The fiber consumed in fruits and vegetables seems to help quiet the overzealous immune system activity that leads to such conditions as irritable bowel syndrome, Crohn's disease, and possibly even colon cancer. Now it appears that a diet rich in fiber may also fend off asthma, an inflammatory condition that constricts the airways of the lung, by changing the way some immune cells are produced in the bone marrow.
When we eat plentiful fruits and vegetables, the bacteria that occur naturally in our intestines help us digest the fiber. The microbes take "soluble" fiber such as pectin—found in apples, pears, berries, citrus fruits, and onions—and ferment it into specific types of fatty acids that interact with immune cells, helping keep inflammation in check. Whether this anti-inflammatory effect extends beyond the digestive tract is less clear. But the fatty acids in question are able to circulate through the bloodstream, perhaps hooking up with immune cells throughout the body.
That could mean that dietary fiber influences other inflammatory diseases, such as asthma. It's known that asthma has increased in westernized countries since the 1960s, during which time the amount of fiber consumed has also declined. Moreover, asthma is not as common in less well-developed areas, such as Africa, where fruits and vegetables form a bigger part of the diet.
To test a possible link, immunologist Benjamin Marsland of the University of Lausanne in Switzerland and colleagues put a group of mice on a low-fiber diet. After 2 weeks, the researchers had the animals sniff an allergen derived from dust mites (a key trigger of human allergy and asthma). These mice showed exaggerated asthmatic responses, including inflammatory compounds in the lungs and the constricted airways that cause the wheezing and shortness of breath so familiar to asthmatic patients.
On the other hand, mice that ate a diet rich in pectin for 2 weeks before getting the dust mite extract showed a reduced inflammatory response. Levels of the immune cells known as eosinophils, and of the antibody immunoglobulin E—both usually increased in allergies and asthma—were almost halved, and the mice showed less constriction of their airways.
To see if the gut bacteria were responsible for the fiber-mediated benefits, the scientists analyzed the feces of mice on normal, low-, and high-fiber diets. In the animals given pectin, the kinds of bacteria best able to produce the anti-inflammatory fatty acids were about twice as prevalent as those of other bacteria more common in a low-fiber diet. On closer examination, the researchers found proportionally higher amounts of the fatty acids not only in the stool of the pectin-eating mice, but also in their blood.
Were the fatty acids in the bloodstream telling the immune system to back off, and was this message enough to call off an asthma attack? To find out, the researchers injected the mice with propionate, one of those fatty acids. After 2 weeks, the rodents again showed reduced inflammatory markers and less constriction  of the airways in response to the dust mite treatment, the team reports online today in Nature Medicine. What's more, key immune cells called dendritic cells behaved differently. Dendritic cells can either scale down immune system activity or ramp up the response, depending on the signals they send to other types of immune cells. In mice on a high-fiber diet, the dendritic cells were less able to turn on the so-called effector cells, which are key players in allergic asthma in mice and humans.
In the final phase of the experiment, the researchers found that the mice given propionate were actually producing more of the immature "precursor" cells that develop into the dendritic cells that protected against asthma. "Our study is the first to show that diet can influence the production of immune cells in the bone marrow, which could have major implications given that immune cell precursors leave the bone marrow and spread to tissues throughout the body, including the lung," Marsland says.
According to Gary Huffnagle, an immunologist at the University of Michigan, Ann Arbor, researchers had expected that if compounds produced by bacteria did influence asthma, they would do so in lung tissue. The chain of events connecting dietary changes, altered metabolism of gut bacteria, a shift in immune cell production in the bone marrow, and relief of asthmatic inflammation is an exciting development, he says. “No one has ever put that all together before. The study is a beautiful convergence of observations."
Rigorous scientific work needs to be done, Marsland believes, to test whether dietary supplements including purified propionate, or some similar fatty acid, might be beneficial for people with asthma or for those who don't have access to fruits and vegetables. In the meantime, he says, a balanced diet rich in fiber is the best way to get the anti-inflammatory benefit.