The fibre consumed in fruits and vegetables seems to help quieten the overzealous immune system activity that leads to conditions like irritable bowel syndrome, Crohn's disease and possibly even colon cancer. Now it appears that a diet rich in fibre may also fend off asthma by changing the way some immune cells are made in the bone marrow.
When we eat plentiful fruits and vegetables, the bacteria that occur naturally in our intestines help us digest the fibre. The microbes take "soluble" fibre 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 fibre influences other inflammatory diseases, such as asthma. It's known that asthma has increased in westernised countries since the 1960s, during which time the amount of fibre 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-fibre diet. After two 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.
On the other hand, mice that ate a diet rich in pectin for two weeks before getting the dust mite extract showed a reduced inflammatory response.
To see if the gut bacteria were responsible for the fibre-mediated benefits, the scientists analysed the faeces of mice on normal, low- and high-fibre 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-fibre diet. 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.
The researchers then injected the mice with propionate, a fatty acid. After two weeks, the rodents again showed reduced inflammatory markers and less constriction of the airways in response to the dust mite treatment, the team reported online 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-fibre 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.
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 said.