A Western diet let people absorb about 116 more calories a day than a fiber-rich one

Healthy adults eating a Western-style diet absorbed about 116 more calories a day than the same kind of volunteers eating a fiber-rich plan, according to a modeling and diet study from Arizona State University published in PLOS One. The fiber-rich group did not report feeling hungrier.

A gap of 116 calories is roughly a small snack, but the interest lies in why it exists. Food on a label is not the same as energy that reaches the bloodstream, and gut microbes sit in the middle of that difference. Both diets in the underlying trial were matched to each participant’s measured energy expenditure, so the gap is not a matter of one group simply eating more.

Two controlled diets, two amounts absorbed

The work, described in an ASU release carried by ScienceDaily, compared two designed eating plans. A “microbiome enhancer” diet was high in fiber and resistant starch, with less processed food and larger food particle size. The Western-style diet was lower in fiber and resistant starch, with more processed food and smaller particles. Those eating the Western diet absorbed approximately 116 more calories per day than those on the fiber-rich diet.

The PLOS One paper reports the underlying trial as a randomized crossover study of 17 healthy men and women, each serving as their own control. Each diet lasted 22 days, 11 at home followed by 11 in a metabolic ward, with a washout of at least 14 days between them, and both were matched to each participant’s measured energy expenditure. The data build on an earlier controlled diet study that the ASU article links as the clinical foundation.

The finding runs against an intuitive guess. The fiber-rich diet produced more microbial activity in the colon, since fermentation of fiber yields short-chain fatty acids that the body can use as fuel, and yet participants on it still absorbed fewer net calories overall. In the researchers’ accounting, colonic short-chain fatty acids contributed about 140 calories a day, roughly 7.4% of usable energy, while about 85% of usable energy was absorbed in the upper gut and about 15% in the lower gut.

The DAMM model and the Atwater problem

The paper’s main product is a model, not the diet result. DAMM, short for Digestion, Absorption, and Microbial Metabolism, tracks digestion in the upper gut, microbial metabolism in the colon, absorption, methane production by methanogens and waste elimination. In a May 2026 ASU News story, Rosa Krajmalnik-Brown, who directs the university’s Biodesign Center for Health Through Microbiomes, called it “a powerful new way to quantify how those microbial partners contribute to human health.”

The comparison target is the conventional Atwater method for estimating how many calories a diet delivers. DAMM’s predictions matched measured absorption more closely than Atwater, which generally underestimated it. In the PLOS One abstract the fit to measured metabolizable energy rose from an R-squared of 88.5% to 95.6%, and mean bias shrank from -22.3 to -2.5 grams of chemical oxygen demand per day, with the Atwater error significant on the Western diet only.

Bruce Rittmann, an ASU Regents Professor and director of the Biodesign Swette Center for Environmental Biotechnology, said the model’s distinctive feature is that it quantitatively links human metabolism with microbial metabolism. Krajmalnik-Brown put the premise simply: “Digestion is not just a human process.”

What 17 volunteers can and cannot support

The authors are open about the limits. The sample of 17 is small, and the model has not been validated in people with metabolic conditions or in weight-loss or weight-gain studies. Short-chain fatty acid absorption was not measured directly, so predicted fecal values were compared with measured ones instead. The methane predictions began too low and needed a fitted correction factor of 2.58, so those results are partly calibrated rather than independent.

Taylor Davis, the first author and a former graduate research assistant at ASU, described DAMM as a framework designed to evolve. The ASU story is explicit that it is a research framework and not a tool for prescribing personalized diets, and it says further studies are needed across populations, eating patterns and microbiome compositions. Nothing in the trial shows that eating fiber changes body weight, and 22 days on each diet is far too short to speak to that.

The ASU story is careful on one point: the 116-calorie difference comes from the clinical diet study, and the article does not say the model itself produced it. Appetite is the other practical question, and the one reported answer is that volunteers on the fiber-rich plan did not feel hungrier, though the release gives no hunger scores. What the result does establish, in a small and tightly controlled group, is that two diets with matched energy content can deliver different amounts of usable energy to the body, and that the difference is large enough to measure. The paper is identified by its DOI, and the gap between label calories and absorbed calories is the part of its argument most likely to be tested next.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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