Effect of amino acid imbalance and deficiency on dietary choice patterns of rats

Detailed dietary choice patterns were determined with a computerized feeding monitoring system in groups of Sprague Dawley rats kept on a 12:12 hr light-dark cycle and offered in sequence a series of dietary choice regimens involving amino acid-imbalanced or deficient diets with threonine as the most limiting amino acid. Animals established their preference for a threonine-basal diet over a threonine-imbalanced or a threonine-devoid (devoid of threonine) diet shortly (within 2-3 hr) after the consumption of small quantities of either diet in the beginning of the first dark-cycle. An intensive sampling process characterized by frequent small bouts was evident throughout the light period. Both the meal size and the meal frequency of the imbalanced or devoid diet were curtailed after prolonged choices. Animals preferred the threonine-corrected (imbalance corrected by threonine supplementation) over the threonine-basal diet initially with an increase in meal frequency. But no clear choice for either diet was observed thereafter. Animals did not establish their preference for the threonine-corrected diet when paired with the threonine-devoid diet until after 5 days with a steady decrease in the meal size of the devoid diet but not the meal frequency. When the protein-free diet was introduced as an alternative for the threonine-imbalanced diet, animals selected the protein-free diet during the first dark-cycle after consuming a small amount of the imbalanced diet. Initially there was a drastic reduction in meal size of the imbalanced diet and subsequently a decrease in meal frequency as well. Nevertheless, animals immediately rejected the protein-free diet and chose the threonine-basal diet when it replaced the imbalanced diet as an alternative. The almost exclusive preference for the basal diet occurred in the beginning of the first dark-cycle with an increase in meal size but no change in meal frequency. The sampling bouts of small quantities, which followed the first introduction of the diets in the choice regimens, may be an inherent investigative behavior whereby the physical or oropharyngeal properties of the diets are recognized. The establishment of the choices for the alternative diets in the present experiments provides additional information about the rapid time course of the food intake control mechanisms in rats fed amino acid-imbalanced or deficient diets.