Gut Bacteria: How Do They Change Calorie Absorption?

How do gut bacteria influence calorie absorption?

October 12 coverage has renewed attention to research connecting microbial metabolism with human digestion. The mathematical study appeared in PLOS One in May, while the human feeding trial underlying it was published in 2023. Distinguishing these dates matters: the latest headline concerns a model that helps explain earlier observations, rather than a newly completed weight-loss trial. [ScienceDaily coverage](https://www.sciencedaily.com/releases/2026/10/261011211613.htm).

The Digestion, Absorption, and Microbial Metabolism model, or DAMM, follows nutrients through the digestive tract. Material that escapes absorption in the small intestine can reach microbes in the colon, where fermentation produces short-chain fatty acids that the body can absorb. The model also tracks energy leaving as waste. Its predictions agreed more closely with the trial measurements than conventional Atwater calculations, which use average energy factors for nutrients. This offers a more detailed account of digestion, although the authors say clinical applications require testing in studies of weight change. [PLOS One study](https://doi.org/10.1371/journal.pone.0347668).

What did the controlled diet study actually show?

The randomized crossover trial involved 17 healthy, weight-stable adults, each eating both study diets in a randomly assigned order. Each diet period lasted 22 days, including time in a metabolic ward. The microbiome-focused diet emphasized fiber, resistant starch, larger food particles and limited processing. Its comparator contained less fiber and more processed foods. Researchers measured food intake, energy expenditure and energy leaving the body, allowing comparisons within the same person. [Trial methods and model validation](https://doi.org/10.1371/journal.pone.0347668).

On average, the microbiome-focused diet produced an additional 116 kilocalories of daily energy loss through stool. Researchers did not detect differences in hunger, fullness, food intake or energy expenditure between diets. Microbial fermentation increased even though less energy remained available to the participant overall. The result illustrates why greater microbial activity does not automatically mean greater calorie absorption. [Randomized clinical trial](https://doi.org/10.1038/s41467-023-38778-x).

However, the intervention changed several food characteristics simultaneously. The findings therefore cannot identify fiber alone, one bacterial species or a single supplement as the cause of the entire effect. The small sample and short observation periods also limit conclusions about sustained weight loss. An NIH research summary describes the work as a step toward understanding diet–microbiome interactions and potentially developing personalized nutrition approaches. [NIDDK research update](https://www.niddk.nih.gov/news/archive/2023/complex-interplay-diet-gut-microbiome-influences-human-health).

Should these findings change how people eat or count calories?

The 116-calorie figure is a group average from a particular experimental diet comparison. It cannot tell an individual how many calories they will absorb from dinner or how much weight they will lose. The study also did not test commercial probiotics or consumer stool tests as weight-management tools. Its practical significance is that food structure, digestibility and microbial activity can influence energy availability in ways that a calorie total does not fully describe. [Randomized clinical trial](https://doi.org/10.1038/s41467-023-38778-x).

Established nutrition guidance already provides a useful starting point. The World Health Organization recommends obtaining carbohydrates primarily from whole grains, vegetables, fruit and pulses, including beans and lentils. For adults, its guidance includes at least 25 grams of naturally occurring dietary fiber daily. These recommendations concern overall health and adequate nutrition; they should not be interpreted as a prescription for reproducing the trial's calorie difference. A varied eating pattern also needs sufficient protein, healthy fats and micronutrients. [WHO healthy diet guidance](https://www.who.int/news-room/fact-sheets/detail/healthy-diet).