Each of us has a unique gut microbiota with a particular composition of gut bacteria, shaped by genetics, environment, lifestyle and diet. Gut bacteria break down compounds in food that our body’s digestive enzymes cannot process, such as dietary fiber.
This intestinal microbiota is reflected in the stool,
each gram of stool comprising 100 billion bacteria. Finally, 3 large groups of bacteria make up the vast majority of human intestinal microbiota, Bacteroides, Ruminococcaceae and Prevotella.
A new key to understanding weight gain
The study consisted, precisely, in evaluating the residual energy in the faeces of 85 participants, in order to estimate the efficiency of their intestinal microbes in extracting energy from food. The composition of gut microbial communities was mapped for each participant. The analysis reveals that:
- regarding 40% of participants belong to a group that, on average, extracts more energy from food than the other 60%; in this group, the microbiota is dominated by Bacteroides bacteria;
- participants who have a gut microbiota that extracts the most energy from food also weigh 10% more than the average participant, which in the study represents approximately 9 additional kilos.
“We have found a new key to understanding why some people gain more weight than others, even when they don’t eat more or eat differently”, comments one of the lead authors, Henrik Roager, professor of nutrition at the University of Copenhagen. In the light of these results, overweight is not only linked to diet and food intake or to the practice of exercise.
Weight gain is therefore correlated with the composition of the intestinal microbiota. Part of the population might thus be harmed by the presence of intestinal bacteria that are a little too efficient at extracting energy. This efficiency would result in more calories extracted and available from the same food intake.
A fair balance makes the “best” microbiota: “That our gut bacteria are good at extracting energy from food is fundamentally a good thing, because the bacteria’s metabolism of food provides energy in the form of, for example, short-chain fatty acids, which are molecules that our body uses as a source of energy. But if we consume more than we burn, the extra energy provided by gut bacteria may also increase the risk of obesity over time.”
Digestion time also matters: from the mouth to the esophagus, passing through the stomach, the duodenum and the small intestine, the large intestine and finally the rectum, the food we eat travels a journey of 12 to 36 hours, before the body has extracted all the nutrients. The evaluation, for all the participants – having overall similar eating habits – of the duration of this process, reveals that a shorter travel time is associated with a better extraction of nutrients (and calories).
- In other words, participants whose microbiota is dominated by Bacteroides bacteria that extract the most energy are also those in whom the passage of food is the fastest through the gastrointestinal system. Findings, in line with previous animal studies, showing that germ-free mice that receive gut microbes from obese donors gain more weight than mice that receive gut microbes from lean donors, despite diet similar.
We will therefore retain that differences in weight gain may at least in part be attributable to communities of gut bacteria that are more efficient at extracting energy from food. Here, “in practice”, the participants whose stools contained the least energy were also those with the highest body weight.