Female athletes are often overlooked in scientific research, including the understanding of their digestive system and its impact on overall health, training, and performance. The gastrointestinal (GI) system plays a critical role in supporting digestion, nutrient absorption, and acting as a protective barrier for the body. While there is some knowledge about the differences in GI structure and function between males and females at rest, there is a lack of understanding regarding how these differences manifest during exercise, especially among female athletes and at different phases of the menstrual cycle.

It has been observed that female athletes commonly report a higher prevalence of GI symptoms compared to males, with many experiencing an increase during menstruation. These symptoms, including bloating, abdominal discomfort, and irregular bowel movements, can significantly disrupt an athlete's training and performance. Elite female athletes have acknowledged that GI symptoms associated with the menstrual cycle have led to altered or disrupted training routines. While these symptoms are commonly recognized to affect a large portion of the female population, few interventions have been studied to assist in managing this discomfort. Some clinicians may direct their patients to take pain-relieving medications without any thoughts on the consequences of chronic use.

One potential strategy to reduce GI symptoms during endurance exercise is probiotic-postbiotic supplementation. Various probiotic supplements have been shown to effectively decrease diarrhoea, constipation stress, and intestinal permeability, as well as impact immunologic outcomes. Postbiotics differ from probiotics in that they are made from microorganisms that are not alive. They allow the modulation of the composition and function of the gut microbiota and enhance the intestinal barrier to prevent unwanted substances from entering the body proper. While these biotics can relieve lower GI symptoms in irritable bowel syndrome (IBS), there is less consensus regarding their efficacy in modulating exercise-associated GI symptoms. Previous research by Pugh et al., 2019 has demonstrated that four weeks of probiotic supplementation was able to reduce GI symptoms during a marathon race, however this work was predominantly conducted in males and did not take menstrual cycle in consideration. This is important given that symptoms are commonly higher in the early follicular phase when a female experiences their period. Therefore, the aim of this study is to assess the effects of postbiotic supplementation on GI symptomology in female athletes.

The aim of this present study is to investigate the influence of postbiotic supplementation on menstrual cycle symptoms and exercise-associated GI symptoms in female endurance athletes while exercising compared to symptoms while taking a placebo.

Inclusion Criteria:

1. Female, cisgender, premenopausal.
2. Minimum age 18 years.
3. Regular, natural menstrual cycle (21-35 days in duration), with 9 or more consecutive periods per year, if possible documented with tracker apps, calendars, or other tools.
4. Willingness and ability to give informed consent and to understand, to participate, and to comply with the clinical study requirements.
5. Body mass index (BMI) ≥ 18.5 and < 24.9 kg/m2
6. Agreeing to maintain lifestyle habits during the study (same dietary and physical activity habits)
7. Having a smartphone and a history of consistent tracking of training and health records via apps.
8. Having regular GI issues during exercise, preferably also during menstruation. During pre-screening, this will be assessed with a shortened GSRS questionnaire, where participants need to have had, in the month preceding pre-screening, the equivalent of at least a score of 2 in a GSRS focus area (diarrhoea, pain, indigestion).
9. No relevant self-reported abnormalities from medical history.
10. Trained athlete (trains at least 3 times per week for more than 1 hour per day and who has been involved in endurance training for at least 2 years in alignment with previous research (Pfeiffer et al., 2010). This will be evaluated by assessing training records (Strava or similar application, certificates of attendance to athletic events, club membership, etc.).
11. Access to a health-care provider, who can be contacted by the CRO, and who reside in areas with available health services.

Exclusion Criteria:

1. Individuals who are pregnant, lactating, or plan to become pregnant before study end.
2. Individuals with secondary amenorrhea.
3. Individuals who use hormonal contraceptives during the last 6 months before (and during) the trial.
4. Individuals who are currently participating in another clinical trial or have participated in a trial within the last two months prior to enrolment.
5. Tobacco smokers and vapers.
6. Individuals with acute illness.
7. Individuals with a current musculoskeletal injury or one within the last month.
8. Individuals taking probiotic supplements or who have taken probiotics two weeks prior to enrolment.
9. Individuals controlling chronic diseases with treatments known to alter the intestinal microbiome or to alter GI symptoms, such as certain pain killers, antacids, proton-pump inhibitors, anti-diarrhoeal medication or supplements, and laxatives.
10. Individuals having received antibiotics treatments within three months of trial start.
11. Individuals having planned major changes in lifestyle (i.e. diet, weight loss program, exercise level, travelling) during the duration of the study.
12. Individuals who have any hypersensitivity/allergy to any of the components of the test product.
13. Individuals who have planned major elective surgery during the study period.