What Does the Microbiome Have to Do With Weight and Obesity?

Though research is in the early stages, we’re getting closer to understanding the relationship between gut bacteria, the food we eat, and the ways it might affect weight management and obesity.

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What you feed the bacteria in your gut may influence how easy or hard it is to lose weight.Getty Images

The human body plays host to trillions of bacteria and other microbes both in the body and on the skin. Collectively, they’re known as the microbiome, and those that live in the gut make up the gut microbiome.

“We all harbor a very specific distribution in the gut that’s fairly adapted to us,” says microbiologist Christian Diener, PhD, a postdoctoral fellow at the Institute for Systems Biology (ISB)?in Seattle. “It’s almost like a fingerprint.”

In the past several decades, a more complete picture of how the gut microbiome works has begun to crystallize. Thanks to initiatives like the federally funded Human Genome Project and others, scientists now better understand how these microbes affect many facets of human health, from certain disease processes to mental health.

But many questions remain, including one related to the role gut bacteria play in weight maintenance. Although researchers have a decent idea of how the food we eat factors into the composition of the microorganisms that live in the gut, research has only just started parsing the reverse, how gut bacteria affect our eating habits, cravings, even our ability to put on or take off weight.

“We’ve known that, in mice, the microbiome can cause obesity,” says Noel Mueller, PhD, an associate professor of epidemiology at Johns Hopkins University in Baltimore. “But which microbes are potentially beneficial or detrimental for metabolic health and obesity has been an ongoing line of investigation. We’re getting a better picture of that now.”

Ways Our Gut Bacteria Can Affect Our Weight and Eating Habits

The evidence that the bacteria in our guts plays a big role in obesity comes from?past research comparing the microbiomes of twins. Researchers found similarities in twins that they call “core microbiome,” indicating that there may be a genetic component to what kind of bacteria populate our digestive tracts. They also discovered some fundamental differences between the microbiomes of a twin who was a healthy weight and the corresponding twin with obesity, which shows that the microbiome can be influenced by environmental factors as well.

Even more interestingly, when researchers transplanted bacteria from human microbiomes into mice that had been bred and raised to be germ-free, mice that received bacteria from the twin with obesity became obese and those that received bacteria from the normal-weight twin stayed at their normal weight. “They were able to replicate that over and over,” says Dr. Mueller. Of course, experts cannot say for certain whether the same results are true in humans without additional study.

In an effort to understand whether the composition of a person’s microbiome might predict weight-loss success, Dr. Diener and colleagues conducted an experiment involving 105 individuals enrolled in a yearlong wellness program. They evaluated the gut bacteria of the participants at the start of the program and again at the end, and found that certain bacterial genes appeared to be associated with the ability to lose weight and others were likely associated with a strong resistance to losing weight. They also learned that people whose gut bacteria reproduce quickly dropped weight more successfully. The results, which were published in October 2021 in the journal mSystems, may explain why some people have a harder time losing weight than others.

Can Gut Bacteria Influence Our Eating Habits?

Those bacteria in our guts may not just influence how readily we gain or lose weight, but what we eat as well. “There’s lot of evidence that diet can be a strong influencer of those microbial communities,” says biologist Brian Trevelline, PhD, a postdoctoral fellow at Cornell who coauthored a paper on the subject that was published in Proceedings of the National Academy of Sciences in April 2022. “We know diet influences microbes, but can microbes influence diet?”

Early evidence seems to suggest they can. Because the study of the microbiome is relatively new, much of what we currently know comes from research on mice. In another experiment using germ-free mice, Dr. Trevelline and colleagues introduced microbial colonies from other mice, each of which had a specific diet: plant-based, omnivorous, or carnivorous. The germ-free mice that received a bacterial transplant from mice on a plant-based diet preferred a high-protein diet over a high-carb, low-protein one, a preference that differed from the other groups. This seems to indicate that the composition of bacteria in the gut may somehow shape food preferences, at least in mice.

Though it’s too early to say whether these results hold for humans, research is ongoing into the effect microorganisms have on appetite and metabolism, as well as on feeling hungry or full.

How to Manipulate Your Microbiome

With the limited knowledge we currently have of the microbiome and its role in obesity, is it possible to alter the bacteria composition in our guts to maintain weight, stave off obesity, and benefit long-term health? While there is no quick and easy solution — after all, bacterial communities differ from person to person, and what works for one may not for someone else, Diener says — experts believe that interventions with some real potential are likely.

One such intervention may involve altering the microbiome by eating fewer processed foods, as emerging research has shown possible links between the Western diet and an imbalance in the gut. “We rely heavily on convenience foods, but they lack nutrition,” says Chanel Love, RD, LD, a Houston-based dietitian who works with people with gut disorders. “When we rely on these foods, we’re not feeding the beneficial gut microbes. And if you’re not continually feeding the beneficial gut microbes, you allow the pathogenic ones to thrive, creating an unhealthy balance of good and bad bacteria.”

Love also recommends upping the variety of plants in a diet, adding not just more of them but a wider range, too. She points to a study showing that people who ate 30 different plant foods weekly had greater diversity in their gut microorganisms than those who’d had 10 or fewer.

This doesn’t mean ditching favorite foods completely. “It’s just diversifying,” she adds. “How can you make that oatmeal a little different every day? Maybe it’s oatmeal and blueberries one day, oatmeal and strawberries the next day, oatmeal and almonds the next.”

Probiotics and prebiotics — when used appropriately and with other dietary modifications — may also help by increasing beneficial gut bacteria and decreasing harmful ones, notes research.

Probiotics are live microorganisms found in fermented foods such as yogurt, and, in some cases, supplements. Simply ingesting these foods isn’t enough, however, Mueller says. For them to survive and outcompete organisms that have lived in the gut for far longer, they’ll likely require accompaniment by the sugars they need to metabolize. Recent research has shown just that. “This is quite spectacular,” he says. “If you give the microbes the right sugar, they stick around.”

Those sugars are known as prebiotics, compounds that act as fuel for specific beneficial microbes and are naturally found in usually plant-based foods such as asparagus, garlic, onions, and banana, according to research.

Then there’s “microbiome-directed foods,” those intentionally formulated to change what’s colonizing the gut. A team at Washington University School of Medicine in St. Louis recently determined that certain types of fiber added to their “snack food prototypes” altered gut bacteria in ways that could benefit long-term health for study participants with overweight and obesity.

As the field progresses further, so, too will the interventions. And, according to Mueller, the proof is mounting that we’re on the right course. “There’s pretty tantalizing evidence,” he says, “suggesting that the microbiome has an important role in the development of overweight and obesity and also potentially for weight loss.”