The Same Calories on Your Plate. Different Calories Into Your Body.
A new study suggests that the composition of our diet, not just total daily calories, may provide a strategy to lose weight.
We usually think of weight control as calories consumed versus calories burned. But there is another part of the equation. The calories listed for a food, on the packaging or in diet tables, are not necessarily the calories your body actually absorbs.
In this study, 17 healthy adults ate two diets with essentially the same calculated calories as well as proportions of carbohydrate, fat, and protein. One resembled a typical Western diet, with more processed foods and less fiber. The other emphasized fiber, resistant starch and less mechanical processing.
The surprising result? On the higher-fiber, less-processed diet, participants lost an additional 116 calories per day in their stools. That meant 116 fewer calories were available to their bodies, despite eating diets with similar calculated calorie content.
Most nutrients are digested and absorbed in the small intestine. But with plant based food, some of these nutrients are trapped inside the fiber, escape digestion, and reach the colon.
Here, the trillions of our gut bacteria take over. They ferment some of this material, fiber and trapped nutrients, producing short-chain fatty acids that the colon absorbs and the body then uses for energy.
So if the high-fiber diet produces more of these fatty acids, shouldn’t it be providing more total calories?
Not necessarily. Bacteria also use some of the food reaching the colon to grow and multiply. Much of this bacterial mass, together with a certain amount of still undigested food, is eventually eliminated in stool. The calories recovered from short-chain fatty acids do not fully make up for the additional calories lost. The result is fewer calories available to your body.
In the study, participants obtained about 95% of a diet’s energy on the Western diet, compared with about 90% on the higher-fiber diet. The researchers estimated that bacterial biomass alone accounted for more than a quarter of the energy in feces on both diets.
Here is another interesting finding. Despite absorbing fewer calories, participants did not report feeling hungrier or compensate by eating more when given the opportunity.
The higher-fiber diet also significantly increased pancreatic polypeptide, a hormone involved in appetite regulation. There was some evidence of increased GLP-1 release, another hormone that promotes satiety, although the overall increase was not statistically significant. These hormonal changes may help explain why participants did not experience increased hunger despite absorbing fewer calories.
Let's put the net loss of 116 calories per day into perspective. Using an approximation of 3,500 calories per pound of body fat, that difference would add up to about one pound of weight loss in a month, assuming the calorie deficit continued.
But don't expect to lose a pound every month indefinitely. As weight decreases, a smaller body burns fewer calories, and metabolic adaptation can further slow weight loss. Eventually, weight loss plateaus. And because this was a short-term study, we don't know whether the same reduction in calorie absorption would persist over time.
And a key qualification: this was not a test of fiber alone. The diets also differed in processing, resistant starch, and food particle size. We cannot say how much of the effect was caused by any one of these changes.
The Bottom Line
Calories still count. But two meals with the same calculated calories may not provide the same usable energy.
Choosing beans, whole grains, vegetables, fruit, nuts, and other minimally processed, fiber-rich foods can change how many calories actually reach your body and how many feeding the bacteria in your colon.
What you eat matters, not just how much you eat.
References
Corbin KD et al. Host-diet-gut microbiome interactions influence human energy balance: a randomized clinical trial. Nature Communications. 2023. https://pubmed.ncbi.nlm.nih.gov/37258525/
Davis TL et al. Modeling the microbial contribution to human energy balance using the Digestion, Absorption, and Microbial Metabolism (DAMM) model. PLOS ONE. 2026. https://doi.org/10.1371/journal.pone.0347668