Severe heat strain arising from intense physical work under climate conditions that does not allow sufficient heat dissipation may lead to heat stroke. This severe conditions is hypothesized to be secondary to increased gut permeability and leakage of bacterial toxins across the gut membrane, stimulating a systematic inflammatory response and associated organ injury. Repeated such sub-clinical increases in gut permeability has been suggested to contribute to the high burden of chronic kidney disease among heat-stressed workers. Many marathon runners experience a transient increase in kidney injury biomarkers while running. Probiotics have been studied as a way to decrease gut permeability and reduce systemic inflammation in many settings, including in athletes . However, no study has measured renal outcomes among workers or athletes performing strenuous activity. This is of interest as it could test the hypothesis that gut-induced inflammation is a driver of kidney injury during heat stress, and could point to a possible intervention to add on to efforts to relieve heat strain.

In the present study, recreational or professional runners will be randomized to take a probiotic supplement or placebo during a 4 week period preceding a strenuous physical exercise (minimum 21 km run). Urine samples will be taken before and after the run, and analyzed for markers of renal injury and inflammation.

Participants registered to run in organised half-marathons, marathons and ultramarathons in Southern Sweden will be recruited. They will be asked to abstain from probiotic supplementation (including functional foods with probiotics) for 2 weeks, and then commence a 4-week period of probiotic or placebo supplementation.

At the end of the 4 week treatment period, the participants run the race. Urine samples are taken before and after the race and analysed for kidney injury markers.

Stool samples are taken by participants at the initiation of the treatment period, last stool before the race, and first stool after the race.

Inclusion Criteria:

• Planning to run one of the long-distance races included.

Exclusion Criteria:

• Not having run at least 21 km previously,
• smoking,
• living too far away from the study area,
• unwilling to abstain from probiotic supplementation during treatment period,
• unwilling to abstain from NSAIDs or gut-permeability lowering supplements before the run,
• regular use of prescribed drugs (including hormonal contraceptives).

No information that could be used to identify individuals will be shared. Considering the small size of the study and that it will most likely be conducted over multiple small runs, it is unlikely that any information on age and sex of participants could be shared without a risk of revealing the identity of the individual. Data on treatment allocation, biological measurement and questionnaire data without such identifying individual participant information will however be shared.

After the researchers have reported on the pre-specified research questions or otherwise have decided to make the data publicly available before this. This may be in approximately 2023-2024. We will keep the records for five years.

Scientifically sound proposal.

The investigator will be provided with numbered boxes containing study products with identical probiotic and placebo capsules. The numbers have been generated using a block randomization (investigator blinded to length of block). Participants in each of the race will be allocated to receive one of the study product boxes depending on 1. Race order (all participants in the first race receive study product before any participant in the second race), and 2. Time of recruitment into study.

There is no care provider in this study of healthy individuals.