Do probiotics cut colds and gut trouble in people who exercise?

Short answer

A little. Multi-strain probiotics cut colds in adults by 10%, and in people who exercise they eased symptoms more than they cut the number of colds. Late-marathon gut discomfort was also milder, but finish times did not differ.

Do probiotics cut colds and gut trouble in people who exercise?
'Probiotics → stronger immunity → fewer colds and upset stomachs' — does this formula hold?
Many people say hard training makes them catch colds easily and gives them stomach trouble at races. Ads often claim probiotics prevent this. We checked colds and gut problems separately.
In adults in general?
Multi-strain probiotics cut colds by 10%
An analysis pooling 107 trials in adults found multi-strain probiotics (16 trials) cut respiratory infections by 10% and shortened each illness by 0.97 days. One type of bifidobacteria (3 trials) cut them by 21%. Side effects did not increase.
What about people who exercise?
Same number of colds, milder symptoms
An analysis of 14 studies in athletes found probiotics did not reduce days of illness, the number of colds, or how long each cold lasted. The combined symptom severity score did drop, though. Two inflammation markers in the blood were also lower.
Where did the number of colds drop?
In a 4-month winter with frequent colds
In a study where 84 people took probiotics for 16 winter weeks (58 finished), colds per person were 2.1 with the dummy pill and 1.2 with probiotics. In a study of 243 people over 20 weeks, colds were rare at 0.6 per person, and there was no difference.
What about gut problems?
Less stomach discomfort late in a marathon
When 24 people took probiotics for 28 days before a marathon, gut symptoms in the last 1/3 of the race were milder than with a dummy pill. Their late-race slowdown was also smaller, 7.9% vs 14.2%. Finish times did not differ.
Pooled, the gut effect was small
An 822-person review could not pick a best product
A review of 10 trials with 822 athletes found probiotics had only a small effect on gut symptoms, and no product stood out as better. In studies of the general public, side effects were mild gut symptoms like gas, diarrhea and stomach pain, and the share of people with side effects did not differ.
Summary: probiotics cut colds and gut trouble a little
They eased symptoms more than they cut the number of colds
In adults, multi-strain probiotics cut colds by 10%. In people who exercise, they eased symptoms more than they cut the number of colds, and the drop in colds came from a winter study when colds were common. Late-marathon gut discomfort was milder, but finish times did not differ.