Functional constipation (FC) in children is a common gastrointestinal (GI) disorder with a worldwide prevalence ranging from 0.7% to 29.6% (1). Complaints include infrequent bowel movement, painful defecation due to hard and/or large stools, faecal incontinence, and abdominal pain (2). Although the condition is rarely life-threatening, it strongly impairs quality of life. Fibres such as inulin and GOS have shown to relief constipation symptoms in young adults and elderly. However, sufficient evidence is lacking linking additional fibre intake to improve symptoms in children with FC. Moreover, the gut microbiota composition, which is highly influenced by dietary intake, has been correlated with various diseases and has become an important therapeutic target. Several studies with fibres but also with faecal microbiota transplants in adults imply a role for the microbiota and modulation thereof in constipated patients. Taking both physiological and microbiota changing properties of these fibres into account, we hypothesize that representative prebiotic fibres might be able to relief symptoms of constipation in children as well. Therefore the objective of the INSIDE study is to investigate the effect of GOS enriched formula milk versus a control milk on stool consistency, frequency, painful defecation, quality of life of the child and parent(s), side effects, faecal incontinence, abdominal pain and school attendance and gut microbiota related outcomes in children of 1-3 years of age diagnosed with FC according to the Rome IV criteria for Functional Gastrointestinal Disorders (FGIDs). The primary objective is to investigate the effect of dietary fibres on measures of clinical efficacy of dietary fibres in the treatment of childhood constipation. Key objectives are 1. To evaluate the early dietary intervention with fermentable soluble fibres on symptoms of constipation in children. 2. To evaluate the fibre fermentative capability of specific beneficial bacteria in supporting SCFA levels. 3. To evaluate the role of early dietary intervention on defecation frequency and stool consistency.