Small amounts of fiber inulin are present in a wide variety of fruits and vegetables, including bananas, asparagus, and garlic.
Inulin is also frequently concentrated in commonly available high-fiber dietary supplements.
A recent study from Weill Cornell Medicine found inulin, commonly used in health supplements and known to have certain anti-inflammatory properties, can also promote an allergy-related type of inflammation in the lung and gut.
They found that dietary inulin fiber alters the metabolism of certain gut bacteria, which in turn triggers what scientists call type 2 inflammation in the gut and lungs.
This type of inflammation is thought to have evolved in mammals chiefly to defend against parasitic worm (“helminth”) infections, and is also part of normal wound healing, although its inappropriate activation underlies allergies, asthma, and other inflammatory diseases.
Previous studies have found that inulin boosts populations of beneficial gut bacterial species, which in turn boost levels of anti-inflammatory immune cells called regulatory T (Treg) cells.
In this study, the researchers examined inulin’s effects more comprehensively.
They gave mice an inulin-based, high-fiber diet for two weeks, and then analyzed the many differences between these mice and mice that had been fed a diet lacking inulin.
A major difference was that the inulin diet also induced markedly higher levels of white blood cells called eosinophils in the gut and lungs.
A high level of eosinophils is a classic sign of type 2 inflammation and is typically seen in the setting of seasonal allergies and asthma.
The researchers found that the eosinophil response was mediated by immune cells called group 2 innate lymphoid cells (ILC2s), which were activated by elevated levels of small molecules called bile acids in the blood.
The team then found that deletion of the bile acid receptor abrogates the inulin-induced inflammation, suggesting that microbiota-driven changes in bile acid metabolism underlie the effects of inulin.
The finding that inulin promotes type 2 inflammation does not mean that this type of fiber is always “bad”. The team found that inulin did worsen allergen-induced type 2 airway inflammation in mice.
But the experiments also confirmed inulin’s previously reported effect at boosting anti-inflammatory Treg cells, which may in many cases, outweigh some pro-inflammatory impact.
Moreover, a type 2 immune response, which in the gut and lungs involves increased production of tissue-protecting mucus, is not necessarily harmful in healthy people.
The researchers found in their mouse experiments that inulin-induced type 2 inflammation enhances the defense against helminth infection.
The study was conducted by Dr. David Artis et al and published in Nature.
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