- When gut microbes break down prebiotic dietary fiber, various molecules can be formed that can modulate the immune system.
- However, scientists do not yet understand the inflammatory effects that can arise when the body metabolizes specific types of dietary fiber.
- A recent study in mice showed that an inulin supplement changed the metabolism of certain gut bacteria, resulting in increased serum levels of bile acids.
- The researchers found that inulin triggered a specific type of inflammatory response, typically seen in response to allergies and parasitic worms, in the lungs and colon.
a recent to study published in Nature suggests that dietary fiber inulin can alter the composition and metabolism of gut microbiotaresulting in a type 2 inflammatory response, typically seen in response to allergic respiratory conditions.
The study questions the relationship between diet, immunity and generally beneficial prebiotics, in this case inulin, a dietary fiber commonly used in anti-inflammatory supplements.
Commenting on the study findings, Dr Sarkis Mazmaniana microbiologist from the California Institute of Technology said:
“The results are conflicting based on previous literature showing anti-inflammatory properties of dietary fiber, although how microbiota-derived metabolites shape type 2 immunity is likely complex and not well understood.”
Dr. Mazmanian said the study offers new insights into how dietary fiber, gut bacteria and the immune system interact.
“Extrapolation to humans, either in terms of biological relevance or potential interventions, remains limited. However, the study raises intriguing new insights that will require replication and further investigation into the mechanisms of action,” he said. Today’s medical news.
The gut microbiota plays an important role in human health and disease. The gut microbiota can modify dietary components and molecules that the body produces, such as bile acids, to produce a wide range of biologically active compounds. These metabolites, the intermediate or end products of bacterial metabolism, can influence host metabolism and immunity.
Diet can influence the composition of the gut microbiome which, in turn, can influence the production of these metabolites.
For example, certain types of dietary fiber can act as prebiotics, foods that can influence the growth and activity of the gut microbiota to produce beneficial health effects. These prebiotic dietary fibers cannot be digested by human enzymes in the small intestine and are broken down by fermentation by gut microbes in the large intestine.
Fermentation of undigested dietary fiber by intestinal bacteria produces metabolites such as short-chain fatty acids, including acetic acid, butyric acid, and propionic acid. Previous studies have shown that these short-chain fatty acids may have a beneficial impact on metabolism and produce anti-inflammatory effects.
However, in addition to short-chain fatty acids, the breakdown of dietary fiber by the gut microbiota also produces other metabolitesincluding branched-chain amino acids, indoles, and bile acids.
There are multiple different types of dietary fiber, including inulin, cellulose, and lignin, and each type of dietary fiber affects the composition of the gut microbiota and the production of microbiota-derived metabolites differently.
Although researchers have extensively examined the effects of short-chain fatty acids on the metabolic and immune systems, the effects of consuming a diet rich in a specific type of dietary fiber on the immune system are not well understood.
In the current study, researchers examined the impact of a diet enriched in inulin, a type of soluble dietary fiber, on the composition and inflammation of the gut microbiota.
Inulin is a storage carbohydrate in various plants and is made up of repeating units of fructose. For example, inulin is present in bananas, onions, artichokes, garlic, wheat, oats, garlic, and radicchio.
Inulin is not digested in the small intestine and is a prebiotic fiber. Animal studies suggest that inulin can stimulate the production of short-chain fatty acids and improve lipid and glucose metabolism.
Furthermore, there is evidence to suggest that dietary supplementation with inulin can promote weight loss in humans. As a result, inulin is used as a dietary supplement and as a bulking agent in processed foods.
Previous studies have shown that inulin can increase levels of regulatory T cells which can suppress the response of the immune system. In the current study, the researchers further examined the impact of inulin on the immune system of mice.
To examine the impact of inulin on the gut microbiota, the researchers used mice that were maintained on an inulin-rich diet or a calorie-equivalent control diet for two weeks.
The inulin-rich diet caused an increase in bacteria belonging to the Bacteroidetes phylum and a decrease in those of the Firmicutes phylum.
The inulin-rich diet also caused changes in the serum levels of several microbiota-derived metabolites. The greatest change in mice maintained on an inulin-rich diet was seen in serum bile acid levels.
Bile acidsthe main constituents of bile, are produced by the liver and can facilitate the digestion and absorption of fats in the small intestine.
