Probiotics did not stop athletes' colds, but they made them milder

Upper respiratory tract infections are a recurring nuisance for athletes, interrupting training blocks and sometimes competition itself. Probiotic supplements are widely marketed as immune support, yet the claims have run well ahead of a clear verdict on what they actually deliver. The practical question is not only whether probiotics stop infections happening in the first place, but whether they change how bad an infection feels once it arrives. This review set out to answer both, and to check whether markers of inflammation in the blood shift alongside the symptoms.

The researchers searched four databases, PubMed, EBSCOhost, Scopus and Web of Science, using terms covering probiotics, exercise, athletes, respiratory infection and inflammation. From 2498 titles and abstracts screened, 14 randomised controlled trials met the inclusion criteria. For each study, two reviewers independently extracted the design, participant characteristics, intervention details, outcome measures and main results. The pooled analysis then examined days of illness, the number and duration of infection episodes, total symptom severity, and blood levels of the inflammatory markers interleukin 6 and tumour necrosis factor alpha.

Probiotic supplementation made no detectable difference to the number of days athletes spent ill, nor to how many infection episodes they suffered or how long each one lasted. Where it did move the needle was severity: the total symptom severity score fell by 0.65 in supplemented athletes. Two inflammatory markers also dropped, with interleukin 6 lower by 2.52 pg/mL and tumour necrosis factor alpha lower by 2.31 pg/mL after supplementation. The authors note the severity benefit was clearest among professional athletes.

The result reframes what probiotics can plausibly do for people who train hard: not a shield against catching infections, but a way of softening them when they strike. That distinction matters, because much of the marketing implies prevention. The authors are clear that more studies with larger groups are needed to pin the effect down, and that the optimal timing, duration, composition and dose of supplementation remain to be determined. Which strains work best, and whether recreational exercisers see the same benefit as elite athletes, are still open questions.

If you train hard and reach for probiotics each winter, this is a reason to recalibrate expectations rather than abandon them. The pooled evidence says you are no less likely to get ill, but the illness you do get may feel milder, and two inflammation markers moved in a favourable direction. Because the clearest effect was seen in professional athletes, how far it extends to ordinary gym-goers is unknown. With strain, dose and timing all unsettled, any product promising a specific formula is ahead of the evidence.