How different cycling training loads change gut bacteria, butyrate, fitness, and overreaching in healthy men

ICARUS: Impact of Training Load on the Gut Microbiome and Its Relation to Exercise Performance, Muscle Phenotype, and Markers of Overreaching in Healthy Men: Phase A - Human Trial

This project will test whether supervised moderate or high indoor cycling changes gut bacteria and butyrate levels and whether those changes relate to fitness, muscle health, and signs of overreaching in healthy recreational men aged 18–45.

Quick facts

• Phase: NA
• Study type: Interventional
• Enrollment: 45 (estimated)
• Ages: 18 Years to 45 Years
• Sex: Male
• Sponsor: KU Leuven (other)
• Locations: 1 site (Leuven, Vlaams Brabant)
• Trial ID: NCT07481578 on ClinicalTrials.gov

What this trial studies

ICARUS is a randomized, controlled, three-arm trial that will enroll 45 healthy recreational men who will be assigned to a non-training control group, an eight-week supervised moderate-load cycling program, or an eight-week supervised high-load cycling program followed by a 10-day taper. The trial's primary focus is on gut microbial short-chain fatty acid production, especially butyrate, measured in blood and feces at four time points (baseline, 4 weeks, 8 weeks, and after taper). Researchers will also collect data on exercise performance, muscle phenotype, gut barrier integrity, and clinical markers of overreaching to link microbiome changes with physiological adaptations or maladaptations. Training is supervised in person at KU Leuven with standardized protocols and stratified randomization by baseline training volume.

Who should consider this trial

Good fit: Healthy, recreationally active men aged 18–45 who exercise about 1–6 hours per week, have BMI 18.5–27.5 kg/m², pass a sports medical screening, and can speak Dutch are ideal candidates.

Not a fit: Women, people outside the age or BMI ranges, competitive athletes, those on recent antibiotics or certain gut-affecting medications, non-Dutch speakers, and people unable to attend in-person supervised cycling sessions are unlikely to benefit or be eligible.

Why it matters

Potential benefit: If successful, the findings could help tailor exercise programs to increase beneficial gut metabolites like butyrate while improving fitness and lowering the risk of overreaching.

How similar studies have performed: Prior observational and small intervention studies show exercise can alter the gut microbiome and increase SCFA levels, but randomized comparisons of different training loads with detailed butyrate and overreaching outcomes are largely novel.

Eligibility criteria

Inclusion criteria:

* Males between 18 and 45 years old.
* Recreationally active individuals, with a weekly exercise training volume between one and six hours per week.
* Good health status confirmed by a sport medical screening, which includes a sport medical questionnaire and resting electrocardiogram.
* Body Mass Index (BMI) between 18.5 and 27.5 kg/m².
* Masters the Dutch language.

Exclusion criteria:

* Engagement in strenuous competitive sports one month prior to or during the study intervention.
* Medication and vaccination: Pro- or antibiotic treatment during the past three months, non-steroidal anti-inflammatory drugs (NSAIDs) or cholestyramine during the past month, drugs interfering with intestinal permeability (e.g., prokinetics, laxatives, lubiprostone, loperamide, anti-spasmodics, linaclotide, proton pump inhibitors) during the past month. Vaccinations within one month prior to or during the study intervention.
* Blood donations within three months or plasma donation within one month prior to or during the study intervention.
* Inflammatory bowel disease (Crohn or colitis ulcerosa) or celiac disease.
* Diagnosed irritable bowel syndrome.
* Intake of any performance-enhancing medication or nutritional supplements known to modulate the gut microbiome in the two months prior to or during the study.
* Substance abuse, including alcohol consumption of more than three units/day (weekly average).
* Any injury or pathology considered a contraindication for performing physical exercise, as determined by the medical doctor overseeing the preparticipation medical screening.
* No access to smartphone and/or computer with internet access.
* No willingness to use the Polar Flow app and MijnEetmeter to collect physical activity, heart rate, sleep, and food intake data.
* Concomitant participation in another interventional trial, without approval from the research team.
* Any other reasons considered by the research team that the participant will not complete the study.

Where this trial is running

Leuven, Vlaams Brabant

• Exercise Physiology Research Group, Department of Movement Sciences, KU Leuven, Leuven, Belgium — Leuven, Vlaams Brabant, Belgium (RECRUITING)

Study contacts

• Principal investigator: Stefan De Smet, Professor — Exercise and Muscle Physiology Research Group, Department of Movement Sciences, KU Leuven, Leuven, Belgium
• Study coordinator: Stefan De Smet

Conditions: Exercise, Overreaching, Butyrate, Gut Microbiome, Exercise Performance, Skeletal Muscle, Glucose Tolerance, Hormones