Online, Lactobacillus gasseri is often presented as the slimming probiotic that melts belly fat. The scientific reality is more nuanced. Studies involve a few specific strains, often in fermented milk, in small groups and over twelve weeks. Here is what they actually show about weight, stress, sleep and vaginal flora, including their limitations, the doses used and safety questions.
Key takeaways
- Lactobacillus gasseri is a human lactic acid bacterium and one of the dominant species in some vaginal microbiotas.
- Weight trials involve two strains (SBT2055, BNR17) over 12 weeks, with modest, inconsistent effects on abdominal fat.
- Meta-analyses of probiotics in general find a difference of less than one kilogram compared with placebo.
- No health claim is authorised for probiotics in the European Union.
What is Lactobacillus gasseri?
It is a lactic acid bacterium that forms part of the human microbiota. It is among the dominant lactobacilli in vaginal flora: in a reference study of 396 women, four of the five main microbiota profiles were dominated by a single Lactobacillus species, including L. gasseri. Certain strains, such as BNR17, have also been isolated from breast milk.
Its classification has changed. In 2018, Japanese researchers showed that several strains labelled L. gasseri actually belonged to a sister species, Lactobacillus paragasseri, almost identical in the gene usually used to identify bacteria. They suggested that many genomes recorded as L. gasseri belonged to this new species. All the more reason to require the exact strain designation.
Why think in terms of strains?
Within the same species, each strain has its own genome and properties. L. gasseri studies involve a handful of strains identified by codes: SBT2055 and BNR17 for weight, CP2305 for stress and sleep. A result obtained with one cannot be applied to another or to a product whose strain is unspecified. The delivery format matters too: fermented milk, a capsule or a tablet of inactivated bacteria are not equivalent.
Lactobacillus gasseri and weight loss: what do studies show?
SBT2055: fermented milk tested in Japan
In an initial double-blind randomised trial, 87 overweight adults with excess abdominal fat drank 200 g of fermented milk daily, with or without strain SBT2055, for 12 weeks. In the probiotic group, visceral fat measured by CT scan fell by 4.6% from baseline, weight by 1.1 kg and waist circumference by 1.7 cm. None of these measures fell significantly in the control group.
A second trial in 210 adults tested doses ten and one hundred times lower. Visceral fat decreased by around 8% from baseline in both probiotic groups, and the effect weakened four weeks after stopping. Both studies were conducted by the fermented milk manufacturer. They mainly compare each group with its own starting point, and the effect appears linked to continued consumption.
BNR17: an effect on visceral fat at a high dose
In Korea, 90 adults with obesity received placebo, 1 billion or 10 billion CFU of strain BNR17 daily for 12 weeks. Only the high dose reduced visceral fat compared with placebo, by around 22 cm², without changing blood markers. An earlier trial in 57 overweight adults receiving 10 billion CFU daily had found only a small weight reduction, not significant compared with placebo. It was funded by a Korean biotechnology company.
What meta-analyses say
Reviews pooling many probiotics find small effects. A meta-analysis of 15 trials (957 overweight adults or adults with obesity, 3 to 12 weeks) observed 0.6 kg more weight loss than placebo, without a significant effect on fat mass. Another, larger review (105 publications, 6,826 participants) found around 0.9 kg less weight and 1.3 cm less waist circumference in overweight people, but not in those with obesity. Improvements mainly came from bifidobacteria and other lactobacilli, and two authors worked for a dairy group.
These differences of less than a kilogram do not replace the established approaches of diet and physical activity. Other ingredients have been studied on related topics, without authorised health claims: gymnema for sugar cravings (see our article on gymnema and sugar cravings) and berberine for metabolism (see our article on berberine).
Stress and sleep: the possibility of an inactivated strain
Strain CP2305 is studied in a particular form: heat-killed bacteria, so they are not alive. In Japan, 60 medical students preparing for their national examination took a CP2305 tablet or placebo daily for 24 weeks. Anxiety and sleep disturbances measured by questionnaires were lower with CP2305 than placebo, and sleep onset was faster in sleep recordings. One author worked for the manufacturer.
A meta-analysis of 6 trials observed an average improvement in sleep quality of 0.8 points on a score ranging from 0 to 21, in mildly to moderately stressed adults. The measured effect is modest and relates to an inactivated strain: it cannot be applied to capsules of live L. gasseri. For other approaches, see our articles on stress and ingredients studied for it and sleep and melatonin.
