Trillions of invisible partners live in your colon, and they're quietly reshaping how many calories your body actually absorbs from every meal. Arizona State University researchers have now created a mathematical model to quantify what nutritionists have long suspected: the number on a food label tells only part of the story.
The innovation matters because we've relied on the same calorie calculation method for more than a century. Food companies multiply the grams of protein, carbohydrates and fat by standard calorie values per gram—a system so simple it ignores the microbial ecosystem that transforms food after it reaches the colon. But digestion, as Professor Rosa Krajmalnik-Brown points out, is not a solo human act. It's a collaboration between your body and the bacterial community living in your gut.
The new model, called DAMM (digestion, absorption and microbial metabolism), follows food through the entire digestive journey. It estimates what the body absorbs directly in the upper digestive tract, what material reaches the colon intact, and crucially, how gut microbes break down remaining food components into products that either get absorbed as additional calories or excreted. The research, published in PLOS One, builds on a controlled diet study that revealed something striking: when healthy adults ate a Western diet high in processed foods, they absorbed about 116 more calories per day than people eating a microbiome-enhancer diet rich in fiber and resistant starch. Yet the high-fiber group, despite absorbing fewer calories, reported no increase in hunger.
That paradox points to something the old Atwater calorie system completely misses. When gut microbes ferment fiber and other undigested food in the colon, they produce short-chain fatty acids—compounds the body can absorb as energy. A fiber-rich diet feeds these microbial partners differently, changing the entire metabolic equation. "DAMM gives us a powerful new way to quantify how those microbial partners contribute to human health and energy balance," Krajmalnik-Brown said, "and also point at the importance of properly feeding our gut microbes."
The research team included Professor Bruce Rittmann, who directs the Biodesign Swette Center for Environmental Biotechnology, and first author Taylor Davis, an ASU graduate research assistant. Their work was carried out in collaboration with AdventHealth Translational Research Institute in Orlando, Florida. Rittmann emphasized that DAMM is genuinely novel: it quantitatively links human metabolism to microbial metabolism in a way that matches clinical results while revealing how the microbial community functions as a true partner with the human host.
The implications extend far beyond curiosity about calorie counts. By showing how different diets affect both the human body and the microbial community in the colon, DAMM could eventually help researchers better understand obesity, diabetes and other metabolic disorders. With further development, it could become a tool for health care providers to design personalized diets tailored to individual patients' microbiota. The model doesn't replace the Atwater system but contextualizes it—acknowledging that what the label says you're eating and what your body actually absorbs are two very different things.