Probiotics for Athletes: What the Evidence Really Shows

Probiotics have moved from the yogurt aisle into sports nutrition. Athletes take them to avoid colds during heavy training, to calm a stomach that rebels during races, and in hopes of better recovery. Some of these uses have reasonable support; others rest on enthusiasm more than data. Our editorial team sorted through the research.

What counts as a probiotic

An international expert panel convened by the International Scientific Association for Probiotics and Prebiotics defines probiotics as live microorganisms that, when administered in adequate amounts, confer a health benefit on the host. The key words are "live," "adequate amounts" and "health benefit": a product must contain enough viable organisms of a strain shown to do something useful.

Effects are strain-specific. Lactobacillus rhamnosus GG and Lactobacillus casei Shirota are both lactobacilli, yet they have different research records. A label that lists only the genus and species, without the strain code, makes it impossible to connect the product to clinical evidence.

Most products contain species of Lactobacillus (many now reclassified into new genera such as Lacticaseibacillus), Bifidobacterium, Streptococcus thermophilus or the yeast Saccharomyces boulardii. Doses are expressed in colony-forming units (CFU), typically ranging from one billion to more than fifty billion per serving.

In the US, probiotics are sold as dietary supplements or foods. They do not require FDA approval before sale, and the agency has not approved any probiotic to treat a disease. That places responsibility for quality largely on manufacturers and, ultimately, consumers.

Why athletes are interested

Heavy training and competition are associated with transient changes in immune function, and endurance athletes frequently report upper respiratory symptoms around major events. Missed training days due to illness are a real cost for anyone preparing for a target race or season.

Gastrointestinal complaints are another driver. Up to half or more of endurance athletes report GI symptoms such as cramping, bloating, nausea or diarrhea during long or intense efforts, partly because blood flow is diverted away from the gut and intestinal permeability rises, particularly in the heat.

The gut microbiome also influences metabolism, inflammation and possibly mood. Studies have found that athletes' microbial communities differ from those of sedentary people, though cause and effect are hard to separate from differences in diet.

Finally, travel to competitions exposes athletes to unfamiliar food and water, raising the risk of traveler's diarrhea. Some probiotics have modest evidence for reducing that risk in the general population.

The evidence on respiratory illness

The strongest athletic evidence concerns upper respiratory tract illness. In a randomized trial by Gleeson and colleagues, endurance athletes who consumed a fermented milk drink with Lactobacillus casei Shirota daily through winter had fewer episodes of upper respiratory infection than those on placebo.

Cox and colleagues gave elite distance runners Lactobacillus fermentum VRI-003 and reported fewer days of respiratory symptoms and less severe illness compared with placebo. Not all trials have been positive, and sample sizes are small, but the overall direction is encouraging.

In the general population, a Cochrane review by Hao and colleagues concluded that probiotics were better than placebo for reducing the number of acute upper respiratory infections, although the quality of evidence was low to moderate.

The International Society of Sports Nutrition position stand, authored by Jäger and colleagues, concluded that certain strains may reduce the incidence, duration or severity of respiratory infections in athletes, while emphasizing that benefits depend on the specific strain and dose.

Gut symptoms, recovery and performance

Studies on GI symptoms during exercise are mixed. Some multi-strain products reduced the severity of symptoms in runners during training or in the heat, while others showed no effect. Differences in strains, doses and outcome measures make comparisons difficult.

Evidence for improved recovery, reduced muscle damage or better protein absorption is preliminary. A few small trials suggest specific strains may increase amino acid absorption from protein or lower markers of muscle damage, but these findings need replication.

Direct performance benefits such as faster times or greater strength are inconsistently reported. Pyne and colleagues, in their review for the European Journal of Sport Science, noted that probiotics are better viewed as a way to maintain health and training availability than as ergogenic aids.

Choosing and using a probiotic

For athletes who want to try probiotics, the practical approach is to match the product to a goal and to strains that have been tested for it. Look for full strain designations on the label and a CFU count guaranteed through the expiration date, not just at manufacture.

1. Start at least two to four weeks before a heavy block, travel or key competition, since several trials used that lead-in period.
2. Take the product daily at a consistent time; follow the manufacturer's storage instructions, as some products require refrigeration.
3. Expect mild bloating or gas during the first week; it usually settles.
4. Never try a new probiotic for the first time on race day.
5. Choose third-party certified products if you are subject to drug testing.

Probiotics are generally safe for healthy people. Rare cases of bloodstream infection have occurred in people who are severely immunocompromised, critically ill or have central venous catheters. Anyone in those groups should take probiotics only under medical supervision.

Food sources such as yogurt with live cultures, kefir and fermented vegetables contribute to dietary diversity, although their strain content and counts are less standardized than supplements.

Conclusions of the Editorial Team

The best-supported use of probiotics in sport is a modest reduction in respiratory infections in athletes under heavy training loads, with specific, well-studied strains. Evidence for gut comfort during exercise is mixed, and claims about recovery or performance are still preliminary.

Probiotics are a reasonable, low-risk option for athletes who frequently get sick or travel, provided they choose a strain-specified, quality-tested product and start well before important events.

Related topics you may find useful include diet and the gut microbiome, how to prevent GI distress during endurance events and vitamin D and immune health in athletes.

References

1. Hill C, Guarner F, Reid G, et al. Expert consensus document: The International Scientific Association for Probiotics and Prebiotics consensus statement on the scope and appropriate use of the term probiotic. Nat Rev Gastroenterol Hepatol. 2014;11(8):506–514.
2. Jäger R, Mohr AE, Carpenter KC, et al. International Society of Sports Nutrition position stand: probiotics. J Int Soc Sports Nutr. 2019;16(1):62.
3. Gleeson M, Bishop NC, Oliveira M, Tauler P. Daily probiotic's (Lactobacillus casei Shirota) reduction of infection incidence in athletes. Int J Sport Nutr Exerc Metab. 2011;21(1):55–64.
4. Cox AJ, Pyne DB, Saunders PU, Fricker PA. Oral administration of the probiotic Lactobacillus fermentum VRI-003 and mucosal immunity in endurance athletes. Br J Sports Med. 2010;44(4):222–226.
5. Pyne DB, West NP, Cox AJ, Cripps AW. Probiotics supplementation for athletes – clinical and physiological effects. Eur J Sport Sci. 2015;15(1):63–72.
6. Hao Q, Dong BR, Wu T. Probiotics for preventing acute upper respiratory tract infections. Cochrane Database Syst Rev. 2015;(2):CD006895.