I was curious about the need for glucose in oral rehydration therapy.
Here's what Wikipedia says about the need for glucose to accompany the salt. (I've bolded a couple of bits)
https://en.wikipedia.org/wiki/Oral_rehydration_therapy
Physiological basis
Fluid from the body enters the
intestinal lumen during
digestion. This fluid is
isosmotic with the blood and contains a high quantity, about 142 mEq/L, of
sodium. A healthy individual secretes 2000–3000 milligrams of sodium per day into the intestinal lumen. Nearly all of this is reabsorbed so that sodium levels in the body
remain constant. In a diarrheal illness, sodium-rich intestinal secretions are lost before they can be reabsorbed. This can lead to life-threatening
dehydration or
electrolyte imbalances within hours when fluid loss is severe. The objective of therapy is the replenishment of sodium and water losses by ORT or intravenous infusion.
[40]
Sodium absorption occurs in two stages. The first is via intestinal
epithelial cells (
enterocytes).
Sodium passes into these cells by co-transport with glucose, via the
SGLT1 protein. From the intestinal epithelial cells, sodium is pumped by
active transport via the
sodium-potassium pump through the
basolateral cell membrane into the
extracellular space.
[41][42]
The
sodium–potassium ATPase pump at the basolateral
cell membrane moves three sodium ions into the extracellular space, while pulling into the enterocyte two
potassium ions. This creates a "downhill" sodium
gradient within the cell.
SGLT proteins use energy from this downhill sodium gradient to transport
glucose across the
apical membrane of the cell against the glucose gradient. The co-transporters are examples of
secondary active transport. The GLUT
uniporters then transport glucose across the basolateral membrane. Both SGLT1 and SGLT2 are known as
symporters, since both sodium and glucose are transported in the same direction across the membrane.[
citation needed]
The co-transport of glucose into epithelial cells via the
SGLT1 protein requires sodium. Two sodium ions and one molecule of glucose (or
galactose) are transported together across the
cell membrane via the SGLT1 protein.
Without glucose, intestinal sodium is not absorbed. This is why oral rehydration salts include both sodium and glucose. For each cycle of the transport, hundreds of water molecules move into the epithelial cell to maintain osmotic equilibrium. The resultant absorption of sodium and water can achieve rehydration even while diarrhea continues.
[40]
