Does L. reuteri cause weight gain? A closer look at the scientific evidence.

November 1, 2025 •

4 min read

According to some studies, higher levels of Lactobacillus reuteri have been associated with obesity in certain human and animal cohorts. This has led many to question: Does L. reuteri cause weight gain? The answer is more complex than a simple yes or no and depends heavily on the specific strain and overall gut ecosystem.

Quick Summary

The link between L. reuteri and weight gain is complex and highly dependent on the specific bacterial strain. Some studies show an association with obesity, while others indicate weight loss benefits. Research is inconsistent, and effects vary by context.

Key Points

Strain-Specific Effects: The impact of Lactobacillus reuteri on weight is not uniform across all strains. Different strains can have neutral, beneficial, or even negative effects on body weight, highlighting the importance of choosing specific, well-researched strains.
Not a General Cause of Weight Gain: The idea that L. reuteri generally causes weight gain is inaccurate. Negative associations often arise from specific contexts, like animal studies or medical interventions (e.g., specific antibiotic use), not typical human probiotic supplementation.
Some Strains Aid in Weight Management: Certain strains of L. reuteri, such as L3 or ZJ617 (in mouse models) and DSM 17938 (in a rat model), have demonstrated positive effects on metabolism, fat reduction, or preventing obesity.
Gut Microbiota's Role in Metabolism: The mechanism linking probiotics and weight involves the gut microbiota influencing energy extraction from food, regulating appetite hormones, and modulating inflammation.
Temporary Bloating vs. Fat Gain: Initial gastrointestinal side effects like bloating or gas can cause a temporary feeling of heaviness but are not the same as fat gain. These symptoms typically subside as the body adjusts to the supplement.

The potential link between probiotics and weight is a topic of significant interest and, at times, confusion. When considering the question, "Does L. reuteri cause weight gain?" it's critical to move beyond the simple name and examine the specific strain and context, as research reveals a nuanced and complex picture.

The Strain-Specific Nature of Probiotics

One of the most important lessons from probiotic research is that effects are highly specific to the strain, not the species. Different strains of Lactobacillus reuteri (L. reuteri) can have opposite or neutral impacts on weight and metabolism, illustrating why broad generalizations are inaccurate.

For example, studies conducted on mice have shown conflicting results for different L. reuteri strains:

One study isolated L. reuteri L3 from normal-weight mice and L. reuteri L10 from obese mice. When administered to high-fat-diet mice, L. reuteri L3 significantly decreased body weight, fat mass, and inflammation. In contrast, L. reuteri L10 had no significant effect on obesity.
Another mouse study found that L. reuteri L6798 was associated with weight gain, while L. reuteri ATCC PTA 4659 was associated with weight loss.
In a recent study from 2025, L. reuteri DSM 17938 was shown to have a protective effect against Western diet-induced obesity in rats by reducing body lipid gain and increasing protein gain.

These findings highlight that the source and specific genetic makeup of a strain dictate its metabolic impact, not just its species name. Probiotics formulated for human consumption are chosen for their specific, often beneficial, effects and are not the same strains used in animal agriculture for promoting growth.

Context is Key: Circumstances that Influence L. reuteri's Impact

Beyond the strain, the conditions under which L. reuteri is observed can also influence its relationship with body weight. Research suggesting an association with weight gain often involves specific, non-generalizable scenarios.

Following Antibiotic Treatment: Some studies have observed an increase in L. reuteri levels after broad-spectrum antibiotic use, such as vancomycin, in humans. In one such case, a patient treated with vancomycin showed a massive increase in L. reuteri along with significant weight gain, potentially linked to the antibiotic's selection of resistant bacteria. This effect is specific to the antibiotic context and not representative of probiotic supplementation.
Kwashiorkor Malnutrition: In malnourished children with Kwashiorkor, an increase in L. reuteri was correlated with weight gain during treatment with ready-to-use therapeutic food. This rapid weight restoration is a desired medical outcome for treating malnutrition and is not applicable to healthy individuals.
Observed Association in Obese Individuals: Some cross-sectional human studies have found higher levels of L. reuteri in the gut microbiota of obese individuals compared to lean counterparts. However, this shows correlation, not causation. It is unclear if the bacteria contributed to the obesity, or if the environment of an obese gut favored the growth of these bacteria.

Potential Mechanisms Behind Probiotic Weight Modulation

Probiotics can influence body weight through several pathways, offering clues as to why different strains have varying effects.

