Absorption of Short-Chain Fatty Acids from the Rat Cecum in Vivo

Absorption of the short-chain fatty acids (SCFA), acetate, propionate and butyrate, was studied in vivo by measuring their disappearance from the washed cecum of anesthetized rats. The SCFA seemed to be absorbed predominantly by simple passive diffusion, because there was linear absorption over a wide concentration range, and no evidence of competition or enhancement of absorption when more than one SCFA was present in the lumen. Absorption of these three SCFA was independent of chain length. Their absorption was clearly limited by the lipid membrane, because permeability through the unstirred water layers was much greater than through the lipid membrane. Absorption was unaffected by altering the pH of the luminal infusate between 5.4 and 7.4. Based on our observation that, during absorption, there were increases in pH, total CO2 and HCO-3 but no change in pCO2, we suggest that SCFA are absorbed primarily via diffusion involving anionic exchange with HCO-3.