Gut bacteria may absorb 116 fewer daily calories on high-fiber diets, study finds

For more than a century, the numbers printed on food packages have told us roughly how much energy our bodies will extract from what we eat. Those calculations rest on a system developed in the 1800s, one that assigns standard calorie values to fats, carbohydrates, and proteins, then adds them up. It is practical. It is widely trusted. It is also incomplete.

Researchers at Arizona State University have now built a mathematical model that reveals why. The model, called DAMM—short for digestion, absorption, and microbial metabolism—tracks not just what we eat, but what happens to it as it moves through our digestive system and encounters the trillions of bacteria living in our gut. The findings suggest that conventional food labels miss a significant part of the story: the role microbes play in converting food into usable energy.

In a controlled clinical study, the researchers compared two groups of healthy adults. One followed a diet rich in fiber and resistant starch—less processed foods, larger particle sizes. The other ate a typical Western diet, with lower fiber content and more processing. Both diets were carefully designed for the study. Yet the results were striking. People eating the Western diet absorbed approximately 116 more calories per day than those on the high-fiber plan. Notably, the people eating more fiber did not report feeling hungrier. The difference in energy absorption came not from how much they ate, but from how their bodies and their gut bacteria processed it.

The DAMM model works by following food through distinct stages. First, it estimates how much energy the upper gastrointestinal tract absorbs directly—the conventional part of digestion that food labels attempt to measure. Then it tracks what remains as it enters the colon, where the real collaboration begins. Gut bacteria break down fiber and other components that escaped earlier digestion, producing compounds called short-chain fatty acids. Some of these fatty acids enter the bloodstream and provide additional calories. According to the model's calculations, these microbial byproducts supply an average of about 140 calories per day—roughly 7.4 percent of total usable energy. That is a meaningful share, one that traditional calorie counts do not explicitly measure.

The model estimates that roughly 85 percent of usable energy comes from the upper gastrointestinal tract, while approximately 15 percent comes from the lower tract, where microbial activity dominates. When the researchers compared DAMM's predictions against actual measurements from the controlled study, the model proved more accurate than the conventional Atwater method, which generally underestimated how many calories participants actually absorbed.

What makes the high-fiber finding particularly interesting is that it appears paradoxical at first. The microbiome-enhancer diet delivered more fermentable material to the colon, which meant gut bacteria produced more short-chain fatty acids. Blood and stool measurements confirmed this increase. Yet participants on this diet absorbed fewer net calories overall. The explanation lies in understanding digestion as a system with competing forces. Although gut bacteria can generate additional usable energy from material that would otherwise pass through the body, the amount of energy absorbed directly from food in the upper digestive tract also matters. A high-fiber diet may support greater microbial activity while simultaneously reducing the total calories the body extracts from the meal.

Rosa Krajmalnik-Brown, director of ASU's Biodesign Center for Health Through Microbiomes, described digestion not as a purely human process but as a collaboration between the body and its microbial partners. The DAMM model, developed with colleagues including Bruce Rittmann and graduate researcher Taylor Davis, offers a way to quantify that partnership. The work was published in PLOS One and involved collaboration with scientists at the AdventHealth Translational Research Institute in Orlando, Florida.

For now, the model remains a research tool rather than a ready-made method for prescribing individual diets. The researchers acknowledge that further studies are needed to test how well it performs across different populations, eating patterns, and microbiome compositions. But the framework is designed to evolve. As scientists uncover more details about how diet, human metabolism, and microbial metabolism interact, those insights can be incorporated into the model. The long-term possibility is tantalizing: nutrition plans tailored not just to what a person eats, but to how their unique microbial community processes it—potentially offering new approaches to obesity, diabetes, and other metabolic disorders.