Healthy participants with self-reported GI complaints will consume the protein supplement that they normally use for recovery purposes after exercise during a 3-week baseline period, followed by a 3-week intervention period in which they maintain usage of their product, but replace a part of this product with a fermented dairy protein with prebiotic fiber. In addition, a group of healthy participants without self-reported GI complaints will serve as a reference group by consuming the protein supplement that they normally use for recovery purposes after exercise during a 3-week period.

A substantial number of athletes suffer from gastro-intestinal problems, which may impair performance and/or subsequent recovery. Also, gut barrier integrity is impaired with high-intensity training. Emerging studies are showing a positive correlation between the gut microbiome and muscle function, athletic performance, body composition, and natural energy levels. This study will perform an exploratory analysis of the well-being and digestive health of well-trained athletes, and assess the potential effect of fermented dairy protein with prebiotic fiber on these parameters.

Question 1: Does the 3-week supplementation of a fermented dairy protein with prebiotic fiber affect the self-reported digestive comfort compared to a 3-week baseline period? And additionally, will the quality of life (self-reported general well-being and states of mood) change with the use of this type of product?

Question 2: Does the 3-week supplementation of a fermented dairy protein with prebiotic fiber show an alteration in gut microbiota in comparison to a 3-week baseline period, while dietary intake remains unaffected?

Question 3: How is the 3-week supplementation of a fermented dairy protein with prebiotic fiber experienced by the participants (taste and concept liking)?

Question 4: Is there a difference in baseline gut microbiota between athletes with and athletes without self-reported GI complaints.

Inclusion Criteria:

• Healthy subjects living in Arizona in the USA
• Women/Men aged between 18-35 years
• Serious athletes (exercise at least 3 x per week)
• With or without GI tract issues due to sport or protein/diet intake
• 18.5 ≤ BMI ≤ 40 kg/m2
• No physical limitations (i.e. able to perform all activities associated with daily living in an independent manner).
• Use of a protein supplement or sports food after at least one training per day 3 x per week
• Willing to slightly increase their protein supplement intake during the intervention phase for 21 consecutive days
• Stable weight during the last 4 weeks, without a focus on body weight reduction through limiting caloric intake
• Written informed consent

Exclusion Criteria:

• Daily use of (sport) supplements containing probiotics and/or prebiotics
• Smoking
• Self-reported cow's milk protein allergy
• Clinical lactose intolerance
• Clinical milk protein allergy
• House dust mite allergy
• Diagnosed GI tract disorders or diseases
• Musculoskeletal disorders
• 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
• Use of antibiotics or anti-inflammatory medication the past 2 weeks
• Blood donation in the past 2 months
• Pregnant/lactating women
• Athletes should not aim to lose body weight by reducing energy intake during the study

Participants can identify as: Male, Female, Non-binary/third gender, or as: Prefer not to say.