Vitamin K2 colonic and ileal in vivo absorption: bile, fatty acids, and pH effects on transport.

Colonic and ileal absorption of vitamin K2 ([2-methyl-3H]menaquinone-9) was investigated in the conscious rat. When the absorption rate was plotted against the perfusate concentration, a linear relationship was found between these two parameters in the ileum and colon. The absorption rate of menaquinone by the ileum was increased as the bile salt concentration, degree of unsaturation of the added long-chain fatty acids, hydrogen ion concentration, and perfusate flow rates were increased. Colonic menaquinone absorption decreased as the bile salt concentration was increased. Menaquinone colonic absoprtion increased as the pH decreased, but no change was noted as the perfusion rate was increased. The present experimental observations in vivo, coupled with prior observations in vitro, indicate that absorption of menaquinone by the ileum and colon occurs by a passive diffusion process that is modified by variations in the perfusate bile salt concentration, the presence of unsaturated fatty acids, and the perfusate pH. The present observations indicate that the mammalian colon and terminal ileum can provide a constant source of vitamin K to aid hemostasis despite episodic lack of dietary vitamin K.