Diarrhea is defined as “the passage of three or more abnormally loose or watery stools in the previous 24 hours”. A persistent or chronic diarrhea is “an episode of diarrhea lasting more than 14 days” .
Which diet in case of diarrhea?
According to the World Gastroenterology Organisation (WGO), food intake must continue during a episode of diarrhea and it is important to stay well hydrated. The recommendations are:
- Frequent and small meals throughout the day, particularly for infants and young children. Infant require more frequent breastfeeding or bottle feedings.
- Energy and micronutrient-rich, mixed foods (grains, eggs, meats, fruits, and vegetables)
- Increasing energy intake as tolerated following the diarrheal episode
- Avoid canned fruit juices 
In addition, consumption of milk can aggravate diarrhea. Indeed, during the diarrhea episode, lactase is often less secreted, leading to difficult digestion of the lactose contained in milk .
Contrary to milk, yogurt is recommended because the lactose it contains is partially digested by bacterial cultures. 
How can yogurt help prevent and treat diarrhea?
Some fermented milks contain probiotics (microorganisms that have shown evidence in providing a health benefit). Thanks to the presence of these microorganisms, those specific probiotic fermented milks may play a role in the prevention and treatment of diarrhea. Indeed, by modulating the intestinal microbiota, these microorganisms might help protect the intestine from gastrointestinal diseases, such as irritable bowel syndrome, infectious diarrhea and allergy gastroenteritis . However, more research on how probiotics in foods can help, treat and prevent symptoms is needed.
In an intervention study comparing several diet in children with persistent diarrhea, the data showed that children fed the yogurt-based diet had a significant reduction in stool output and in the duration of diarrhea . Another intervention study showed that consumption of yogurt was an effective method for reducing the incidence of antibiotic-associated diarrhea (AAD) in children (1-12 years old) . In elder people, the prevention of AAD by standard yogurt and fermented milk has not be proven .
The WGO Global Guidelines recommend yogurt containing certain strains of probiotics to prevent diarrhea such as the Clostridium difficile associated diarrhea. The same guidelines precise that yogurt also prevent symptoms of lactose maldigestion.