Rationale: constipation-related complaints are prevalent in 5-20% of the population.

Dietary fibers play a crucial role in improving and maintaining gut health, increasing stool weight, stool frequency and improvement of stool consistency. Currently, very few adults meet the recommendation of 30 (females) or 40 (males) grams of fiber per day. Personalized dietary advice may be the solution to increase dietary fiber intake and reduce constipation-related complaints in large populations. Objective: To investigate the effectiveness of personalized dietary advice (PDA) in reducing constipation-related complaints, by increasing dietary fiber intake in people with constipation-related complaints. Study design: This study has a one-group pre-test post-test design with a run-in period.

The duration of the study is 8 weeks, which includes a 4-week run-in phase and a 4-week intervention period. All subjects receive the PDA. Study population: adult subjects with constipation-related complaints, defined as predominant Bristol stool form between 1-4 and not satisfied with their bowel habits (scale ranging from 1-10, cut-off <6). Possibly stool frequency ≤4 stools per week will be included as a definition. Subjects need to have a relatively low dietary fiber intake defined as <26 grams (females) or <33 grams (males), which is ≥15% below the recommendation of fiber intake. Intervention: personalized advice based on their habitual food pattern (as assessed using a food frequency questionnaire) and preferences. Based on a special algorithm, the PDA provides high fiber alternatives for low-fiber products that subjects currently use, close to their current eating behavior, to help increase dietary fiber intake. This PDA will be provided using an online web-portal.

Main study parameters/endpoints:primary outcomes are stool pattern, gastrointestinal complaints and constipation quality of life and severity. Secondary parameters include dietary fiber intake, physical activity, body weight, psychological questionnaires, and fecal microbiota composition and metabolite levels. Furthermore, the PDA will be evaluated.

Inclusion Criteria:

• Age 18- 55 years

• BMI <30 kg/m2

• Has constipation related complaints: response <6 to the question "how satisfied are you with your stool pattern", which is rated on a visual analog scale (VAS) from 1-10, together with at least one of the following criteria:

  1. Habitual stool form of Bristol stool type 1-4 or
  2. ≤4 defecations per week

• living in the surroundings of wageningen (max. 50km)

• in possession of a computer with chrome browser and a mobile phone compatible with applications Note: these criteria are less stringent than the official "constipation" definition, because we preferably want to include participants that do not use laxatives (yet), and we assume that in this 'mild constipation' group increasing fiber intake can have the most beneficial effects. Moreover, we chose not to use the Rome IV criteria as definition of constipation-related complaints, due to the proven inability to distinguish functional constipation from Irritable Bowel Syndrome. If there are sufficient eligible participants, the participants with the lowest stool frequency will be selected

• Relatively low fiber intake (females <26 grams, males <33 grams). When enough eligible participants are available, we will choose the participants with the lowest dietary fiber intake.

• signed informed consent

Exclusion Criteria:

• Subjects with a disease that may interfere with the personal dietary advice or outcomes, such as a known autonomic disorder, inflammatory bowel disease, coeliac disease, cancer, dialysis patients, chronic kidney failure, depression or hypothyroidism.
• Currently following strict diet and unwilling or unable to change; for example a gluten free diet or a "crash diet" using meal substitutes
• Use of medication that can interfere with the study outcomes, including diuretics, antidepressants, codeine, antibiotics or fiber supplements. Preferably laxative use is excluded, but if recruitment is hurdled by this, we at least exclude specific laxatives like resolor, relistor and constella. These are laxatives who have a significant impact on bowel movements, and are only available on prescription. Osmotic (over the counter available) laxatives will be included if recruitment is difficult. Subjects will then be asked to keep their laxative use stable or reduce, but not to increase usage from their habitual pattern (unless necessary, but then report it to the researchers). Moreover, additional questions regarding laxative usage will be included to estimate the effect of laxatives on stool pattern.
• Female subjects: currently pregnant or breastfeeding, or intending to become pregnant during the study, as this can affect stool patterns and wellbeing.
• Are simultaneously participating in another study.
• Unwilling or unable to fulfil the study criteria.
• Student or employee working at Food, Health and Consumer Research from Food and Biobased Research, or Department of Human Nutrition & Health, Wageningen University.

participants are not informed about the content and focus of the personalized dietary advice