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Gut bacteria utilize dietary fiber to release beneficial nutrient with positive effects on metabolism

Researchers have demonstrated that certain human gut microbes can mine dietary fiber to extract a beneficial nutrient that otherwise would remain inaccessible to the human body.

Oranges and orange slices on a white background

Many factors—including population growth, climate change, and societal disruptions caused by the COVID-19 pandemic—have focused attention on food production and the massive amount of waste generated during food manufacturing. Fiber byproducts such as peels, rinds, and seeds discarded from fruits and vegetables have potential nutritive value and may be an untapped source of biomolecules that promote human health. Here, a team of researchers first studied groups of male mice harboring common human gut microbes as well as “germ-free” mice that were raised and maintained without any microbes. The mice were fed a high-fat, low-fiber diet, characteristic of what many people in the United States and other developed countries eat, with or without supplementation of fiber from oranges. The researchers discovered that when mice were fed orange fiber, a molecule called N-methylserotonin was released in high abundance—but this only happened in mice that harbored the collection of human gut microbes. Testing dozens of different human gut bacteria and utilizing innovative laboratory analyses, they identified a bacterial species that can produce enzymes that act like molecular scissors to break down the fiber and release the nutrient. When the researchers added N-methylserotonin to the drinking water of germ-free mice on a high-fat, low-fiber diet without any fiber supplementation, the mice had reduced body fat, improved sugar metabolism in the liver, and more rapid gut motility, suggesting beneficial effects of this biomolecule. Importantly, in a small, all-female, human study, the researchers showed that people who consumed orange fiber snacks had increased levels of N-methylserotonin in their stool samples compared to people who ate pea-fiber containing snacks, indicating the release of this molecule occurs in humans and is fiber specific. Moreover, the study participants who ate the orange snacks also had increased levels of gut microbial genes that produce enzymes that break down fibers entrapping N-methylserotonin and allow for its release.

Although more research is needed to better understand the actions of N-methylserotonin in humans, this study highlights the relationships between food science, nutrition, and the microbiome. It also identifies a potential framework for establishing affordable and sustainable sources of dietary fiber—and nutrients within these—by utilizing food production byproducts that would otherwise be discarded. These results indicate potential therapeutic applications such as supplementing the human diet with select fibers for personalized nutrition.

Han ND, Cheng J, Delannoy-Bruno O, …Gordon JI. Microbial liberation of N-methylserotonin from orange fiber in gnotobiotic mice and humans. Cell 185: 2495-2509, 2022.

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