“[The study reveals that] A modified fiber diet modulates type 2 immunity through effects on bile acids, defining a new diet-microbiome axis that affects inflammation in the gut and lungs of mice.”
— Dr. Sarkis Mazmanian
After their synthesis from cholesterol, bile acids are conjugated with the amino acids glycine and taurine in the liver. On traveling to the intestine, bile acids can be transformed by enzymes produced by intestinal bacteria. For example, the enzyme bile salt hydrolase secreted by the gut microbiota deconjugates bile acids from glycine and taurine in the small intestine.
In the current study, the researchers found an increase in the levels of the main types of unconjugated bile acids, such as cholic acid, in the serum.
Previous studies have shown that bile acids can modulate the immune response. Therefore, the researchers examined the effects of inulin on the immune response.
Consistent with previous studies, inulin produced a modest increase in regulatory T cell levels. Furthermore, mice maintained on an inulin-rich diet showed an increase in the level of eosinophilsa type of white blood cell, in the colon and lungs.
These elevated levels of eosinophils are a feature of a specific type of immune response called type 2 inflammationwhich is typically seen in allergies, asthma and eczema.
Furthermore, inulin also caused an upregulation of immune cells and inflammatory proteins involved in mediating a type 2 inflammatory response.
The researchers then examined the role of the gut microbiota in mediating the type 2 inflammatory response induced by inulin. They found that germ-free mice, that is, mice that lacked the microbiota, kept on an inulin-rich diet did not show increased levels of eosinophils in the colon.
Furthermore, germ-free mice inoculated with a single bacterial strain and fed an inulin-rich diet showed elevated levels of serum cholic acid and promoted a type 2 inflammatory response. This inulin-induced type 2 inflammatory response was abolished in germ-free mice inoculated with the same bacterial strain lacking a functional bile salt hydrolase enzyme.
In addition, oral administration of cholic acid, but not short-chain fatty acids, for 2 weeks also elicited a type 2 inflammatory response similar to that seen with inulin.
These data show that the gut microbiota is required to mediate the effects of inulin on type 2 inflammation.
Furthermore, the requirement for a functional bacterial bile salt hydrolase enzyme to trigger the type 2 inflammatory response suggests an essential role for microbial bile acid metabolism in mediating inulin-induced type 2 inflammation.
The inulin-rich diet also caused changes in microbiota composition, increased serum bile levels, and activation of type 2 inflammatory response in germ-free mice transplanted with human microbiota.
These effects were similar to those seen in mice with a diverse microbiota maintained on an inulin-rich diet. This shows that inulin had a similar impact on the human and mouse microbiota.
The researchers found that inulin consumption led to a more severe type 2 inflammatory response in mice that responded to allergens, such as house dust mites and the food additive papain.
For example, exposure to house dust mites caused an enhanced type 2 inflammatory response in the lungs and increased airway resistance, indicating decreased lung function.
In contrast to these adverse effects, the elevated type 2 inflammatory response due to inulin had a protective effect against parasitic worms, resulting in their more rapid clearance.
Therefore, although inulin could potentially exacerbate allergic reactions in susceptible people, a diet rich in inulin does not necessarily have a negative impact in healthy people.
The author of the study Dr Mohammad Arifuzzamanpostdoctoral researcher at Weill Cornell Medical College, noted: “It could be that this inulin-to-type 2 pathway of inflammation represents an adaptive and beneficial response to infection by endemic helminth parasites, although its effects in a more industrialized, helminth-free environment they are more complex and more difficult to predict.”
molly rapozoregistered dietitian, nutritionist, and brain health coach at the Pacific Neuroscience Institute at Providence Saint John’s Health Center in Santa Monica, CA, said dietary fibers can be beneficial in multiple ways, and “so the positive effects could certainly outweigh any negative.” “.
However, he cautioned that more research is needed, especially in humans.
“The inflammatory effect of inulin in this study does not necessarily outweigh the benefits of dietary fiber in general, however, this is an opportunity to look at our fiber sources of inulin,” he said.
He also advised choosing whole food sources of prebiotic fibers like inulin, rather than choosing highly processed foods and supplements.
Get inulin from food
“Choose more whole-food sources of inulin, such as Jerusalem artichokes, onions and leeks, garlic, chicory root (in coffee and coffee substitutes), dandelion greens, jicama, asparagus, flaxseed, oats, wheat, and barley ”.
— Molly Rapozo, registered dietitian
