Your gut microbes might be turning fiber into extra calories

The collection of microbes living in your digestive tract is known as the gut microbiome. While everyone has one, some people's microbiomes produce large amounts of methane, whereas others produce very little.

Microbes and the Energy Hidden in Fiber

The study found that people whose microbiomes generate more methane tend to extract more energy from high-fiber foods. This may help explain why the same meal can provide different calorie counts for different individuals once it reaches the colon.

Researchers emphasized that high-fiber foods remain beneficial. People generally absorb more calories from a typical Western diet high in processed foods, regardless of methane levels. Even so, calorie absorption on a fiber-rich diet varies depending on how much methane a person's gut produces.

These findings suggest that gut methane could become a key factor in personalized nutrition -- a future where diets are tailored to the unique microbial activity in each person's digestive system

"That difference has important implications for diet interventions. It shows people on the same diet can respond differently. Part of that is due to the composition of their gut microbiome," says Blake Dirks, lead author of the study and graduate researcher at the Biodesign Center for Health Through Microbiomes. Dirks is also a PhD student in ASU's School of Life Sciences.

Meet the Methane Makers

Published in The ISME Journal, the study identifies the key players: methane-producing microbes known as methanogens. These microorganisms appear to be linked with more efficient digestion and higher energy absorption.

A major job of the microbiome is breaking down food that the body cannot digest on its own. Microbes ferment fiber into short-chain fatty acids (SCFAs), which provide a valuable energy source. During this process, hydrogen gas is released. Too much hydrogen can slow fermentation, but other microbes prevent this by consuming hydrogen -- keeping the digestive chemistry in balance.

Methanogens are the hydrogen consumers. As they feed on hydrogen, they release methane as a byproduct. They are the only microbes in the human gut that produce this gas.

"The human body itself doesn't make methane, only the microbes do. So we suggested it can be a biomarker that signals efficient microbial production of short-chain fatty acids," says Rosy Krajmalnik-Brown, corresponding author of the study and director of the Biodesign Center for Health Through Microbiomes.

How Microbes May Shape Metabolism

The ASU researchers found that the interactions between these microbes may directly affect metabolism. Participants who produced more methane also had higher levels of short-chain fatty acids, indicating that more energy was being created and absorbed in the gut.

To test these effects, each participant followed two different diets. One included highly processed, low-fiber foods, while the other emphasized whole foods and fiber. Both diets contained equal proportions of carbohydrates, proteins, and fats.

The research was conducted in collaboration with the AdventHealth Translational Research Institute, which provided access to a specialized facility. Each participant spent six days in a sealed, hotel-like room called a whole-room calorimeter. This environment allowed researchers to precisely measure metabolism and methane output.

Unlike traditional methods that rely on a single breath test, this setup continuously captured methane released through both breath and other emissions (ahem), providing a more accurate view of microbial activity.

"This work highlights the importance of the collaboration between clinical-translational scientists and microbial ecologists. The combination of precise measures of energy balance through whole-room calorimetry with ASU's microbial ecology expertise made key innovations possible," says Karen D. Corbin, a co-author and associate investigator at the institute.

Tracking Energy and Microbial Activity

Data collected from blood and stool samples revealed how much energy participants absorbed from their food and how active their gut microbes were. Researchers then compared people with high methane production to those with lower levels.

Almost all participants absorbed fewer calories while eating the high-fiber diet compared to the processed-food diet. However, those with higher methane production absorbed more calories from the fiber-rich foods than those with less methane in their systems.

A Step Toward Personalized Health

The findings lay important groundwork for future studies and medical applications.

This research creates a foundation for future studies and medical treatments.

"The participants in our study were relatively healthy. One thing that I think would be worthy to look at is how other populations respond to these types of diets -- people with obesity, diabetes or other kinds of health states," Dirks says.

Although the study did not aim to induce weight loss, some participants did lose a small amount while following the high-fiber diet. Future research may explore how methanogens influence weight-loss efforts or specialized nutrition programs.

"You can see how important it is that the microbiome is personalized," Krajmalnik-Brown says. "Specifically, the diet that we designed so carefully to enhance the microbiome for this experiment had different effects on each person, in part because some people's microbiomes produced more methane than others."

Other members of the ASU research team include Professor Bruce Rittmann and graduate researcher Taylor Davis.

This project was funded by the National Institute of Diabetes and Digestive and Kidney Diseases of the National Institutes of Health.