Oral Rehydration Solution: a four-ingredient formula, decoded

ORS works by exploiting SGLT1 (sodium-glucose cotransporter 1), a protein in the small intestine that binds one sodium ion and one glucose molecule and pulls them across the gut wall together; water follows by osmotic gradient (Wright & Loo, 2000). This is significantly faster than passive water absorption, and crucially it keeps working even when the gut wall is damaged by acute infection — which is why ORS works for cholera and dysenteric illness where IV fluid is unavailable. The discovery of cotransport's role in oral rehydration in the 1960s (in the cholera wards of Dhaka) was described by The Lancet as potentially the most important medical advance of the century, and ORS is credited with saving tens of millions of lives.

The WHO formula has gone through one major revision. The original 1969 ORS used a 90 mmol/L sodium concentration with 111 mmol/L glucose, designed for cholera. In 2002, WHO and UNICEF jointly revised it to a "reduced osmolarity" formulation — 75 mmol/L sodium, 75 mmol/L glucose, 65 mmol/L chloride, 20 mmol/L potassium, and 10 mmol/L citrate, with total osmolarity around 245 mOsm/L. The reduced-osmolarity version produced better outcomes in non-cholera diarrheal illness (which is most of the global ORS use case) by reducing the risk of stool volume increase and modest hyponatremia. It is the current global standard.

A WHO reduced-osmolarity ORS sachet contains sodium chloride, potassium chloride, trisodium citrate, and glucose anhydrous, dissolved in 1 liter of water — delivering roughly 1,725mg sodium, 780mg potassium, and 75 mmol glucose per liter. The bulk-mineral cost is only a few cents per liter, which is part of why ORS is so cost-effective. Commercial products like Pedialyte, DripDrop, and Liquid I.V. borrow from this formula family with different sugar, sodium, zinc, magnesium, flavor, and package choices.