Colonic absorption: the importance of short chain fatty acids in man.

The human large intestine absorbs Na+, Cl- and water from its lumen and secretes HCO-3 and some K+. The primary event in absorption is thought to be the active transport of Na+ ions out of the cell and across the baso-lateral cell membrane, by the energy requiring Na+-K+ ATPase. This leads in turn to Na+ entry into the cell via its luminal border and the creation of a potential across the mucosa which drives the transport of other ions. Cl- is coupled to HCO-3 secretion through a common carrier and K+ enters the intestinal lumen partly through an active secretory pathway. Most ions probably cross the epithelium by both transcellular and paracellular (shunt) pathways, water moving in response to solute transport. However the colon is not normally perfused by a saline-bicarbonate solution. It contains an active microflora which ferment 30 g or more of carbohydrate daily, derived from diet and intestinal secretions, with the production of at least 300 mmol of short chain fatty acids (acetic, propionic and butyric acids). About 6 g of urea is also degraded to NH3. These metabolic processes result in the generation of solutes which are then transported across the mucosa and which alter the pattern of water and electrolyte transport significantly. Short chain fatty acids are rapidly absorbed by passive diffusion as the undissociated acids, although anion transport, possibly through a paracellular route, is also feasible. Their absorption leads to the accumulation in the lumen of HCO3, a rise in pH, fall in pCO2 and stimulation of Na+ and water transport. The effect on Na+ transport is thought to indicate the presence of a Na+/H+ exchange in the cell membrane. The amounts of these organic solutes produced in the colon each day are probably greater than the total numbers of inorganic ions such as Na+, K+, Cl- and HCO-3 and as such must be taken into account in any understanding of overall transport processes in the large intestinal epithelium.