Most likely, we’ve all heard this before: eating a high-fiber diet is good for you. But now a new study published in Nature Communications reveals some potential mechanisms for WHY. In a nutshell, when it comes to calories from high-fiber whole foods versus ultra-processed junk food, the latter are more quickly absorbed in your upper GI tract, meaning that your body gets more calories from them. High-fiber foods don’t get absorbed as easily, so they are able to make it further down the digestive tract to the large intestine, where, lying in wait, are trillions of bacteria that make up your gut microbiome. When you eat a fiber-rich diet, you’re not just feeding yourself but your intestinal microbes as well, effectively reducing the body’s calorie intake. A caloric “tug-of-war” exists between body and biome and the more we understand it, the better we can eat and the healthier we can be!
Researchers wanting to understand how the gut microbiome regulates weight and metabolic health designed a small but rigorous clinical trial. The typical Western diet includes foods like white bread, cheese, ground beef, cold cuts and sugary snacks and fruit juices. The “microbiome enhancer” diet includes high-fiber foods like oats, beans, lentils, steak, whole grains, nuts (whole nuts, not nut butter), fruits and vegetables. In the latter case, gut microbes break the food down through a process called fermentation and create healthy by-products which are good for metabolic health.
The study recruited healthy men and women who were fed both diets for 22 days (so, 22 days on one diet; 22 days on the other). For half the time, participants lived in the lab where scientists could track everything eaten, while also controlling physical activity levels. They also spent 6 days in a tiny, airtight metabolic chamber to determine how many calories were burned, while collection and analysis of bowel movements provided insight to amount of energy and bacteria expelled in the waste.
While the two diets were polar opposites, they provided each participant with the same amount of calories and similar amounts of protein, fat and carbohydrates. One diet (the Western one) was completely digested while the other (high-fiber one) made it down to the colon, feeding the microbes. The results showed that participants absorbed significantly fewer calories on the fiber-rich diet compared to the processed diet. The “lost” calories showed up in stool as significantly more bacterial “biomass” and short-chain fatty acids, a sign that the gut microbes were multiplying and fermenting.
It takes energy to make bacteria and, instead of that energy going to us, it expands the microbial community. That creates benefits for us, including increases in a hormone called GLP-1, which promotes a feeling of fullness. In the end, participants on the fiber-rich diet lost more weight and body fat and, despite absorbing fewer calories, did not show signs of increased hunger. The next steps include expanding the study to older adults, those with metabolic diseases, and those with obesity.