Benefits
Respiratory symptom days in endurance athletes (strain VRI-003, sold as PCC)
In 20 elite male runners followed through winter training, strain VRI-003 cut reported respiratory symptom days to 30, versus 72 on placebo. A larger 11-week trial of the same strain in 99 cyclists found less lower respiratory symptom load in men but a higher, very uncertain load in women, and a mixed picture overall. Results apply to this strain only.
Immune response to flu vaccination in adults (strain CECT5716)
In a small randomized trial of 50 healthy adults, taking strain CECT5716 for 2 weeks before and after a flu shot was followed by a rise in natural killer cells and in flu-specific IgA, changes not seen in the placebo group. These are immune markers from one small study with a company author, not proof of protection from flu.
Gut and airway health in formula-fed infants (strain CECT5716)
In a randomized trial of infants aged 6 to 12 months, a follow-on formula with strain CECT5716 was linked to fewer gut infections than the same formula without it. The respiratory and total infection results are less certain because the published statistics for them do not fit together. This is evidence in babies fed formula and does not transfer to adults taking a capsule.
Digestive comfort during antibiotic therapy (strain MN-LF23)
In 94 adults on prescribed quadruple therapy for H. pylori, strain MN-LF23 did not change whether the infection cleared, but digestive symptom scores, constipation and indigestion were lower than on placebo. It is one trial, and the infection itself was treated by prescription drugs.
Blood antioxidant markers in healthy adults (strain ME-3)
In small 3-week studies, healthy adults taking strain ME-3 as capsules or in fermented goat milk showed higher blood antioxidant activity, and the milk form lowered markers of oxidized LDL. These are lab markers in a few dozen people, studied by the university that developed the strain.
Breast comfort in nursing women (strain CECT5716)
Strain CECT5716 was first isolated from breast milk and has been tested in nursing women with painful breastfeeding, and in a 3-week trial run by its maker, breast pain scores were lower than with placebo and Staphylococcus counts in milk fell from baseline. Europe's food safety authority judged the evidence for lowering breast infection risk as not established. See the CECT5716 page for detail.
Blood cholesterol (strains PCC and ME-3)
In a 10-week placebo-controlled trial in 46 adults with raised cholesterol, strain PCC did not change LDL, total cholesterol, HDL or triglycerides more than placebo. A 4-week study of a product combining ME-3 with red yeast rice and other ingredients reported lower cholesterol, but it had no control group, so the change cannot be credited to the probiotic.
Mechanism of action
Each strain has its own traits
L. fermentum is a species, and the strains within it differ in their genes and lab behavior. ME-3 was chosen for antioxidant activity, CECT5716 was isolated from breast milk, and VRI-003 is supplied by an Australian company. A result for one strain cannot be assumed for another, or for an unnamed L. fermentum on a label.
Passes through and adds to gut lactobacilli
In human studies, taking ME-3 or VRI-003 raised the number of lactobacilli found in stool, although ME-3 was not always recovered itself. This shows the bacteria survive the gut journey for a time; it does not show lasting colonization.
Signals to the immune system
In the athlete and vaccine trials, strains VRI-003 and CECT5716 were linked to a higher share of natural killer cells and T-helper type 1 markers, and to a trend toward better-kept interferon gamma levels. These are marker changes that may help explain the symptom findings, not proven causes.
Antioxidant activity of strain ME-3
ME-3 was selected because it showed antioxidant activity in laboratory tests. In small human studies it was linked to better blood antioxidant markers and a lower glutathione redox ratio, but how the bacteria cause these changes has not been worked out in people.
Clinical trials
Double-blind, placebo-controlled crossover trial of strain VRI-003 (PCC) capsules, 1.26 x 10^10 CFU/day, over 4 months of winter training (Cox et al. 2010, Br J Sports Med)
20 healthy elite male distance runners in Australia.
Runners reported 30 days of respiratory symptoms while on the probiotic versus 72 days on placebo (p < 0.001). Illness severity was lower, but this did not reach significance (p = 0.06). Salivary IgA, IL-4 and IL-12 did not differ, and treadmill running performance did not change. A small study of trained men only.
