Scientists talked about the effect of intestinal microbes on calorie intake

Intestinal microorganisms are involved in extracting energy from food, so the calorie content on the package does not always match the amount of energy that the body receives. Arizona State University scientists have developed a mathematical model to account for this contribution. It was tested on experimental data in which, with a fiber-rich diet, the body received an average of about 116 calories per day less than with the Western type of diet. Why bacteria change the calculation of calories and how this knowledge can help in the study of metabolism — in the Izvestia material.

What remains behind the number on the package

The usual calorie counting begins with the composition of the product: how many proteins, fats and carbohydrates are in it. For more than 100 years, a system based on Atwater coefficients has been used to assess energy value. In its common version, 1 g of protein and 1 g of carbohydrates give about four calories each, and 1 g of fat — nine.

However, the path of food inside the body is more complicated than this calculation. Some of the nutrients are digested and absorbed in the small intestine. The other part reaches the colon, where it is processed by microorganisms. The body can use some of the products of this processing as an energy source, and some of the material is eliminated.

The result is influenced by the composition of the diet, the degree of processing of products and the availability of nutrients for digestion. It also matters how many food components the intestinal microbial community gets.

The Atwater coefficients give an average estimate. The contribution of microbial processes in it is not calculated separately: it is combined with other factors. The new work suggests tracing these processes in more detail and assessing how they change the energy balance.

Two diets and a difference of 116 calories

To test the model, the researchers used data from a previously conducted clinical experiment, the results of which were published in Nature Communications in 2023. It involved 17 healthy adults. Everyone consistently followed two diets, so scientists could compare one person's reaction to different nutritional conditions.

The participants followed each diet for 22 days: 11 days at home and 11 more at the research hospital. The order of the rations was determined randomly, with a break of at least two weeks between the stages. Nutrition was carefully monitored, and the energy needs of the participants were assessed using energy consumption measurements.

The first diet was designed so that more food components reach the intestinal microbes. It contained more fiber and resistant starch, included less processed foods and foods with larger particles. Resistant starch is poorly broken down in the small intestine and can enter the large intestine for further processing by microorganisms.

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The second diet corresponded to the Western type of diet: it contained more processed foods, less fiber and resistant starch. According to standard calculations, the diets were comparable in terms of energy value and the total content of essential nutrients.

Nevertheless, on a diet with a lot of fiber, an additional 116 kcal per day was consumed on average. Accordingly, the body received less available energy. The participants did not report increased hunger. This difference characterizes the absorption of energy in a particular experiment. It does not measure the extra calories spent on physical activity or other processes.

How bacteria extract energy from fiber

Fiber enters the colon and becomes a substrate for microorganisms. During fermentation, they form short-chain fatty acids. The body is able to absorb some of these compounds and use them in metabolism.

Thus, digestion continues even after the food has passed the main stages of digestion and absorption in the small intestine. Microbes convert some components inaccessible to human digestive enzymes into substances from which the body can obtain energy.

According to the calculations of the new model, the absorbed short-chain fatty acids provided an average of about 140 kcal per day. Under the conditions studied, this corresponded to about 7.4% of the energy available to the body.

These 140 kcal are included in the overall energy balance of the diet. They cannot be automatically added to the figure on the package: the standard calculation already gives an average estimate of the available energy, and the model specifies which processes it comes from. In addition, we are talking about the average value obtained by modeling specific diets. It does not establish the same "microbial bonus" for every person and every menu.

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Why More Fermentation Doesn't Mean More Calories

At first glance, the result looks contradictory. A diet with a lot of fiber actively supplies microbes with food. They produce more short-chain fatty acids, which can become a source of energy. However, in the experiment, the body as a whole received fewer calories.

The explanation is due to the fact that the energy balance consists of several flows. It is important how many nutrients were absorbed into the colon, how much was processed by microorganisms, what part of their metabolic products entered the body and how much energy was spent with secretions.

In a clinical study, stool losses were associated with undigested material, microbial biomass, and metabolic products. Therefore, increased microbial processing of food can be combined with a decrease in the total amount of energy available to humans.

This result shows why the fiber content and the degree of processing of foods are important for assessing nutrition. Comparable calorie figures do not yet describe all the differences in how the body and microbes process the two diets. The work helps to explain in more detail why the energy value of a diet depends on its composition and processing in the digestive system. Intestinal microbes are involved in this process, but their contribution is determined by which substances reach them and what happens to their metabolic products.

In the future, researchers expect to use similar models to study obesity, diabetes, and other metabolic disorders. A more detailed description of digestion can help you understand why different diets have different effects on energy balance.