It is widely believed that the fiber in vegetables and fruits can help the body calm an overactive immune system. An overactive immune system can cause a variety of diseases, such as irritable bowel syndrome and Crohn’s disease, and can even lead to colorectal cancer. Researchers from the University of Lausanne (Switzerland) recently found that a fiber-rich diet can also alter immune cell production in the bone marrow, helping the body fight off asthma. Asthma is an inflammatory disease that causes constriction of the airways in the lungs. A related research paper was published in the recent issue of Nature Medicine. When we eat vegetables and fruits, naturally occurring bacteria in our intestines help us digest fiber. These microbes take up “soluble” fiber (such as pectin in apples, pears, strawberries, citrus and onions) and ferment it to produce specific types of fatty acids. These fatty acids interact with immune cells and help them control inflammation. Previously, it was not clear whether this anti-inflammatory effect could extend beyond the digestive tract. However, these fatty acids do circulate in the bloodstream and are likely to have an effect on immune cells in the body. This means that dietary fiber may also affect other inflammatory diseases, such as asthma. It has been found that fiber consumption in Western countries has slowly decreased since the 1960s, while the incidence of asthma has gradually increased. In less developed regions (such as Africa), where fruits and vegetables are still the mainstay of the diet, asthma is also less prevalent. In this study, the scientists fed mice a diet low in fiber. After two weeks, the researchers exposed the mice to allergens from dust mites, a key trigger in human allergy and asthma. They found that the mice exhibited an excessive asthmatic response, including the appearance of inflammatory material in the lungs and tightening of the airways. Airway constriction is a common symptom in asthmatics and causes shortness of breath and shortness of breath. The researchers fed another group of mice a pectin-rich diet for two weeks before exposing them to dust mite extract. The study showed that the inflammatory response in these mice was reduced, their levels of immune cells (eosinophils) and IgE were almost halved, and the mice’s airway tightness was significantly improved. Eosinophil and IgE levels are usually elevated in the presence of allergy or asthma. The researchers then divided the mice into three groups: normal, low-fiber and high-fiber diets, and analyzed their feces. They found that the bacteria most capable of producing anti-inflammatory fatty acids were more prevalent in the mice fed pectin, about twice as much as in the mice on the low-fiber diet. Further studies showed high levels of anti-inflammatory fatty acids in both the feces and blood of the high-fiber diet group. So is it the fatty acids in the blood that are suppressing the immune system? Is this suppression enough to quell the asthma response? To do this, the researchers injected mice with one of the anti-inflammatory fatty acids, propionate (propionate). After two weeks, inflammatory markers in these mice were reduced and airway tightness was improved in the face of dust mite extract. In addition, dendritic cells in the mice were altered. Dendritic cells are a key immune cell that can both reduce the activity of the immune system and enhance the immune response, depending entirely on the signals that dendritic cells send to other immune cells. The study showed that in mice on a high-fiber diet, dendritic cells were less likely to initiate effector cells, which play a key role in allergic asthma. Finally, the researchers found that the mice injected with propionate generated more precursor cells that differentiated into dendritic cells that fought off asthma. “Our study shows for the first time that diet can affect immune cell production in the bone marrow. This will help us further understand the process by which immune cell precursors leave the bone marrow and spread to the tissues,” Marsland said. The researchers intend to further investigate whether purified propionate or other similar fatty acids could be used as a dietary supplement for asthma patients. They stress, however, that a balanced diet rich in fiber is the best way to fight inflammation.