Absorption and metabolism of octanoate by the rat colon in vivo: concentration dependency and influence of alternative fuels.

BACKGROUND Compared with short and long chain fatty acids, medium chain fatty acids (MCFAs) have been shown to provide the highest colonic absorption of substrate carbon. Moreover, colonic epithelial cells fulfil their basic energy requirements as easily from MCFAs as from short chain fatty acids. AIMS To further characterise octanoate as a colonic luminal substrate, we determined in vivo the influence of (i) substrate concentration and (ii) alternative luminal fuels, on rat colonic transport and metabolism. METHODS Segments of rat proximal colon (8 cm) were cannulated and perfused for 100 min with (14)C labelled octanoate. The right colic vein was also cannulated and venous blood analysed for total (14)C, (14)CO(2), and metabolites by scintillation counting and high performance liquid chromatography. RESULTS Tracer appearance in mesenteric blood stabilised after 20-40 minutes of perfusion. Increasing luminal octanoate concentrations from 2 to 40 mM resulted in linear increases in total carbon absorption. Maximum CO(2) production was reached near 10 mM. A substantial proportion of octanoate was absorbed without being metabolised (59-94%). The luminal presence of a mixture of alternative fuels had no influence on either octanoate transport or metabolism. CONCLUSIONS This study demonstrated substantial concentration dependent colonic absorption of octanoate, rendering this MCFA a potential and much needed high energy substrate for patients with compromised small bowel function. Moreover, octanoate meets the basic energy requirements of colonic epithelial cells in vivo as well as butyrate. This study also demonstrates the divergence of in vitro and in vivo data regarding fatty acid absorption and metabolism in the colonic epithelium.