Thirty-three seconds doesn't sound like much until you remember who was running. In a new randomized crossover trial, elite athletes who took the multi-strain probiotic SLAB51 for one week lasted about 33 seconds longer on a treadmill test than when they took a placebo. A single dose did nothing. VO2max, the number most endurance athletes obsess over, didn't budge either. So what changed, and should you care?
What Is SLAB51?
SLAB51 is a branded, multi-strain probiotic. It combines eight strains of lactic acid bacteria and bifidobacteria in a single formulation. That matters because probiotic effects are strain-specific. A yogurt culture, a generic "gut health" capsule and a defined eight-strain blend are different products, and results from one can't be assumed for the others.
The trial, published in Nutrients in 2026 by a team at Korea National Sport University (Lee, Kim, Park, Yun and Yoon), set out to test whether SLAB51 could influence aerobic performance and oxidative stress in elite athletes. That population is rarely studied in probiotic research.
How the Study Worked
The design is what makes the result worth discussing. It was a randomized, double-blind, placebo-controlled crossover trial in human male elite athletes. Each participant took both SLAB51 and placebo, one week each, with a two-week washout between them. Because every athlete served as their own control, differences between individuals matter far less than in a standard two-group trial.
Testing happened at three points: baseline, after a single intake, and after one week. Performance was measured with the treadmill-based Bruce protocol, a staged test that steadily increases speed and incline until exhaustion. The researchers also collected physiological and oxidative stress markers. Analysis used two-way repeated-measures ANOVA, followed by post hoc linear mixed-effects models that account for the crossover structure.
The Numbers
- Exercise duration: After one week, athletes lasted 33.32 seconds longer on SLAB51 than on placebo (95% CI 17.42–49.22 s, p < 0.001).
- Time above RER 1.0: The time spent above a respiratory exchange ratio of 1.0, a marker of high-intensity effort, also differed significantly in favor of SLAB51.
- Superoxide dismutase (SOD): Pre-exercise activity of this antioxidant enzyme was higher after one week.
- No change: Maximal oxygen uptake, lactate removal and malondialdehyde (a marker of lipid oxidative damage) showed no significant treatment effects.
- Single dose: No performance effect appeared after one intake.
The confidence interval is worth noting. Even the low end, about 17 seconds, stays above zero. But the benefit shows up as endurance time, not as a higher aerobic ceiling.
Why Would Gut Bacteria Affect Endurance?
The authors frame exercise performance as an interplay between energy metabolism and redox regulation, the balance of reactive oxygen species (ROS) produced during hard training and the antioxidant defenses that keep them in check. Too much ROS can impair muscle function and recovery. SOD is one of the body's front-line antioxidant enzymes, converting superoxide radicals into less harmful products.
The finding of higher pre-exercise SOD activity is consistent with the idea that SLAB51 may shift the body's antioxidant readiness. The proposed link to oxygen-related metabolism is plausible, but the study doesn't prove it. Malondialdehyde didn't change, so the data don't show reduced oxidative damage. The mechanism remains an open question, and the authors themselves call for cautious interpretation.
The unchanged VO2max fits with this picture. If SLAB51 helps, it may be by improving how long athletes can sustain high-intensity work, not by raising their oxygen-processing ceiling.
Practical Takeaways
Strain specificity matters. These results apply to SLAB51 specifically. Don't assume any probiotic with "Lactobacillus" on the label will do the same. Look for products that name their strains and have human trials behind them.
Give it at least a week. Nothing showed up after a single dose. If you're testing a probiotic for performance, a one-off pre-race capsule isn't supported by this evidence. A week of consistent intake is the timeframe where effects appeared.
Know who it might be for. The evidence applies to elite male athletes doing treadmill-based aerobic work. Endurance athletes chasing small margins may find the idea appealing. Recreational exercisers can't assume the same effect size.
Keep expectations realistic. Thirty-three seconds on a graded test to exhaustion is a modest, measurable edge, not a transformation. Training, nutrition, sleep and recovery still do the heavy lifting.
Limitations
The study was small, and it enrolled only elite male athletes, so the findings can't be generalized to women or non-elite populations. The intervention lasted only one week per condition, leaving open whether the effect grows, plateaus or fades. The mechanism is unclear, and the oxidative stress picture is mixed: SOD rose while malondialdehyde didn't change. The headline 33-second figure also comes from the post hoc mixed-model analysis, which the authors describe as additional to the original ANOVA, and that deserves some caution. The treadmill result also isn't the same as a race result.
Bottom Line
SLAB51 is one of the few probiotic formulations to show a measurable endurance signal in elite athletes under a rigorous crossover design. The effect appeared after a week, not a single dose, and without any change in VO2max. It's a promising lead, not a verdict. Larger, longer trials that include women and track race-relevant outcomes will show whether those 33 seconds hold up.
