Diurnal rhythmicity of absorption of a lipid compound (vitamin K-1) in vivo in the rat.

The possibility of rhythmicity in the intestinal absorption of lipids was explored by assessing the absorption of vitamin K-1 by the unanesthetized rat at 6 PM, 12 and 6 AM, and 12 PM. A marked variability in the absorption rate of vitamin K-1 was found throughout the 18-hr period. The highest rates of absorption occurred at midnight (139.8 +/- .22 and 134.4 +/- 9.1 pmol/min/10 cm of jejunum and ileum, respectively). The lowest rates of absorption occurred at 6 AM (54.5 +/- 1 and 81.4 +/- 7.4 pmol/min/10 cm of jejunum and ileum, respectively). Absorption rates at noon were not different from absorption at 6 AM but an initial increase in absorption was noted at 6 PM. Synchronization of the absorptive rate with time is most likely related to the time of feeding and not to changes in the pattern of illumination. The possibility of marked diurnal variability in the absorption rate should be considered in the design and execution of intestinal absorption experiments.