Energy Harvesting and Storage: The gut microbiome influences the body's ability to extract energy (calories) from food. Some bacteria might be more efficient at this, potentially contributing to weight gain in susceptible individuals.
Metabolite Production: Gut bacteria ferment dietary fiber to produce short-chain fatty acids (SCFAs), which are linked to appetite regulation and energy use. Some L. reuteri strains, like ZJ617 in mice, produce metabolites (e.g., spermidine precursors) that confer anti-metabolic syndrome benefits.
Hormone Regulation: Certain probiotics can influence the release of appetite-regulating hormones such as glucagon-like peptide-1 (GLP-1) and peptide YY (PYY), which affect feelings of satiety and may help burn calories.
Inflammation: Gut bacteria can also influence inflammation levels, and chronic, low-grade inflammation is a contributing factor to obesity. Some L. reuteri strains have anti-inflammatory properties that may indirectly benefit weight management.

Comparison of Probiotic Effects on Weight (Examples from Research)


Feature	L. reuteri L3 (mouse study)	L. reuteri L10 (mouse study)	L. reuteri L6798 (mouse study)	L. reuteri DSM 17938 (rat study)
Effect on Body Weight	Decreased body weight and fat mass.	No significant effect on obesity.	Caused weight gain.	Mitigated weight gain by reducing lipid gain.
Effect on Inflammation	Decreased pro-inflammatory cytokines.	No significant effect.	Not applicable.	Reduced systemic and hepatic inflammation.
Mechanism	Suppressed adipogenesis and fatty acid synthesis genes.	Lack of anti-obesity effects.	Unclear mechanism, possibly related to energy harvest.	Protected liver from metabolic derangement.
Application	Shows potential anti-obesity effects, but research was in mice.	Highlights importance of strain specificity; demonstrates not all strains are effective for weight.	Demonstrates that some strains, especially in specific contexts, can contribute to weight gain.	Shows potential as a preventative strategy against diet-related diseases.

Can Taking L. reuteri Supplements Cause Weight Gain?

For the average person, the answer is generally no. The association between some L. reuteri strains and weight gain comes from specific, non-applicable contexts, like animal agriculture or treating specific medical conditions. Most commercial probiotic supplements containing L. reuteri strains are developed for specific health benefits, and many have been studied for their potential positive effects on metabolic health.

If you experience bloating, gas, or other gastrointestinal discomfort when starting a probiotic, this is a common, temporary side effect as your gut adjusts, not fat gain.

Conclusion: It's Complicated and Context-Dependent

The question, "Does L. reuteri cause weight gain?" does not have a simple answer. The effect of L. reuteri is not uniform across all strains, with research showing some strains are associated with anti-obesity effects, while others are linked to weight gain in specific contexts. The key lies in the strain and the individual's metabolic context. For those concerned about their weight, consulting a healthcare professional is the best course of action before starting any new supplement. It's also important to remember that probiotics are just one piece of a larger health puzzle, which includes a balanced diet, exercise, and other lifestyle factors.

To learn more about the use of probiotics for weight management, see this informative review on the topic: Use of probiotics in preventing and treating excess weight and obesity.

Frequently Asked Questions

For most people, taking L. reuteri supplements will not cause weight gain. The idea that this probiotic causes weight gain is primarily based on specific studies in animals or unusual medical contexts. The effect is highly dependent on the particular strain, and many human-targeted strains are not associated with weight gain.

Yes, some studies, particularly in animal models, have shown that specific strains of L. reuteri can have anti-obesity effects. For instance, L. reuteri L3 decreased body weight and fat mass in mice, and L. reuteri DSM 17938 mitigated lipid gain in rats on a Western diet.

Associations between L. reuteri and obesity often come from specific contexts. This includes studies in animal agriculture (using different strains for growth promotion), observations of malnourished children gaining weight during treatment, or correlation studies in obese individuals where it's unclear if the bacteria cause obesity or thrive in an obese gut environment.

Strain specificity is crucial because different strains within the same species can have vastly different outcomes. A strain's unique genetic makeup and origin determine its specific health effects. Therefore, you cannot assume that all L. reuteri will have the same effect on weight.

Yes, the gut microbiota plays a significant role in body weight regulation by affecting how the body harvests energy from food, regulating appetite hormones, and modulating inflammation. Probiotic supplementation can alter this microbial balance, potentially affecting weight.

No, temporary bloating or gas is a common and usually harmless side effect of starting a probiotic as your digestive system adjusts. It is not an indication of fat gain. The discomfort typically resolves within a few days to a couple of weeks.

To choose a probiotic for weight management, look for products with specific strains that have been studied in humans for their effects on weight, fat mass, and metabolism. Consult with a healthcare professional to determine the best approach for your individual needs.