Schedule of Protein Ingestion, Nitrogen and Energy Utilization and Circadian Rhythm of Hepatic Glycogen, Plasma Corticosterone and Insulin in Rats

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Abstract
Apparent protein and lipid synthesis was studied both in growing rats and in depleted-repleted adult rats. The circadian rhythm of liver glycogen and of plasma glucose, corticosterone and insulin were studied in adult rats. Animals either received a mixed diet ad libitum (8% protein) or a protein meal (1.1 g protein) given at different hours in the day (05, 09, 13, 17, 19 and 23 hours) with free access to a protein-free diet (separately fed). When the protein meal was ingested during the lighted period (07 to 19 hours), growth, weight repletion and nitrogen retention were lower (P < 0.05) than in rats consuming protein during darkness (19 to 07 hours). Moreover, in the latter case, tissue and liver lipid levels were increased (P < 0.05) and the circadian rhythm of liver glycogen was modified (shorter glycogenolysis and extended glycogen synthesis). The circadian rhythms of plasma corticosterone and insulin were essentially similar in rats fed the mixed diet or protein separately. Since ingestion of protein at different times throughout the day did modify protein and lipid deposition as well as the circadian rhythm of liver glycogen, we conclude that a close reciprocal correlation must exist between the mode of feeding, the utilization of nutrients and the rhythm of metabolic activity.