Gut bacteria clearly influence body weight in mice, but human evidence is far less certain. A fecal transplant trial in adults with obesity found no meaningful metabolic change, and probiotics have no FDA-approved health claims. Diet is the best supported lever.
Your gut microbiome, the community of bacteria and other microbes living in your digestive tract, does appear to play a role in body weight, but the strongest evidence comes from mice, not people. Animal studies show that gut bacteria can change how much energy is extracted from food and can transfer weight-related traits from one animal to another. In humans, the relationship is real but tangled up with diet, and so far attempts to change body weight by changing the microbiome directly have not delivered. The most reliable way to support your gut bacteria is still what you eat.
The microbiome is the community of microorganisms that live in and on the body, with the largest and most diverse population in the lower digestive tract. The National Center for Complementary and Integrative Health (NCCIH) puts it simply: although we tend to think of bacteria as harmful germs, many bacteria actually help the body function properly. Studies described below show that gut bacteria can help break down complex carbohydrates from food and may interact with the immune system.
Two landmark studies shaped the field.
Energy harvest (Nature, 2006). Turnbaugh and colleagues at Washington University reported that obesity in mice and in human volunteers was associated with shifts in the relative abundance of two major bacterial groups, the Bacteroidetes and the Firmicutes. In mice, the microbiome associated with obesity had an increased capacity to harvest energy from the diet. When germ-free mice (raised with no gut bacteria) were colonized with an "obese microbiota," they gained significantly more body fat than mice colonized with a "lean microbiota."
Human twins, mouse recipients (Science, 2013). Ridaura and colleagues took gut bacteria from pairs of adult female twins in which one twin had obesity and the other did not, and transplanted them into germ-free mice. Mice receiving bacteria from the twin with obesity gained more total body mass and fat. When those mice were housed with mice carrying the lean twin's bacteria, the lean-associated bacteria moved in and prevented the extra weight gain.
The crucial detail is that this protection was diet-dependent. It occurred when the mice ate a diet modeled on the lower range of US saturated fat intake and the upper range of fruit and vegetable intake, but not on a diet high in saturated fat and low in fruits and vegetables. In other words, the bacteria and the food worked together.
Germ-free mice start with no gut bacteria at all, which is very different from people, who already carry an established microbiome. Human weight is also influenced by many factors at once. It is also difficult to tell cause from effect: diet changes the microbiome, and the microbiome may change how the body handles diet.
The clearest test so far is a direct one. In the FMT-TRIM pilot trial, published in PLOS Medicine in 2020, Yu and colleagues randomized 24 adults with obesity and mild to moderate insulin resistance to weekly capsules containing gut bacteria from healthy lean donors, or placebo capsules, for six weeks. Donor bacteria took hold in most recipients and persisted for at least 12 weeks. Even so, there was no statistically significant improvement in insulin sensitivity, and no significant differences in most other metabolic measures, including lean mass and fat mass. The authors noted the small size of the study, but concluded they did not observe clinically significant metabolic effects.
Where human research is more encouraging is in showing that diet reshapes the microbiome, and that people respond differently to the same foods.
Fermented foods and fiber. A 2021 randomized study in Cell assigned healthy adults to a high-fiber diet or a high-fermented-food diet for 17 weeks, with 18 people per group. The high-fermented-food diet, which included foods such as yogurt, kefir, kombucha and fermented vegetables like kimchi, steadily increased microbiome diversity and decreased several markers of inflammation. The high-fiber diet increased the microbiome's capacity to break down complex carbohydrates, although overall diversity stayed stable. Weight loss was not the purpose of this study, and it was small, but it shows that everyday foods can measurably change gut bacteria.
Personal responses to food. In a 2015 Cell study, Zeevi and colleagues continuously monitored blood sugar in 800 people and measured responses to 46,898 meals. They found high variability in how different people responded to identical meals. An algorithm that included gut microbiome features, along with blood tests, habits and activity, predicted individual blood sugar responses and was validated in a separate group of 100 people. This research focused on blood sugar, not weight, but it helps explain why one diet does not suit everyone equally.
Probiotic supplements are widely marketed for weight and gut health. According to the NCCIH:
The NCCIH recommends talking with a health care provider before starting a probiotic supplement, and not using one as a reason to delay care for a health problem.
In mice, yes: bacteria from obese donors led to more fat gain than bacteria from lean donors. In humans, gut bacteria are associated with weight, but it has not been shown that changing them directly causes meaningful weight change.
The NCCIH states that the FDA has not approved any health claims for probiotics and that strong evidence is lacking for most uses. If you are considering one, check with your health care provider first, especially if you have a medical condition.
No. In a 2020 randomized pilot trial in adults with obesity, donor bacteria from lean people took hold but did not produce significant metabolic or body composition changes. It has not been shown to work for weight, so it should not be thought of as a weight loss option.
Eat a varied diet rich in plant foods. In the studies described here, diet consistently shaped gut bacteria and their effects, and a plant-rich pattern was the one linked to protection against weight gain in mice.
If you have ongoing digestive symptoms, are considering supplements, or want help building an eating plan that supports your weight goals, a licensed provider or registered dietitian can help you sort out what is worth trying. Learn more about our weight loss programs. This article is educational and does not determine eligibility for any treatment.
Written by the NewDay Wellness editorial team from the sources above and checked against them before publication, and medically reviewed by Dr. Sean Arora and Dr. Shannon Arora of Arora Health & Aesthetics; see our editorial standards. This article is educational and does not diagnose conditions, establish eligibility for any medication, or replace care from a licensed provider. Compounded medications are not FDA-approved.
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