Many people drink orange juice for vitamin C when they feel a cold or flu coming on, but if work on rodents proves transferable, they might get more benefit from eating the rind. The discovery is something of a surprise because previous work led scientists to think that high-fiber diets might actually decrease immunity. Instead, something more complex and intriguing happened.
Dietary fiber might seem unrelated to the immune system, but previous research has indicated a connection. It is thought this occurs through the production of short-chain fatty acids (SCFAs) by gut bacteria in response to soluble fiber in the diet.
"The beneficial effects of dietary fiber and SCFAs on a variety of chronic inflammatory diseases, including asthma and allergies, have received substantial attention in recent years,” said Professor Benjamin Marsland of Monash University in a statement. "But we were concerned that these treatments might lead to a general dampening of immune responses and could increase susceptibility to infections."
Marsland exposed two groups of mice, one on a diet high in the soluble dietary fiber inulin, and the other fed insoluble cellulose, to the influenza A virus. Far from the inulin-eaters getting sicker, as might be expected if the SCFAs suppressed their immune systems, they actually appeared healthier. He told IFLScience; “The control mice huddled together, their fur ruffled, they looked like they had the flu. The mice on the high inulin diet ran around the cage and looked healthy.” Blood and lung tests confirmed appearances.
Surprised, Marsland and his colleagues ran the experiment again with new mice, and a normally lethal dosage of the flu virus. The high-inulin mice got sick but survived.
“Dietary fiber was selectively turning off part of our immune system, while turning on another, completely unrelated part of our immune system," Marsland concluded. The aspects of the immune system that cause inflammation were being dialed down, reducing damage to lung cells, just as occurs for allergies and asthma in the presence of SCFAs. On the other hand, Marsland reported in Immunity, the T-cells that kill cells infected by pathogens became far more active, providing a powerful viral defense.
Marsland stressed to IFLScience it is important to be careful extrapolating from rodent research, but said we know human gut bacteria also produce SCFAs when we increase our soluble fiber consumption. What Marsland hopes to test is how much extra fiber we need to gain a meaningful benefit, and which sorts of soluble fiber work best.
Inulin is already added to some processed foods and is found in garlic, onions, and Jerusalem artichokes. The peel of citrus fruits is a particularly rich source, so break out the marmalade.