The effects of different foods to normal bowel function in adult population will be investigated. A randomised controlled intervention trial will be conducted on about 120 (30 per group) adult (18-70 years) subjects with functional constipation and irritable bowel syndrome with constipation according to the IV. Rome criteria. At least one week before inclusion in the study, patients will stop taking laxatives and other anti-constipation medications. Subjects will be randomly selected to consume one of the test foods (fresh apples, dried apples, sauerkraut) or the active control (psyllium) in the equivalent amount of 3 g of fiber per day for 4 weeks. Subjects will be asked not to change their lifestyle habits, such as diet and physical activity level, and not to take oral laxatives during the period of participation in the study.

1. Inclusion criteria:

   • Signed informed consent of the participant to participate in the study
   • Age between 18 and 70 years
   • Functional constipation or irritable bowel syndrome
   • Expressed willingness to eat the investigated food daily (sauerkraut, apples, dried apples or psyllium; period of four weeks)

2. Exclusion criteria:

   • Known allergy to the investigated foods (fresh apples, dried apples, sauerkraut or psyllium)
   • Pregnancy or breastfeeding
   • Previous gastrointestinal surgery with the exception of cholecystectomy and appendectomy
   • Other gastrointestinal diseases (chronic inflammatory bowel disease, celiac disease, gastroparesis, etc.)
   • Serious chronic diseases
   • Introduction of extensive changes in diet or physical activity in the last 4 weeks before the inclusion examination
   • Regular use of laxatives or inability to stop using laxatives for the duration of the study
   • Concurrent participation in another study

Intervention include consumption of specific foods, therefore masking to participant and investigatior is not possible. Collected data will be masked for accessor of outcomes.