“Stomach flu” is a common illness, but it can cause dehydration that can be serious, resulting in hospitalization. The good news is that a study of children with stomach flu shows that giving diluted apple juice is an effective way to prevent serious complications.
The study of about 650 children between 6 months and 5 years of age who had stomach flu, compared two different types of treatment. Half the children were given half-strength apple juice, followed by whatever types of fluids they preferred to drink. The other half of the children were assigned to drink an apple-flavored electrolyte drink. The study was published in the prestigious medical journal of the American Medical Association, JAMA, in 2016. 
The Canadian researchers who conducted the study concluded that for children with mild gastroenteritis (stomach flu) who were in the study with only slight dehydration, drinking diluted apple juice and other drinks that they liked was more effective than electrolyte drinks.
- Freedman, Willan, Boutis, Schuh: Effect of Dilute Apple Juice and Preferred Fluids vs Electrolyte Maintenance Solution on Treatment Failure Among Children With Mild Gastroenteritis: A Randomized Clinical Trial, 2016. 10