New insight into how your gut bacteria could be helping you gain weight

A team of researchers from the University of Copenhagen has looked to the microbiome to understand what role gut bacteria may play in weight gain. The findings suggest certain populations of bacteria are more efficient at extracting energy from food, and this could be why some people easily pile on the pounds from relatively healthy diets.

Over the last couple of decades, a handful of studies found fascinating links between obesity and the gut microbiome. For example, fecal transplant studies saw thin mice gain substantial weight when colonized with gut bacteria from obese mice. And one hypothesis suggested the key factor differentiating obese and lean animals was their microbiome's capacity for harvesting energy from food.

We know the bacteria in out gut plays an important role in digestion. Metabolites from different bacteria can help degrade food into carbohydrates and lipids, so when we eat we are also treating the microbes in our gut to a feast. And those microbes do influence our metabolism but it's unclear exactly how much of a role this all plays in obesity.

To investigate, researchers looked at 85 middle-aged, overweight human volunteers. Fecal samples were studied, not only for microbiome analysis but also to track the energy density of the stool as a measure of a person's gut microbial energy extraction.

The study also investigated an alternative hypothesis for obesity – intestinal transit time. Instead of gut microbes being responsible for more energy extracted from food, it has been suggested slower transit times for food through the gut could play a role. The idea is that the longer food spends moving through our guts the more time we have to extract energy from that food.

Interestingly, the researchers found those participants with the shortest intestinal transit time were unexpectedly extracting the most energy from food. It seemed the microbiome was most influential on energy extraction, with a microbial population dominated by Bacteroides bacteria doing the best job.

"We thought that a long digestive travel time would allow more energy to be extracted," said Henrik Roager, co-author on the new study. "But here, we see that participants with the B-type gut bacteria that extract the most energy, also have the fastest passage through the gastrointestinal system, which has given us something to think about."

Connecting all this to weight gain, the researchers found those subjects who were extracting the most energy from their diet weighed around 10 percent more than those who were not as efficient at getting energy from food. This equated to about a 9-kg (20-lb) weight difference between the most and least efficient energy extractors.

"The fact that our gut bacteria are great at extracting energy from food is basically a good thing, as the bacteria's metabolism of food provides extra energy in the form of, for example, short-chain fatty acids , which are molecules that our body can use as energy-supplying fuel," said Roager. "But if we consume more than we burn, the extra energy provided by the intestinal bacteria may increase the risk of obesity over time."

While the researchers are confident their findings do point to a causal link between how much energy is extracted from food and certain populations of gut bacteria, the leap to whether this accounts for significant weight gain is still hypothetical. According to Roager, it's certainly plausible that the weight differences between groups in the study are related to the amount of energy extracted from food. But that specific question is one to be explored more robustly in future research.

The new study was published in Microbiome.

Source: University of Copenhagen