- A new model suggests calorie labels do not tell the full story of how much energy people actually absorb from food.
- Gut microbes can break down undigested food in the colon and produce short-chain fatty acids that the body can use as extra energy.
- The findings could eventually help personalise diets for obesity, diabetes and other metabolic conditions.
Calories on food labels look precise, but digestion is not that simple.
A new model from Arizona State University suggests the gut microbiome may affect how much usable energy the body actually takes from food.
The model is called DAMM, short for digestion, absorption and microbial metabolism.
It follows food through the digestive tract and estimates what is absorbed directly, what reaches the colon and what gut bacteria do with the leftovers.
That matters because gut microbes do not just sit there.
They ferment fibre and other food components that escape digestion in the upper gut.
In doing so, they produce short-chain fatty acids, which can be absorbed and used by the body.
The researchers estimate that these microbial products contribute around 140 calories a day on average.
That is roughly 7.4% of usable energy.
Most energy still comes from the upper digestive tract, but the microbial contribution is clearly not irrelevant.
The model was tested against data from a controlled diet study.
People eating a more typical Western diet absorbed about 116 more calories a day than those eating a microbiome-enhancing diet rich in fibre and resistant starch.
Interestingly, the high-fibre group did not feel hungrier.
That is important because it suggests a better gut-focused diet may reduce net calorie absorption without simply making people feel deprived.
The model also showed that high-fibre diets deliver more fermentable material to the colon, increasing microbial activity and short-chain fatty acid production.
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So the story is not simply “more microbes means more calories”.
It depends on the diet, the gut environment and what is absorbed or excreted.
The practical message is that calories are not just about the food on the plate.
They are also about the gut ecosystem processing that food.
This could eventually help clinicians design more personalised diets, especially for people with obesity, diabetes or metabolic disease.
