Your gut bacteria could be changing how many calories you absorb

The calorie count printed on a food package might seem like a straightforward measure of how much energy you will get from eating it. But the number on the label does not necessarily match the calories your body actually absorbs. One reason is that digestion involves more than just breaking down food.

It also depends on trillions of microorganisms living inside your digestive system. Researchers at Arizona State University have developed a mathematical model that explores this often overlooked part of digestion. Known as DAMM, short for digestion, absorption and microbial metabolism, the model tracks how food moves through the digestive tract, how much energy the body absorbs directly, and how gut microbes process whatever remains.

It also estimates which products of digestion are absorbed and which leave the body as waste. By accounting for the relationship between diet, human digestion, and gut bacteria, the researchers hope to improve our understanding of obesity, diabetes, and other metabolic disorders. With further development, the approach could eventually help health care professionals create diets tailored to individual patients.

"Digestion is not just a human process -- it is a collaboration between our bodies and trillions of microbes living in the gut," said Professor Rosa Krajmalnik-Brown. "DAMM gives us a powerful new way to quantify how those microbial partners contribute to human health and energy balance, and also point at the importance of properly feeding our gut microbes." Krajmalnik-Brown is director of ASU's Biodesign Center for Health Through Microbiomes and a professor in the university's School of Sustainable Engineering and the Built Environment.

The research team also included ASU Professor Bruce Rittmann, director of the Biodesign Swette Center for Environmental Biotechnology and a Regents Professor of environmental engineering, along with first author Taylor Davis, an ASU graduate research assistant. Scientists at the AdventHealth Translational Research Institute in Orlando, Florida, collaborated on the project.

The findings were published in PLOS One . Why Traditional Calorie Counts Miss Part of the Picture For more than 100 years, scientists have used a system based on Atwater parameters to estimate how much energy people can obtain from food. This method assigns average calorie values to each gram of fat, carbohydrates, and protein, then uses those values to calculate the total available energy.

Although this approach is practical and widely used, it cannot fully explain what happens during digestion. In particular, it does not account for how different foods nourish gut bacteria or how those bacteria transform undigested material into substances the body can use for energy.

One important example involves short-chain fatty acids. These compounds are produced when microbes in the colon break down fiber and other food components that pass through the upper digestive tract without being absorbed. Some of these fatty acids can enter the bloodstream and provide additional calories.

To better understand these interactions, the researchers built on an earlier controlled diet study investigating how the gut microbiome influences human energy balance. The gut microbiome consists of the enormous community of bacteria and other microorganisms that inhabit the digestive system.

A High-Fiber Diet Led to 116 Fewer Calories Absorbed Per Day In the clinical study, healthy adults followed one of two carefully designed eating plans. The first was a microbiome-enhancer diet containing substantial amounts of fiber and resistant starch (less processed foods and foods with larger particle size).

Resistant starch is a type of carbohydrate that escapes much of the usual digestion in the small intestine, allowing it to reach the colon, where microbes can break it down. The second eating plan resembled a typical Western diet, with lower amounts of fiber and resistant starch (more processed foods and smaller particle size).

Despite both diets being designed for the study, participants absorbed noticeably different amounts of energy. Those consuming the Western diet absorbed approximately 116 more calories per day than those eating the fiber-rich diet. Perhaps just as interesting, the people following the higher-fiber eating plan did not report feeling hungrier.

The findings suggest that the amount of energy people obtain from food depends partly on its composition and how it moves through the digestive system. Differences in food processing, fiber content, and microbial activity can all influence the final outcome. "What is truly unique about the DAMM model is that it quantitatively links human metabolism to the metabolism of the microorganisms in the colon in a way that matches the results from the clinical study and provides fundamental insight into how the microbial community works in partnership with the human host," Rittmann said.

Gut Bacteria Can Supply About 140 Calories a Day The DAMM model offers a more detailed way to follow the energy contained in food from the moment it enters the digestive system. First, it examines the protein, carbohydrate, and fat content of a person's diet and estimates how much usable energy is absorbed in the upper gastrointestinal tract.

It then tracks the remaining material as it moves into the colon. Here, gut microbes break down nutrients that escaped earlier digestion, producing compounds such as short-chain fatty acids. The body can absorb some of these compounds through the colon and use them as another source of energy.

The model also includes methane production by certain microorganisms called methanogens, which generate methane as part of their metabolism. According to the researchers' calculations, short-chain fatty acids absorbed from the colon supplied an average of approximately 140 calories per day.

That represents about 7.4% of the total usable energy obtained from food. More broadly, the model estimated that roughly 85% of usable energy came from the upper gastrointestinal tract, while approximately 15% came from the lower gastrointestinal tract, where microbes are especially important.

These findings highlight why traditional calorie calculations cannot fully describe human digestion. A meaningful share of the energy available to the body is influenced by microbial processes that conventional food-label calculations do not explicitly measure. When the team compared DAMM's predictions with actual measurements from the controlled diet study, the model produced estimates that were closer to the observed calorie absorption than those generated using the conventional Atwater approach.

The older method generally underestimated how many calories participants absorbed. By including microbial metabolism, DAMM provided a more accurate estimate of the energy obtained from the different diets. How Fiber Changes the Calorie Equation One of the model's most revealing features is its ability to account for differences between diets.

The microbiome-enhancer diet delivered more fiber and other fermentable material to the colon. This gave gut microbes additional substances to break down, increasing their production of short-chain fatty acids. DAMM predicted this increase, consistent with the pattern observed in the controlled clinical trial.

Participants consuming the microbiome-enhancer diet had higher levels of short-chain fatty acids in measurements taken from their blood serum and feces. These compounds are important because they illustrate how closely microbial activity and human metabolism are connected. Gut bacteria can transform food components that would otherwise pass through the digestive system into molecules that the body can absorb and use for energy.

However, producing more of these compounds does not necessarily mean absorbing more calories overall. In fact, participants following the microbiome-enhancer diet absorbed fewer net calories than those eating the Western diet, even though the higher-fiber diet supported greater microbial activity and increased short-chain fatty acid production.

This apparent contradiction becomes easier to understand when the different stages of digestion are examined separately. Although gut microbes can generate additional usable energy, the amount absorbed directly from food in the upper digestive tract also matters. DAMM helps researchers distinguish between these contributions.

It estimates how much energy the body extracts before food reaches the colon, how much energy microbes produce from the remaining material, and how much ultimately gets absorbed or eliminated. Toward More Personalized Diets and Better Nutrition By bringing human digestion and microbial metabolism into a single mathematical framework, the researchers have created a tool that could help explain why different diets produce different effects on the body's energy balance.

The approach may eventually improve scientists' understanding of how diet influences obesity, diabetes, and other metabolic conditions. It also raises the possibility of designing nutrition plans that consider not only the nutrients in food but also how those nutrients interact with the microbes living inside each person's gut.

For now, the model provides a research framework rather than a ready-to-use method for prescribing personalized diets. Further studies will be needed to determine how well it performs across different populations, eating patterns, and microbiome compositions. The researchers also expect the model to become more sophisticated as scientists uncover additional details about how microorganisms and the human body exchange nutrients and energy.

"The DAMM model is more than just a tool for characterizing diet," Davis said. "It's a framework designed to evolve. As we discover more on how diet, metabolism and the microbes interact, the new insights can be incorporated into the model, allowing it to grow with us as we learn."