Double-blind, randomized, placebo-controlled trial of at least 1 x 10^9 CFU/day of strain VRI-003 PCC for 11 weeks, funded in part by Chr. Hansen and Probiomics, which employed two authors (West et al. 2011, Nutr J)
99 competitive cyclists (64 men, 35 women), average age about 35.
Results split by sex. In men, the load of lower respiratory symptoms was about one third of placebo, and gut symptoms were less severe, more so at higher training loads; in women, lower respiratory symptom load rose instead, though that estimate was very uncertain. Mild gut symptoms were about twice as frequent on the probiotic. The effect on upper respiratory illness was too uncertain for a firm conclusion.
Randomized, double-blind, placebo-controlled trial of strain CECT5716, 1 x 10^10 CFU/day, for 2 weeks before and 2 weeks after an intramuscular flu vaccine; first author from the company Puleva Biotech (Olivares et al. 2007, Nutrition)
50 healthy adults (31 men, 19 women), average age 33.
Two weeks after vaccination the share of natural killer cells rose in the probiotic group but not in the placebo group, and the probiotic group had larger rises in some T-helper type 1 markers. Flu-specific IgA rose in the probiotic group, while no antibody response could be detected in the placebo group. Influenza-like illness over the next 5 months was reported as lower with the probiotic. Immune markers in a small group, not a test of flu protection.
Randomized, double-blind, controlled trial of follow-on formula with strain CECT5716 plus galactooligosaccharides versus the same formula with galactooligosaccharides only (Maldonado et al. 2012, J Pediatr Gastroenterol Nutr)
Infants enrolled at 6 months of age and fed formula until 12 months, in Spain.
The probiotic formula group had a 46% lower rate of gut infections (rate ratio 0.54). Upper respiratory infections were reported 27% lower and total infections 30% lower, but the abstract gives the same confidence interval (0.46 to 1.38, crossing 1) for both, which does not fit their p values, so those results are uncertain. These are formula-fed babies, a population that does not transfer to adults taking capsules.
Randomized, double-blind, placebo-controlled trial of strain MN-LF23 (sachets of 1 x 10^10 CFU) during 14 days of standard quadruple therapy and for 4 weeks after; two authors affiliated with Mengniu Dairy (Zhao et al. 2025, Nutr J)
94 adults with confirmed H. pylori infection in China (47 per group).
The probiotic did not improve eradication (82.98% vs 85.11% on placebo, P = 1). Digestive symptom scores (GSRS), constipation and indigestion were lower in the probiotic group than on placebo, and some gut bacteria rose. Inflammatory markers did not differ. The infection itself was treated with prescribed drugs.
Two 3-week trials: an open placebo-controlled study of ME-3 fermented goat milk and a double-blind randomized placebo-controlled study of ME-3 capsules (9.2 log CFU/day), from the University of Tartu group that developed the strain (Songisepp et al. 2005, Nutr J)
45 healthy volunteers in Estonia (21 in the milk study, 24 in the capsule study).
Both forms raised total fecal lactobacilli and improved blood total antioxidative activity and total antioxidative status. The glutathione redox ratio improved only with the fermented milk. ME-3 itself was recovered from stool only in the milk group. Short studies measuring lab markers rather than health outcomes.
Pilot study comparing 150 g/day of goat milk fermented with strain ME-3 against plain goat milk for 21 days (Kullisaar et al. 2003, Br J Nutr)
21 healthy adults in Estonia.
The fermented milk group showed lower oxidized LDL, peroxidized lipoproteins and 8-isoprostanes, longer resistance of lipoproteins to oxidation and higher total antioxidative activity. Gut lactobacilli also shifted. A small pilot of blood and urine markers, not of heart or vascular outcomes.
Single-center, double-blind, placebo-controlled parallel trial of L. fermentum PCC, 2 capsules twice daily (2 x 10^9 CFU each), for 10 weeks (Simons et al. 2006, Nutr Metab Cardiovasc Dis)
46 adults in Australia with total cholesterol of 4 mmol/L or higher.
LDL cholesterol drifted down 7.0% on the probiotic and 5.2% on placebo, with no significant difference between groups. Total cholesterol, HDL and triglycerides did not change. Liver enzymes and safety tests were unchanged; one person withdrew because of bowel discomfort and three others reported it.