Vaginal flora: a key species, disappointing trials
The fact that L. gasseri dominates certain vaginal microbiotas does not mean supplying it is enough to rebalance them. In Denmark, 74 women with an unfavourable microbiota before fertility treatment received vaginal capsules of L. gasseri and L. rhamnosus, or placebo, for 10 days. Flora improved in 29% of the probiotic group versus 40% with placebo, a non-significant difference: flora often changes on its own.
In China, 67 women with bacterial vaginosis received an antibiotic, with or without a 30-day oral probiotic containing L. gasseri and L. crispatus. The cure rate did not improve, and swallowed strains were not found in vaginal flora; the authors emphasised a better microbiota score in women who recovered, a secondary outcome. For intimate symptoms, consult a doctor.
What Lactobacillus gasseri doses have studies used?
| Strain | Population | Dose and duration | Result |
|---|---|---|---|
| SBT2055 | 87 overweight adults | Around 20 billion CFU/day in fermented milk, 12 weeks | Visceral fat −4.6% from baseline |
| SBT2055 | 210 adults | 200 million to 2 billion CFU/day, 12 weeks | Visceral fat −8%, effect weakened after stopping |
| BNR17 | 90 adults with obesity | 1 or 10 billion CFU/day, 12 weeks | Visceral fat reduced at high dose |
| BNR17 | 57 overweight adults | 10 billion CFU/day, 12 weeks | No significant weight difference |
| Inactivated CP2305 | 60 stressed students | 1 tablet/day, 24 weeks | Anxiety and sleep disturbances reduced compared with placebo |
Does Lactobacillus gasseri pose a risk?
In healthy adults, trials have not identified serious adverse effects: one BNR17 trial reported no particular or serious adverse reaction. Infections caused by lactobacilli exist but are rare. Of 48 cases published between 2019 and 2021, only one involved L. gasseri and another L. paragasseri. They occurred mainly in people with diabetes or impaired immunity, with risk factors such as a prosthetic heart valve.
Caution is therefore needed in cases of immune deficiency, serious illness, a catheter or an artificial heart valve: medical advice is essential. The trials presented here involved adults; data are lacking in children and during pregnancy.
Which health claims are authorised?
None. In the European Union, no specific health claim has been validated for a probiotic, and the Commission considers the word “probiotic” itself an implied claim. An L. gasseri supplement therefore cannot promise weight loss, a “flat stomach” or an effect on vaginal flora. Only yoghurt cultures have a claim relating to lactose digestion.
Frequently asked questions
How long does it take to see an effect?
Trials measured results after 12 weeks for weight and up to 24 weeks for sleep. No study has shown an effect in a few days, and measured effects remained modest.
When should you take Lactobacillus gasseri?
No study has compared different times of intake. In Japanese SBT2055 trials, the strain was consumed daily in fermented milk, and in a Korean trial capsules were taken before meals. For a particular product, follow the label directions.
Which foods contain L. gasseri?
It is not used to make conventional yoghurt. It is mainly found in the human microbiota, and some fermented milks contain specific strains, such as those used in the SBT2055 trials.
Is Lactobacillus gasseri suitable for men?
Yes; the species is not exclusive to women, even though it is associated with vaginal flora. The stress trial, for example, involved 41 male and 19 female students.
L. gasseri or L. rhamnosus for weight loss?
No trial has compared them directly. In the large 2019 meta-analysis, improvements mainly came from other bacteria, and average differences from placebo remained below one kilogram.
Should L. gasseri be refrigerated?
That depends on the product: follow the label. Live bacteria are sensitive to heat and moisture, hence the value of suitable shells for moisture-sensitive powders, as explained in our article on capsules and their shells.
Conclusion
Lactobacillus gasseri is a real bacterium in our microbiota, but its image as a slimming probiotic rests on a few strains, tested in small groups over three months, often by their manufacturers. Effects on abdominal fat exist in these trials, remain modest and weaken after stopping. Findings for stress and sleep concern an inactivated strain, and trials on vaginal flora are disappointing. Before interpreting a study, check the strain, dose, delivery format and population.
Sources
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Food supplements do not replace a varied, balanced diet or a healthy lifestyle. Ask a healthcare professional for advice if you are taking medication, pregnant or breastfeeding.
