Although often considered a benign disease, acute gastroenteritis remains a major cause of morbidity and mortality in children around the world, accounting for 1.34 million deaths annually in children younger than 5 years, or roughly 15% of all child deaths.  As the disease severity depends on the degree of fluid loss, accurately assessing dehydration status remains a crucial step in preventing mortality. Luckily, most cases of dehydration in children can be accurately diagnosed by a careful clinical examination and treated with simple, cost-effective measures. Although dehydration technically refers to pure water loss and can be associated with euvolemic or even hypervolemic states in certain pediatric disorders, the term is used throughout this article in its more general sense to mean overall fluid or volume loss due to diarrhea.
Adequate fluid balance in humans depends on the secretion and reabsorption of fluid and electrolytes in the intestinal tract; diarrhea occurs when intestinal fluid output overwhelms the absorptive capacity of the gastrointestinal tract. The 2 primary mechanisms responsible for acute gastroenteritis are (1) damage to the villous brush border of the intestine, causing malabsorption of intestinal contents and leading to an osmotic diarrhea, and (2) the release of toxins that bind to specific enterocyte receptors and cause the release of chloride ions into the intestinal lumen, leading to secretory diarrhea. 
Even in severe diarrhea, however, various sodium-coupled solute co-transport mechanisms remain intact, allowing for the efficient reabsorption of salt and water. By providing a 1:1 proportion of sodium to glucose, classic oral rehydration solution (ORS) takes advantage of a specific sodium-glucose transporter (SGLT-1) to increase the reabsorption of sodium, which leads to the passive reabsorption of water. Rice and cereal-based ORS may also take advantage of sodium-amino acid transporters to increase reabsorption of fluid and electrolytes. 
Children in the United States experience, on average, 1.3-2.3 episodes of diarrhea each year. Overall, acute gastroenteritis accounts for than 1.5 million outpatient visits, 220,000 hospitalizations, and direct costs of more than $2 billion each year in the United States alone. 
A study by Hullegie et al investigated the effects of first-year daycare attendance on acute gastroenteritis incidence and primary care contact rate up to age 6 years. The study found that first-year daycare attendance advances the timing of acute gastroenteritis infections, resulting in increased acute gastroenteritis disease burden in the first year and relative protection thereafter. The study also added that protection against acute gastroenteritis infection persists at least up to age 6 years. 
Worldwide, children younger than 5 years have an estimated 1.7 billion episodes of diarrhea each year, leading to 124 million clinic visits, 9 million hospitalizations, and 1.34 million deaths, with more than 98% of these deaths occurring in the developing world. [1, 5, 6, 7]
Although the prevalence of acute gastroenteritis in children has changed little over the past 4 decades, mortality has declined sharply, from 4.6 million in the 1970s to 3 million in the 1980s and 2.5 million in the 1990s.  One of the most important reasons for this decline has been the increasing international support for the use of oral rehydration solution (ORS) as the treatment of choice for acute diarrhea, with the proportion of diarrheal episodes treated with ORS rising from 15% in 1984 to 40% in 1993. 
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