In the past 50 years, as fruits and vegetables have featured less and less in the Western diet, rates of allergic asthma have gone up. Now a new study suggests these trends are not coincidental, but causally linked.
Funded by the Swiss National Science Foundation (SNSF), and led by Benjamin Marsland, an assistant professor at Lausanne University Hospital (CHUV) in Switzerland, researchers report their work in a recent online issue of Nature Medicine.
Using laboratory mice, they found that when gut bacteria digest dietary fiber, such as that contained in fruits and vegetables, they release fatty acids into the bloodstream, and these affect how the immune system behaves in the lungs.
The finding builds on knowledge that has been around for some time: that having a rich and diverse mix of microbes in the gut that digests and ferments fiber, helps prevent cancer of the intestines.
Prof. Marsland says:
"We are now showing for the first time that the influence of gut bacteria extends much further, namely up to the lungs."
For their study, he and his colleagues tested three groups of lab mice. They put one group on a low-fiber diet, comparable to a Western diet, averaging no more than 0.6% fiber, another group on a standard diet comprising 4% fermentable fiber, and the third group on a standard diet enriched with fermentable fibers.
To provoke an allergic response, the researchers exposed the mice to an extract of house dust mites.
Mice on low-fiber diet had stronger allergic reaction to dust mites
They found that the mice on the low-fiber diet had a much stronger allergic reaction - with more mucus in the lungs - than the mice on standard diet with more fiber.
And the mice on the enriched fiber diet showed an even stronger protective effect than the mice on the standard diet.
Researchers found that mice on a high fiber diet had a reduced allergic reaction to dust mites, suggesting that high fiber foods, such as fruit and vegetables, may have a protective effect against asthma.
On further investigation, the team found that the protective effect is the result of a series of reactions that ensue when the fibers reach the intestines, are fermented by gut bacteria and transformed into short-chain fatty acids.
The short-chain fatty acids enter the bloodstream and affect the development of immune cells in the bone marrow. Once the dust mites are detected in the lungs, the immune cells are summoned, where they trigger an allergic response, the strength of which depends on the effect of the short-chain fatty acids.
Prof. Marsland suggests these findings are clinically relevant on two counts: the low-fiber diet of the mice with the strongest allergic response is comparable to our Western diet, and also because the parts of the immune system involved in mice is almost indistinguishable from that of humans.
Although he says there are still a lot of questions to resolve before we can be certain how a diet enriched with fermentable fibers affects allergies and inflammations, he believes this evidence gives another reason for us to eat more fruits and vegetables.
There is also evidence that when children are exposed to dogs in the home early in infancy, their risk for allergies and asthma goes down. Now, another mouse study published recently in the Proceedings of the National Academy of Sciences suggests this is also linked to gut microbes.