Effects of dietary protein alterations on circadian rhythms of gastrointestinal peptides in rats.

Altered protein diets and circadian rhythms of gastrin and cholecystokinin (CCK) were investigated in 126 male and 126 female Sprague-Dawley rats acclimated for two weeks to a 12:12 hr light-dark cycle. Rats were divided equally and fed low-protein (8%), high-protein (64%) or normal protein (27%) diets for four weeks. All animals were fasted for 24 hr prior to blood collections. Blood samples were collected at 4-hr intervals for 24 hr for determination of plasma gastrin and CCK using specific radioimmunoassays. A significant rhythm for gastrin was detected in males on normal and low-protein diets (P less than 0.03) and in females on low-protein diets (P less than .02). A significant rhythm for CCK was detected (P less than 0.05) in rats of both sexes fed normal and high-protein diets. Mean plasma levels of both peptides were lower in females than males. In a separate study, food intake and body weight were monitored in male rats receiving the three diets over 21 days. Animals on the low-protein diet exhibited reduced food intake and body weight compared to rats fed the normal or high-protein diets.