Could a daily shake soothe Athletes' stomach troubles?

Copied word for word from the study's registry entry, so the wording is the study team's rather than ours.

Aside from having participants fitting the inclusion and exclusion criteria below, potential participants will be screened using the gastrointestinal symptom rating scale (GSRS). Based on the outcome they will be assigned to the Part I (i.e. no self-reported GI tract issues (GSRS questionnaire score GI complaints, based on GSRS screening with all items scored \<4 (severe) and a total GSRS score ≤25), or Part II (i.e. Severe self-reported GI tract issues (GSRS questionnaire score GI complaints, based on GSRS screening with at least one item scored ≥4 (severe) and a total GSRS score ≥30). If they don't fit these criteria (i.e., having a GSRS score ranging from 26-29) they will be excluded from participation. Inclusion Criteria: * Healthy subjects living in the Phoenix area in AZ, USA during the study * Women/Men aged between 18-50 years (50-50%) * Recreational competitive and non-competitive athletes (exercise at least 2x per week with a minimal total duration of 2 hours/week) * BMI value between ≥18.5 and ≤ 30 kg/m2 * No physical limitations due to an injury (i.e., able to perform all activities associated with daily living and exercise in an independent manner). * Willing to slightly increase their energy intake via a supplement during the study period on daily basis in the morning before breakfast. * Willing and comfortable to perform multiple physical (stress) tests on a treadmill during the study. * Participants without any contraindications when scoring the Physical Activity Readiness Questionnaire (PAR-Q+). * Written informed consent * For part I: Self-perceived lactose intolerance, and/or omission of dairy products, and/or identifying as Hispanic/Latino or Asian * For part I: Determined to be lactase non-persistent (LNP) CC genotype by qPCR, and/or positive lactose intolerance test (\>20ppm increase from baseline when 25g of lactose administered) Exclusion Criteria: * Users of (sport) supplements containing probiotics and/or prebiotics and those unwilling to temporarily stop use of these supplements at least 3 weeks prior to start of the intervention * Women who plan to start or stop the use of contraceptives before or during study period * Women that have additional or worse GI symptoms during their period that can be classified as severe * Smoking * Clinical lactose intolerance * Clinical or self-perceived milk protein allergy * Diagnosed GI tract disorders or diseases * Musculoskeletal disorders * Diagnosed metabolic disorders (such as diabetes) * Use of any medications known to affect protein metabolism (i.e., corticosteroids, non-steroidal anti-inflammatories, or prescribed acne medications). * Chronic use of gastric acid suppressing medication or anti-coagulants * Chronic use of anti-inflammatory medication * Use of antibiotics the past 0-6 months * Blood donation in the past 2 months * (Intending to become) Pregnant/lactating women * Significant changes in body weight during the past 4 weeks (i.e., due to reduction of body weight through limiting caloric intake, illness,) * Athletes should not aim to lose bodyweight by reducing energy intake during the study * For Part I: A hydrogen breath peak of less than 20 ppm over baseline during a lactose tolerance test