Intestinal absorption of vitamin D sterols: Differential absorption into lymph and portal blood in the rat

Radiolabeled vitamin D sterols (5 nmol) were administered intraduodenally (in fat emulsion) into conscious rats with mesenteric lymph fistula and portal vein cannulation. The rats were kept for 4 h in restraining cages, and lymph and blood samples were collected at half-hour intervals. Vitamin D₃ was absorbed mostly into the mesenteric lymph (6.5% of administered dose vs. < 1%/4 h into the portal blood). 1,25-Dihydroxyvitamin D₃ was absorbed mainly into the portal blood (~7%/4 h and only 1.6%/4 h into the mesenteric lymph). 25-Hydroxyvitamin D₃ had a similar rate of absorption into the lymph and portal vein (5%/4h and 7%/4 h, respectively). Vitamin D₃ absorption started after 2 h, whereas the more hydroxylated metabolites were more rapidly absorbed and appeared in the lymph and portal blood within the first half-hour. In the lymph the vitamin D sterols were transported mainly on chylomicrons and lipoproteins (> 50%), whereas in blood less than 20% were associated with plasma lipoproteins. In rats where lymph was not diverted, the increment of lymphatic contribution was observed in plasma levels for all vitamin D sterols. The absorbed sterols in lymph and blood were mostly unchanged with the exception of 25-hydroxyvitamin D₃, about one-third of which underwent esterification during absorption into the mesenteric lymph. We conclude that 25-hydroxyvitamin D₃ and 1,25-dihydroxyvitamin D₃, in particular, are absorbed directly into portal blood independent of fat absorption, and thus may be prescribed as oral supplements in most patients with fat malabsorption.