find him on Twitter @Lung_doctor.