Why Some People Are Better Calorie Utilizers

Special Microbes in the Gut Flora Improve Fiber Degradation in the Gut

Mystery Solved?

Previously little-noticed microbes in our gut could explain why some people are better "food converters" than others. An experiment reveals that certain archaea help the rest of the gut flora to break down fiber more efficiently and completely. As a result, the food provides the hosts of these microbes with more energy and calories. A hallmark of such "super-utilizers" in the gut is increased methane production – this could be used as a biomarker.

The intestinal flora contributes significantly to our digestion. pixabay

If two people eat the same amount of the same food, they have consumed the same amount of calories. However, not the same number of energy-providing calories necessarily reach the metabolism. This is because some people's intestines utilize food more effectively and extract more calories than others. But why?

Analysis of Human "Exhaust Gases"

A team led by Blake Dirks from Arizona State University has now investigated this. To do this, they had 17 test subjects stay in a sealed, hotel-like room for two six-day periods. Sensors in this so-called whole-room calorimeter continuously measured which and how much gases the subjects exhaled and farted. The scientists also analyzed blood and stool samples and had the participants swallow a sensor that measured pH, pressure, and temperature along the digestive tract.

This data provided information about the subjects' metabolism. It showed how much energy the participants' bodies absorbed from food, how active their intestinal flora was, and which microbes were present in it. The team specifically compared data from people whose intestinal microbiome produced high and low levels of methane.

During the experiment, the subjects were given two different diets one after the other: one with a lot of highly processed foods and little fiber, and one with a lot of whole foods and a lot of fiber. The foods were different, but both diets contained the same overall proportion of carbohydrates, proteins, and fats.

More Methane, More Calories

The experiment showed that the test subjects generally consumed fewer calories from the high-fiber diet than from processed foods. Some participants unintentionally lost weight during the experiment as a result. This confirms the observations of previous studies that a high-fiber diet tends to be healthier and less fattening than a Western diet.

However, significant individual differences were also evident. Some test subjects consumed more calories from the high-fiber diet than others. This was more pronounced the more methane their gut flora produced. This could mean that methane-producing microbes in the gut make digestion more efficient, thus enabling higher energy absorption from food.

Hydrogenotrophic processes in the human colon when the macronutrient input is carbohydrate. Fiber that reaches the colon is hydrolyzed and fermented by the microbiome to produce SCFAs, H2, and CO2. Most SCFAs are absorbed by the host or excreted in feces. Although some H2 is released through the breath or flatus, most is metabolized in the colon by homoacetogenesis, dissimilatory sulfate reduction, or methanogenesis. Acetate produced though hydrogenotrophy is absorbed by the host or excreted in feces. CH4 and some hydrogen sulfide produced through hydrogenotrophy are released through the breath or flatus. Figure created using biorender. (Source:Methanogenesis associated with altered microbial production of short-chain fatty acids and human-host metabolizable energy / The ISME Journal)

Hydrogen Inhibits Food Degradation

But why is this? When the bacteria in the gut flora break down and ferment fiber, they produce hydrogen as a byproduct in addition to usable short-chain fatty acids. However, if this hydrogen accumulates in the intestine, it inhibits further fiber degradation, and the remaining fiber is excreted undigested. This mechanism limits our utilization of these food components.

But there are also some microbes in the gut that can consume hydrogen and convert it into methane. These oddballs, especially Methanobrevibacter smithii, belong to the archaea family, some of which were only recently discovered. As Dirks and his team have now discovered, these archaea apparently help their bacterial neighbors in the intestinal flora digest fiber by disposing of the resulting hydrogen.

Archaea as Utilization Helpers

These previously neglected archaea could therefore explain why some people digest food more effectively than others: Methane producers are particularly numerous in these people's intestinal flora. In the experiment, half of all subjects had such archaea. "People who follow the same diet can react differently. Part of this is due to the composition of their gut microbiome," says Dirks.

"The human body itself doesn't produce methane; only archaea do. Therefore, methane could be a biomarker signaling efficient microbial production of short-chain fatty acids," adds senior author Rosa Krajmalnik-Brown of Arizona State University.

Personalized diet for weight loss?

Using methane measurements as biomarkers, personalized diets could be developed in the future to help with weight loss – for both healthy and sick people. However, follow-up studies with more participants are needed to first clarify how the intestinal flora of people with obesity, diabetes, or other metabolic diseases responds to high-fiber diets.

(The ISME Journal, 2025; doi: 10.1093/ismejo/wraf103)

Source: Arizona State University