The intestinal immune system is an indispensable intermediary in the complex association between diet and metabolism: without it, the dietary fibers present in fruit and vegetables cannot play a proper part in regulating blood sugar levels in the body. Researchers at Inserm and Sorbonne University have now demonstrated that a certain type of immune cell is essential for the beneficial effect of dietary fibre on carbohydrate metabolism. These findings are published in the journal Nature Communications.
The health benefits of dietary fiber, found in particular in fruit and vegetables, are now well documented: it contributes to weight management, carbohydrate and lipid balance in the body, and is thought to play a protective role against colon cancer. According to previous studies, fibers are aided in their task by the intestinal immune system. The immune system comprises different populations of immune cells which, for example, ensure food tolerance or fight infectious agents in the intestinal wall. However, its precise role in relation to dietary fiber remains poorly understood.
A team led by Inserm researcher Emmanuel Gautier at the Cardiovascular and Metabolic Diseases Research Unit (Inserm/Sorbonne University) wanted to find out more. The scientists worked on a mouse model fed a high-fat, low-fiber diet, mimicking a "Western" diet. For four weeks, half of these animals were additionally supplemented with fructo-oligosaccharide (FOS)-type fiber, sold commercially for dietary purposes.
The scientists observed that, although all the animals were overweight, those supplemented with dietary fiber showed improved glucose uptake by the body, resulting in better glycemic control.
The role of different immune cells
To gain a better understanding of the mechanisms involved, the team compared the compositions of the microbiota and the immune system of the intestine of animals supplemented with and without fiber.Mice not supplemented with fiber had an impoverished microbiota with lower bacterial diversity. In addition, at least two immune cell populations were deficient in the intestine: Th17 lymphocytes, involved in protecting the intestinal barrier, and peripheral regulatory T lymphocytes (pTreg) contributing to tolerance of the intestinal microbiota. These alterations suggest a weakening of the local immune system, linked to microbiota depletion.
Conversely, in fiber-supplemented mice, more bacterial species were maintained in the microbiota, in particular bacteria known to stimulate the production of Th17 immune cells. In fact, this lymphocyte population appeared preserved, as did that of pTreg lymphocytes.
This could be explained by the contribution of fiber to the enrichment of the intestinal microbiota in bacterial species that support the differentiation of certain immune cells," explains Adelaide Gelineau, first author of this study. However, the mechanism explaining the association between these bacteria and an enrichment in certain immune cells is not yet fully understood, especially in response to dietary variations ", she adds.
Finally, the team discovered the importance of a third immune cell population called cDC2 dendritic cells. These cells are known to support the development of Th17 cells and participate in pTreg cell function. The research team therefore set out to study their role in the context of a fatty diet with or without fiber supplementation, using a cDC2-deficient mouse model. In this way, they were able to confirm that they are essential to the beneficial effect of fiber on glycemic control.
"Without these cells, fiber intake is insufficient to preserve Th17 cells and correct carbohydrate imbalance. This central role of cDC2 dendritic cells in controlling the immune and metabolic effects of fiber was previously poorly understood", Emmanuel Gautier points out.
Here, with a single ingredient, FOS-type fiber, we succeeded in preserving intestinal flora, local immunity and carbohydrate metabolism in animals on a fatty diet," adds the researcher. With this work, we provide insight into the cellular mechanisms by which dietary fiber has a beneficial impact on glucose metabolism. Understanding these interactions between diet, immunity and metabolism is a prerequisite for advancing knowledge in nutrition, in particular for assessing the impact of diets on the body and establishing recommendations", he concludes of these results, which now need to be confirmed in humans.
