Balancing your gut microbiome might be the key to shifting the scale.
Eat less, move more is common weight loss advice for good reason: Restricting calories can help you lose weight. But emerging research suggests it’s much more complicated than that. Recent studies show that certain gut bacteria can influence weight and make weight loss harder.
Gut health “is about having the right variety of microorganisms,” says Steven Shamah, M.D., a leader in gastroenterology and Director of Endoscopy at Lenox Hill Hospital in New York City. “The gut doesn’t work right if there’s a low diversity of microorganisms, affecting gut integrity, hormone production, and metabolism.” All of which make weight loss more difficult, he explains.
About the Experts
Emeran Mayer, M.D., is a world-renowned and award-winning gastroenterologist, neuroscientist, and UCLA brain-gut microbiome expert.
Steven Shamah, M.D., is an internationally recognized leader in gastroenterology and internal medicine and the Director of Endoscopy at Lenox Hill Hospital in New York City.
Resham Uttamchandani, M.D., is a double board-certified physician with specialized training in obesity medicine.
Your gut is home to trillions of microorganisms. In return for a balanced diet, these microorganisms produce substances that influence gene expression, immunity, and metabolism.
Research suggests that your gut microbiota—the community of microorganisms in your gut—also influences metabolism, energy balance, and weight (1). Having the right balance of bacteria in your gut may help weight loss efforts, while the wrong mix can hinder them.
A healthy gut has a diverse gut microbiota, primarily from the Firmicutes, Bacteroides, Proteus, Actinomycetes, Fusobacteria, and Verrucomicrobia bacteria families (1). Firmicutes and Bacteroides are the most abundant, accounting for 90 percent of the microbial population (1, 2).
People with obesity typically have a higher ratio of Firmicutes to Bacteroidetes (the family of bacteria that includes Bacteroides), explains obesity medicine specialist Resham Uttamanchandi, M.D. That may be because Firmicutes are better at absorbing calories from food than Bacteroidetes, which may contribute to weight gain (2, 3).
A high Firmicutes to Bacteroidetes ratio also weakens the gut barrier, leading to what many refer to as a “leaky gut.” Some experts suspect this condition can trigger a host of health problems that cause the body to absorb and store more fat and increase appetite, such as insulin resistance, inflammation, and metabolic syndrome.
When gut bacteria consume nondigestible carbohydrates from food, they produce vitamins, anti-inflammatory agents like butyrate—a metabolite that interrupts the pathway that produces inflammatory cytokines—and other short-chain fatty acids (SCFAs).
Some studies show high levels of SCFAs contribute to obesity by providing extra calories that get stored as fat.
However, research is conflicting. Other studies show SCFAs may help control appetite, decrease fat production, increase fat burning, reduce inflammatory compounds, and help regulate insulin (1). They may also increase appetite-suppressing hormones like PYY and GLP-1 (4).
Emeran Mayer, M.D., a gastroenterologist and neuroscientist who specializes in the brain-gut connection, explains that these hormones work in two ways to control appetite. First, they activate the vagus nerve that runs from the gut to the brain to suppress hunger. Then, they travel to the brain through the blood, saturating the appetite control center and shutting down hunger.
Further complicating matters: how SCFAs behave in the gut—increasing fat storage or decreasing appetite—may be influenced by diet and lifestyle (5).
Low-grade chronic inflammation is a side effect of obesity that can lower metabolism, increase appetite, and cause weight gain (6, 7).
Poor gut health worsens these impacts. A bacterial imbalance reduces the gut’s ability to produce enough anti-inflammatory agents to combat chronic inflammation (1, 8). A leaky gut may worsen the inflammation by allowing harmful substances to enter the body, including more stored fat, says Shamah (9).
Low-grade inflammation caused by a lack of diversity of bacteria in the gut may also “desensitize satiety cell receptors,” so you’re always hungry, adds Mayer.
One consistent finding in studies is how the ratio of enterotypes, groups of microorganisms with similar composition in the gut, affects weight and how this ratio is influenced by what you eat and digest. Prevotella are enterotypes associated with diets high in complex carbohydrates and fiber, while Bacteroides are enterotypes associated with diets high in fat, animal protein, and sugar (10, 11).
