Intestinal water absorption

The small intestine absorbs the majority of water, while the large intestine further concentrates the residual material by absorbing the remaining water and electrolytes.^[2][3] Roughly 9 liters of water enter the small intestine daily (this fluid is a mix of ingested water and gastrointestinal secretions),^[4][5] where about 8 - 8.5 liters are absorbed.^[5] The remaining water is absorbed by the large intestine, with only around 100 ml being lost in the feces.^[6]

The main mechanism for intestinal water absorption is osmosis, where water flows from an area with lower solute concentration (and thus higher water concentration) to an area with higher solute concentration.^[1] Sodium is involved in this procedure. Through active transport, it enters sodium channels and sodium/potassium pumps across the intestinal lining, creating an osmotic gradient.^[1] Other electrolytes are also involved, such as potassium, chloride, and bicarbonate, which also affect the osmotic gradient.^[7][8] Water can also move through passive transport across cell membranes, following the concentration gradient without requiring energy.^[9]

Aquaporins, such as AQP3 in the colon, are specialized water channel proteins which significantly increase water permeability across cell membranes to facilitate water absorption.^[10] Co-transporters, such as NKCC1 (Na+-K+-2Cl- cotransporter) and NHE3 (Na+/H+ exchanger) indirectly contribute to water absorption by delivering sodium and other electrolytes into cells and establishing the osmotic gradient for water to follow.^[9] Finally, glucose can enhance sodium and water absorption in the small intestine, aiding in rehydration.^[11]

Intestinal water absorption imbalance of the normal process of water and electrolyte absorption results in excessive fluid loss (diarrhea), insufficient fluid removal (constipation),^[3] as well as more serious complications like dehydration and electrolyte imbalances.^[12][13] Other complications include weight loss, steatorrhea (fatty stools), and deficiencies in essential vitamins and minerals. In rare cases, the imbalance can contribute to kidney stones (nephrolithiasis) or gallstones.^[13][14]

This imbalance can stem from a variety of reasons such as infections, inflamatory conditions (Crohn's disease or celiac disease), pancreatic pathologies, certain medications and dietary habits.^[15][16] Treatment of intestinal water absorption imbalance can be done by addressing the underlying cause (malabsorption, diarrhea, or other causes), rehydrating with oral rehydration solutions (ORS) in mild to moderate dehydration, intravenous fluids in severe dehydration, and, in some cases, using medications to manage symptoms.^[13][17] In cases of malabsorption, dietary changes, supplemental nutrition, and digestive enzyme replacement may be helpful.^[17][18]