Verdict: Worth considering for athletes prone to colds or race-day gut problems, using a strain tested in trials, but not as a performance booster
Safety: Generally safe for healthy people; avoid without medical advice if you are immunocompromised or seriously ill
Key takeaways
Probiotic effects depend on the exact strain and dose, so results from one product cannot be applied to another
Probiotics may modestly reduce the number of upper respiratory infections, though evidence in athletes is mixed
A 2022 meta-analysis found a small improvement in aerobic performance tests in trained people, based on small and varied studies
Endurance athletes have two good reasons to care about their gut. Hard training, travel and crowds raise the risk of colds, and long races, especially in the heat, often bring stomach cramps and urgent toilet stops. Probiotics are marketed for both problems. The evidence is real but modest, and it depends heavily on which bacteria you take.
What it is
Probiotics are "live microorganisms that, when administered in adequate amounts, confer a health benefit on the host", the definition used by the International Society of Sports Nutrition (ISSN). Most products contain Lactobacillus (some now renamed, such as Lacticaseibacillus) or Bifidobacterium species, in capsules, powders or fermented dairy drinks. The dose is counted in colony forming units (CFU).
The key point from the ISSN position stand (Jäger et al., 2019) is that health benefits are strain- and dose-dependent. A good product label should list the genus, species and strain of each microbe, plus the CFU count at the end of shelf life. For the bigger picture on gut bacteria and fitness, see the gut microbiome and VO2max.
Does it raise VO2max?
There is no good evidence that probiotics raise VO2max in humans. VO2max is limited mainly by the heart and blood (see what limits VO2max), and nothing in the gut changes that quickly. Probiotics might help indirectly by keeping you healthy enough to train consistently, which is what actually builds fitness.
Does it improve endurance performance?
Santibañez-Gutierrez and colleagues (Nutrients, 2022) pooled 12 studies with 232 trained participants doing tests lasting more than 5 minutes. Probiotics had a small positive effect (standardised mean difference 0.29). Subgroups suggested larger effects with doses of at least 30 billion CFU, single-strain products, supplementation of 4 weeks or less, and tests to exhaustion, but the meta-regression did not confirm that dose or duration predicted the effect. The studies were small and used many different strains, so this is a weak signal.
The most practical finding comes from the gut. In a trial of 24 recreational runners, 28 days of a multi-strain probiotic reduced the severity of gastrointestinal symptoms in the final third of a marathon, and runners slowed down less over the race. Finish times, however, did not differ (Pugh et al., 2019). The ISSN also notes that specific strains can improve gut barrier function during intense exercise in the heat. More on this in gut microbiome and athletic performance.
Colds and upper respiratory infections
A 2022 Cochrane review of 23 trials and about 6,950 people (mostly non-athletes) found probiotics may reduce the number of people with at least one upper respiratory infection (risk ratio 0.76) and may shorten episodes by about a day, with low-certainty evidence.
In athletes, results show why strain matters but also why strain is not enough. One trial of Lactobacillus casei Shirota in endurance athletes found fewer infection episodes over a 16-week winter (Gleeson et al., 2011). A larger 20-week trial of the same strain in university athletes found no difference, possibly because infections were rare during the study (Gleeson et al., 2016).
How it is used in studies
- Duration: at least 4 weeks for performance and gut trials, and often 3 months or more for infection trials.
- Dose: most infection trials used about 1 billion or 100 billion CFU a day of one or two strains. The marathon trial used 25 billion CFU.
- Timing: start well before a key race or a winter training block. Try it in training, not for the first time on race week.
Side effects, safety and anti-doping
For healthy people, probiotics are generally safe. Some people notice bloating or wind in the first days. People who are immunocompromised, critically ill, have a central line or have had recent surgery should not take probiotics without medical advice, because rare serious infections have been reported in these groups. If you have a medical condition or take medication, talk to your doctor first. Probiotics are not banned, but supplements can be contaminated, so athletes who compete should choose third-party tested products. This page is not medical advice.
Who might benefit
- Athletes who get frequent colds during heavy training, travel or winter.
- Runners and triathletes who struggle with gut symptoms in long races, alongside gut training with carbohydrate.
- People after a course of antibiotics, on medical advice.
They are not a substitute for sleep, enough energy intake and a fibre-rich diet. Low energy intake and poor sleep raise infection risk far more than a missing capsule (see VO2max and sleep).
Bottom line
Probiotics are not a performance supplement. Their best-supported uses for athletes are a modest reduction in respiratory infections and, for some strains, fewer gut problems during long races. Choose a product with a named, studied strain and give it at least four weeks. Compare other options on the supplements page.
Frequently asked questions
Are probiotics good for gut health?
Some strains help with specific problems, such as reducing certain digestive symptoms, but there is no single best probiotic for gut health. Benefits are strain- and dose-specific.
Do probiotics help athletes and endurance runners?
A meta-analysis of 12 small studies found a small benefit in aerobic performance tests, and one marathon trial found fewer gut symptoms late in the race. Finish times did not change.
Do probiotics prevent colds and upper respiratory infections?
A Cochrane review found probiotics may reduce the number of people who get upper respiratory infections, with low-certainty evidence. Athlete trials are mixed, even with the same strain.
How long do probiotics take to work?
Most athlete trials supplemented for at least four weeks, and infection trials often ran for three months or more. Benefits in trials were measured while people kept taking them.
Sources
- Jäger R, et al. International Society of Sports Nutrition position stand: probiotics. J Int Soc Sports Nutr. 2019;16:62. doi:10.1186/s12970-019-0329-0
- Santibañez-Gutierrez A, et al. Effects of probiotic supplementation on exercise with predominance of aerobic metabolism in trained population: a systematic review, meta-analysis and meta-regression. Nutrients. 2022;14(3):622. doi:10.3390/nu14030622
- Zhao Y, Dong BR, Hao Q. Probiotics for preventing acute upper respiratory tract infections. Cochrane Database Syst Rev. 2022;8:CD006895. doi:10.1002/14651858.CD006895.pub4
- Pugh JN, et al. Four weeks of probiotic supplementation reduces GI symptoms during a marathon race. Eur J Appl Physiol. 2019;119(7):1491–1501. doi:10.1007/s00421-019-04136-3
- Gleeson M, et al. Daily probiotic's (Lactobacillus casei Shirota) reduction of infection incidence in athletes. Int J Sport Nutr Exerc Metab. 2011;21(1):55–64. doi:10.1123/ijsnem.21.1.55
- Gleeson M, Bishop NC, Struszczak L. Effects of Lactobacillus casei Shirota ingestion on common cold infection and herpes virus antibodies in endurance athletes: a placebo-controlled, randomized trial. Eur J Appl Physiol. 2016;116(8):1555–1563. doi:10.1007/s00421-016-3415-x