The ratio of Prevotella to Bacteroides in the gut may help identify the best diet for weight loss, according to findings from a 2018 study published in the International Journal of Obesity (11). In the study, people with a high P/B ratio lost significantly more weight following a high-fiber diet than people with a low P/B ratio following the same diet. Researchers think individuals with a high P/B ratio might lose more weight because they produce more SCFAs when eating a high-fiber diet, improving energy use or appetite control.
Mayer points out that there are several strains of Prevotella and a lot we don’t know about enterotypes. Meaning: it’s too soon to definitively say whether the ratio of these family of microorganisms can determine the best diet for weight loss. In the future, gut microbiota may help personalize the best diet—high-protein or high-complex carb—for weight management. But there’s still much to learn.
While scientists figure out exactly how gut bacteria influence weight, focusing on lifestyle changes—eating a balanced diet, getting more exercise, managing stress, and getting enough sleep—currently seems to be the best way to boost gut health (and weight loss).
The following gut-health strategies may make diet and exercise more effective, says Uttamchandani.
The microorganisms in fermented foods like yogurt, kimchi, and kombucha contain beneficial bacteria that may enhance gut diversity. A 2021 study published in Cell showed increased microbial diversity in a group of people who ate a diet high in fermented foods (12). While this study didn’t focus on weight, it did show reduced inflammatory markers, which is promising, says Mayer.
Resistant starches are indigestible carbohydrates found in foods like brown rice, lentils, kidney beans, and potatoes. They move through your small intestine relatively unscathed. In the large intestine, microorganisms ferment and consume the starches, creating by-products like vitamins and SCFAs (13).
A 2024 clinical study published in Nature Metabolism found that supplementing with 40 grams of resistant starch improved gut microbiota, decreased inflammation, and led to significant weight loss in people following a calorie-controlled diet (14).
Intermittent fasting (IF) can directly help with weight loss (15). It may also improve the richness and diversity of your gut microbiota, suggests a 2024 review published in Frontiers in Nutrition (16).
Shamah recommends IF to boost weight loss and gut health, but he cautions what you eat during your eating window can help or hurt your gut bacteria. Emphasize whole grains, colorful fruits and vegetables, and foods rich in prebiotics, the fiber your gut bacteria feast on.
Avoid intermittent fasting plans with short eating windows, like 20:4. Limiting your eating time to four hours a day may not allow for enough calories, Shamah explains. Very low-calorie diets stress the body and negatively affect gut bacteria balance (17).
Both Mayer and Uttamchandani point out that Akkermansia muciniphila, a bacteria that helps maintain the intestinal barrier, is lower in people with obesity (18). Clinical studies show that supplementing with the probiotic improves insulin function, lowers cholesterol, and promotes a slight decrease in weight (19).
Supplementing with A. muciniphila may improve microbiota balance, but it will not lead to significant weight loss without lifestyle changes such as eating a balanced diet, getting exercise, and reducing stress.
If you’re struggling with cravings or difficulty losing weight, gut health may be a factor. A gut health test may help you understand where your gut stands in terms of diversity and balance.
That said, Mayer and Shamah warn that many commercial stool tests may not provide accurate information and might push supplements to fix “problems” discovered by the test.
Shamah says the best supplement for gut health and weight loss should contain Saccharomyces, Lactobacillus acidophilus, and Lactobacillus rhamnosus. These highly studied strains have been shown to improve gut diversity and support weight management, he adds.
A. muciniphilia is a popular probiotic marketed for weight loss, but research is limited. While it may help populate bacterial levels, it may not lead to significant changes in weight.
Still, there’s no best probiotic for weight loss. There’s still a lot researchers don’t know and standard over-the-counter (OTC) probiotics may not work for all people, adds Mayer. Most of the multistrain probiotics on the market also haven’t been clinically studied to evaluate their effectiveness.
Currently, the American Gastroenterological Association (AGA) says the evidence is weak for making probiotic recommendations to support the treatment of any gastrointestinal disorders (20). The same is true for probiotics and weight loss.
If you’re going to invest in a probiotic, look for a reputable brand with third-party testing that contains the strains recommended by Shamah. Or talk to your doctor for individualized recommendations.
Boosting gut health can support weight loss, but it’s not a quick fix with a supplement. The same strategies recommended for weight management also improve gut diversity: eating a balanced diet, getting more exercise, managing stress, and getting enough sleep.
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