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Fifty milliliters of a liquid diet were administered daily to rats in three different ways: 1) orally, beginning at 0000 h; 2) by continuous intravenous infusion; and 3) by discontinuous intravenous infusion from 0000 to 1400 h. Animals were killed every 6 h over a 24-h period. Activity profiles as a function of time of day were determined for the following small intestinal parameters; monosaccharide transport; five disaccharidases; alkaline phosphatase; gamma-glutamyltransferase; leucylnaphthylamide hydrolyzing activity; villus height and width; and number of columnar cells lining a villus section. Circadian rhythmicity as previously reported was observed for all parameters in rats fed orally for 7 days but was not observed in any parameters in rats fed by continuous infusion for 9 days. Rats fed by discontinuous infusion for 10 days maintained circadian rhythmicity in the following functions: monosaccharide transport; disaccharidase activities; and columnar cell number. Thus, rhythmicity in these functions can exist without nutrient delivery to the alimentary tract and presumably arises from involvement of a neuro-endocrine component. The other activities tested appear to require the alimentary tract for the existence of circadian rhythmicity.

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Restricted feeding at two distinct times of the day was associated with different adaptive responses in rats. Animals meal-fed from 1400 to 1800 hours for 6 days demonstrated feeding-cued circadian rhythms in liver weight and in liver transaminase and lactic dehydrogenase enzyme activities. Daily fluctuations in the levels of blood immunoreactive insulin and gastrin were also found to be cued by food intake. By comparison, rats fed from 0700 to 1100 hours for 6 days exhibited altered liver function and blood immunoreactive gastrin patterns. It is suggested that meal-feeding in rats is associated with certain adaptive responses which are dependent upon the timing of the meal. The observed metabolic alterations consequent to meal-feeding are much more apparent in morning-fed than in afternoon-fed animals.