Diarrhoea is classified as (1) acute diarrhoea if it lasts seven days or less and as (2) chronic or functional diarrhoea if it lasts longer than 14 days (Guarino et al., 2018). Diarrhoea is a common gastrointestinal disorder in infancy and characterised by the passage of unusually loose or watery stools, generally more than three times a day. The most frequent reasons are viral or bacterial infections associated with acute gastroenteritis (AGE), an inflammation of the stomach and intestine accompanied with diarrhoea and often vomiting. Other reasons are food intolerance, allergies, poisoning or colonic microbioal dysbiosis as consequence of antibiotic treatment (NICE, 2009, Brady, 2018).

According to Rome IV the diagnostic criteria for functional diarrhoea must include all of the following (Benninga et al., 2016):

• "Daily painless, recurrent passage of 4 or more large, unformed stools
• Symptoms that last more than 4 weeks
• Onset between 6 and 60 months of age
• No failure to thrive if caloric intake is adequate"

(1) AGE and the associated diarrhoea is frequently caused by viral infection with Rotavirus, Cytolomegavirus, Adenovirus or Norovirus. Also bacterial infections with Salmonella, Shigella, Campylobacter or pathogenic Escherichia coli can lead to AGE and are less common in industrialised countries. Each year, approximately 1 in 10 children under five years are referred to healthcare services due to gastroenteritis (NICE, 2009). AGE-associated diarrhoea can have serious consequences such as dehydration, a potentially life-threatening condition. Signs of dehydration are irritability or lethargy, decreased urine output, sunken eyes, absence of tears, warmth of extremities, dry mucous membranes and reduced skin turgor (Brandt et al., 2015, NICE, 2009). Acute gastroenteritis may or may not be accompanied by vomiting, which contributes to fluid loss and increases dehydration risk (NICE, 2009).
Vomiting is defined as the forceful ejection of stomach contents up to and out of the mouth and must be distinguished from the normal phenomenon of regurgitation (→).

(2) In contrast, functional diarrhoea is influenced by dietary factors: these include general overfeeding, excessive fructose consumption often associated with excessive fruit juice intake, low fat intake and also overly sorbitol (or other sugar replacer) intake (Zeevenhooven et al., 2017). For functional diarrhoea underlying organic causes such as an impairment of small intestine transport or disfunctional water and electrolyte secretion could not be identified (Benninga et al., 2016). The prevalence of functional diarrhoea in infants (≤ 12 months) is around 2% and in children (13 - 48 months) about 6% (Steutel et al., 2020).

(1) In diarrhoea caused by AGE maintenance of fluid management and adequate diet are priorities. This can prevent or break the cycle and support the healing process instead. Breastfeeding should be continued. In addition, oral rehydration solutions (ORS) are recommended as first line intervention to counteract fluid losses and prevent dehydration states independent of feeding mode (Brandt et al., 2015, NICE, 2009, Guarino et al., 2018). When formula-feeding, particular attention should be paid to preparation and hygiene practices such as cleanliness of bottles and spoons, accurate dosing, and water quality. Manufacturer's preparation instructions should be followed diligently (WHO/ FAO, 2007). Further recommendations for cases of ACE include (NICE, 2009):

• "continue breastfeeding and other milk feeds
• encourage fluid intake
• discourage the drinking of fruit juices and carbonated drinks, especially in those at increased risk of dehydration
• offer oral rehydration salt (ORS) solution as supplemental fluid to those at increased risk of dehydration"

(2) In contrast, medical interventions are generally not needed for the management of functional diarrhoea. Caregivers should be reassured and concerns addressed stating that the disorder is usually self-limiting and, if the energy intake is adequate, harmless. Diet and defaecation diaries can help to eliminate concerns about food allergies and dietary advice regarding the excessive consumption of fruit juices and fructose intake in older children should be supplied. Breastfeeding should be continued (Benninga et al., 2016, Zeevenhooven et al., 2017). In formula-fed infants lactose-free or –reduced infant formula may be tried if the diarrhoea lasts longer than 14 days (Guarino et al., 2018).