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Study has found different bacteria is linked to obesity risk. adamkaz/Getty Images
  • Researchers have discovered specific gut bacteria associated with obesity, while some appear to be protective against it.
  • The specific types of bacteria associated with obesity differed between men and women.
  • A clear causal link between gut bacteria and obesity has not yet been clearly established in humans, but experts say that encouraging a healthy gut is still a good idea.

In a new study, researchers have discovered that not only can bacteria in the gut be associated with obesity, but that different bacteria affect men and women differently.

Prior research has established a connection between the gut microbiome — the community of microorganisms, bacteria, and viruses that reside in the gastrointestinal tract — and obesity.

Some strains of bacteria and the like appear to promote obesity, while others may be protective against it.

In a forthcoming study that will be presented at this year’s 2024 European Congress on Obesity, researchers have made additional discoveries about this complex process.

Using metabolomic and metagenomic data from the Spanish population, researchers identified specific bacteria for men and women that appear to either promote or protect against obesity.

“Having a study that can assess different microbiome components and seeing differences between those who may have higher risk for obesity or with a higher index of obesity versus those with a lower index is impressive,” Nakul Bhardwaj, DO, MPH, an Assistant Professor of Medicine at Johns Hopkins University, told Healthline. He wasn’t affiliated with the research.

On the other hand, Karen D. Corbin, PhD, RD, a Spokesperson for The Obesity Society and Associate Investigator at the AdventHealth Translational Research Institute, was more measured in her response to the research, telling Healthline, “It adds some knowledge to the field about a possible connection between the gut microbes and obesity…What we need more of is data that shows whether the microbes actually cause obesity or if they are a reflection of your weight status and whatever’s going on metabolically in your body. That was not addressed in this paper.”

Corbin was not involved in the research.

Both Bhardwaj and Corbin stressed that the study only shows association, not causation.

For their study, Spanish researchers recruited 361 adults with an average age of 44 years old. More than two-thirds (251) were women. Participants were separated into two classes based on a measurement called the “obesity index”: low level of obesity or high level of obesity.

While many studies on obesity rely on body mass index or BMI alone, the investigators took a multifaceted approach with their obesity index. The obesity index is not a standardized gauge for obesity but one that includes three variables: BMI, fat mass percentage, and waist circumference.

Corbin told Healthline that she appreciated the novelty of the index, but was unfamiliar with it as a tool: “I like the fact that they were trying to include something beyond BMI. My question would be: Have they validated this particular index compared to something like the traditional BMI alone?” she said.

After classifying participants using the obesity index, researchers analyzed two types of data.

They looked at metagenomic data, which is genetic material from a collection of microorganisms in a sample — in this case, stool samples. In addition, they looked at metabolomic data to analyze small molecules known as metabolites produced during cellular metabolism.

These two types of data, when analyzed in concert, can give a very precise picture of gut health and metabolism.

Using all of this data, researchers profiled some of the specific strains of bacteria found in the guts of the participants.

The researchers then looked at the microbiome for people classified as “high” on the obesity index compared to those classified as “low” on the index.

They found certain bacteria were linked with obesity risk, but that it was different for men versus women.

In the study, the gut microbiome of both men and women who ranked high on the obesity index is characterized by a lack of a potentially protective bacteria known as Christensenella minuta.

Interestingly, women and men who were obese had distinct gut microbiota profiles from one another. In men, two other forms of bacteria associated with obesity were prolific: Parabacteroides helcogenes and Campylobacter canadensis.

“Different microbes can be protective or they can increase risk for obesity. The way that they do that is by stimulating different aspects of our metabolic response and of our immune response…There are microbes such as E. coli that have been shown to increase risk of obesity because they’re known to be pro-inflammatory,” Mariana X. Byndloss, DVM, PhD, the Co-director of the Vanderbilt Microbiome Innovation Center at Vanderbilt University Medical Center, told Healthline. She wasn’t affiliated with the research.

In the study the bacterium species Prevotella micans, Prevotella brevis, and Prevotella sacharolitica were associated with obesity in women but not in men.

“What I found most exciting was that they attempted to look at differences by sex, which I think it’s important to consider sex, ethnicity and race in what we’re seeing,” said Corbin, but also noted that prior studies have also investigated this.

Research on the gut microbiome and metabolic diseases like obesity has not yet yielded a clear treatment, but there are definite ways to improve your overall gut health.

The microorganisms that make up the ecosystem of the gut need food, just like you, so what you put in your body affects their health as well.

“The thing people can do today is eat diets that feed your microbes by increasing dietary fiber, such as whole grains, vegetables, and fruits. And also foods that have not just fiber, but resistant starch,” said Corbin.

Resistant starches are, as their name suggests, harder for the body to digest and are passed through the stomach and small intestine down to the colon where they feed beneficial gut bacteria.

“Every time you look at your plate of food, ask yourself, have I fed my microbiome today? And if you haven’t, make a switch or make an addition to make sure there’s something in there that will literally directly feed the gut microbes,” said Corbin.

There’s no quick fix, like a probiotic, either.

“I’ll often have patients coming in who want a quick fix: a medication or a probiotic that will increase their ability to lose weight and treat obesity. The reality is we’re not at that stage yet. We haven’t had enough randomized control trials that really tell us that prescribing a particular probiotic or regimen to increase particular bacterial strains in the microbiota will lead to benefits,” said Bhardwaj.

New research indicates that certain strains of bacteria are associated with obesity, while others may be protective against it.

Furthermore, it appears that the types of bacteria linked with obesity are different in men and women.

A clear causal link hasn’t yet been proven between these bacteria and developing obesity in humans, but research continues to show associations between gut health and metabolic disease.