We now know that a vast community of microbes lives in our gastrointestinal tract, aiding in the digestion of food. That gut microbiome also has a significant impact on our health and well-being, and researchers are learning more about how those things are connected. New work has focused on the upper digestive tract, which is not well-studied. The work, reported in Cell Host & Microbe, has shown how a Western-style diet, rich in calories, can increase the number of bacteria that absorb fats from food.
It has been shown that within two days of eating high-fat foods, microbes in the bowels multiply. They apparently facilitate the release of enzymes that break down fats in the diet as well as bioactive molecules. That stimulates cells in the intestine to get those fats ready for absorption. These microbes may cause over-nutrition and obesity after awhile.
"These bacteria are part of an orchestrated series of events that make lipid absorption more efficient," said the senior author of the work, Eugene B. Chang, MD, the Martin Boyer Professor of Medicine and director of the NIH Digestive Diseases Research Core Center at the University of Chicago Medicine. "Few people have focused on the microbiome of the small intestine, but this is where most vitamins and other micronutrients are digested and absorbed."
"Our study is one of the first to show that specific small-bowel microbes directly regulate both digestion and absorption of lipids," he added. "This could have significant clinical applications, especially for the prevention and treatment of obesity and cardiovascular disease."
To learn more about the microbes that are involved in fat absorption, the researchers used germ-free mice, which don’t carry any microorganisms, as well as mice that carry non-pathogenic gut bacteria, called "specific pathogen free (SPF)” mice.
The investigators fed the mice a diet high in fats and determined that the germ-free mice could not digest or absorb any fatty foods, and did not gain any weight. They did have elevated levels of fats, or lipids, in their stool. The SPF mice gained weight on the high-fat diet though, and certain microbes in their guts proliferated.
In the SPF mice, the team found that a microbe from the Clostridiaceae family specifically impacted fat absorption. The levels of other bacterial families like Bifidobacteriacaea and Bacteriodacaea were reduced when exposed to the high-fat diet; those microbes are also typically associated with being lean.
The researchers exposed the germ-free mice to the microbes facilitating fat absorption, and they were then able to absorb lipids.
"Our study found that, at least in mice, a high-fat diet can profoundly alter the microbial makeup of the small intestine," Chang said. "Certain dietary pressures, such as calorie-dense foods, attract specific bacterial strains into the small intestine. These microbes are then able to allow the host to digest this high-fat diet and absorb fats. That can even impact extra-intestinal organs such as the pancreas."
The authors believe that this work will aid in developing therapies that manipulate microbial levels, and can combat obesity.
"I would say the most important takeaway overall is the concept that what we eat--our diet on a daily basis--has a profound impact on the abundance and the type of bacteria we harbor in our gut," said the lead author Kristina Martinez-Guryn, Ph.D., now an assistant professor at Midwestern University. "These microbes directly influence our metabolism and our propensity to gain weight on certain diets."
This research is still in the preliminary stages, but she added that "our results suggest that maybe we could use pre- or probiotics or even develop post-biotics (bacterial-derived compounds or metabolites) to enhance nutrient uptake for people with malabsorption disorders, such as Crohn's disease, or we could test novel ways to decrease obesity."
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