Calories can look simple on a food label. A serving has a number, and that number is based on how much protein, fat and carbohydrate the food contains.
Inside the body, the story is more complicated. A new mathematical model from Arizona State University suggests that how many calories people absorb from food may depend not only on the food itself, but also on what happens as that food moves through the digestive tract and interacts with gut microbes.
The model, called DAMM, short for Digestion, Absorption and Microbial Metabolism, was published in PLOS One. It estimates how much energy is absorbed directly in the upper digestive tract, how much food material reaches the colon and how gut microbes process that remaining material into compounds that may be absorbed or excreted.
The findings do not mean calorie labels are meaningless or that gut microbes override the basics of energy balance. Instead, the model adds detail to a familiar idea: the calories listed on a package are useful estimates, but human digestion is not a simple math equation.
“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, director of the Biodesign Center for Health Through Microbiomes at Arizona State University. “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.”
For more than a century, scientists have relied on Atwater factors to estimate the energy people get from food. The system assigns average calorie values to protein, carbohydrate and fat. It is practical and widely used, but it does not fully capture what happens when food reaches the gut microbiome, the community of bacteria and other microbes living in the digestive tract.
That matters because some food components, especially fiber and resistant starch, are not fully digested in the upper gastrointestinal tract. Instead, they can travel to the colon, where gut microbes ferment them and produce short-chain fatty acids. Some of those fatty acids can be absorbed and used by the body as energy.
In the new model, researchers estimated that short-chain fatty acids absorbed from the colon contributed an average of about 140 calories per day, or about 7.4% of total usable energy. The model estimated that about 85% of usable energy came from the upper gastrointestinal tract, while about 15% came from the lower gastrointestinal tract, where microbial activity plays a larger role.
The model was tested against data from a controlled diet study in healthy adults. In that earlier study, participants ate two carefully designed diets: a microbiome-enhancer diet rich in fiber and resistant starch, and a more typical Western-style diet lower in those components. The diets were designed to provide similar calories and proportions of protein, fat and carbohydrates.
Even so, participants absorbed fewer calories on the fiber-rich diet. According to Arizona State University, people eating the Western-style diet absorbed about 116 more calories per day than those eating the high-fiber diet, while the high-fiber group did not report feeling hungrier.
That finding helps explain why the structure and quality of food may matter beyond the calorie number alone. A less processed, fiber-rich diet may deliver more material to the colon, where microbes use some of that energy for their own growth and activity. A more processed diet may be absorbed more efficiently earlier in digestion.
The DAMM model also predicted higher short-chain fatty acid production on the microbiome-enhancer diet, which matched the general pattern seen in the clinical study. That detail is important because short-chain fatty acids are not simply “extra calories.” They are part of a broader microbial process that can influence how energy moves through the gut.
“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,” said Professor Bruce Rittmann, director of the Biodesign Swette Center for Environmental Biotechnology at Arizona State University.
The model could eventually help researchers better understand obesity, diabetes and other metabolic conditions by showing how different diets affect both the body and the gut microbiome. It may also support future work on more personalized nutrition approaches.
For now, though, DAMM is a research tool, not a consumer test or a diet plan. It does not prove that one person will lose weight on a certain diet based on their microbiome, and it does not replace basic nutrition guidance. The study also does not show that people should ignore calorie labels.
Instead, it points to a more realistic view of food and metabolism. Calories still matter, but food form, fiber, processing and gut microbes may influence how those calories are handled inside the body.
“The DAMM model is more than just a tool for characterizing diet,” said Taylor Davis, first author of the study and an Arizona State University graduate research assistant. “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.”
This project was funded by the National Institute of Diabetes and Digestive and Kidney Diseases of the National Institutes of Health.