Mechanism of Appetite Control in Rats Consuming Imbalanced Amino Acid Mixtures

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Abstract
The hypothesis was tested that reduced food intake following ingestion of an imbalanced amino acid mixture is a consequence of abnormally low tissue concentrations of the free limiting amino acid. Rats were fed diets containing only L-amino acid mixtures as their protein source. Groups of animals received ad libitum a basal diet, a histidine-imbalanced diet, or a corrected diet. Controls included the histidine-imbalanced diet force-fed to match the ad libitum consumption of the corrected diet and the corrected diet pair-fed to match the ad libitum consumption of the imbalanced diet. In experiments of this type free concentrations of all amino acids were determined at short time intervals in liver, muscle, and plasma, and food consumption was measured. Marked depression of the concentration of the limiting amino acid occurred in 3 to 5 hours, the reduction in diet consumption appearing some 20 hours later. In experiments where animals that were adjusted to an imbalanced mixture were supplemented with the limiting amino acid, there was again a lag of 18 to 20 hours between correction of the tissue levels of the free limiting amino acid and an increase in diet consumption. These data suggest that it is not tissue levels of free limiting amino acid per se that suppress or stimulate intake of the imbalanced diet, but rather some metabolic consequence of these tissue concentrations. Growth experiments established that almost all the growth depression resulting from the imbalanced group is accounted for by the reduced food intake.