Learned preference for the limiting amino acid in rats fed a threonine-deficient diet.

Preference for an odor associated with protein is only seen in protein-deprived rats. We hypothesized that rats depleted of the amino acid threonine (THR) would prefer a flavored solution paired with a THR replete diet. Rats were given a flavored drink paired with THR-deficient (DEV) diet followed by a second flavor paired with a corrected (COR) diet. In choice testing, the animals clearly selected the COR-paired flavor, while control rats preferred the other flavor. This did not, however, differentiate between aversion to the DEV-paired flavor and learned preference for the COR-paired flavor. In subsequent tests, an unpaired flavor was given rather than the DEV-paired flavor. The COR-paired flavor was included in the test as before. In the second and third trials, using either DEV or a less profoundly deficient diet, animals depleted of THR selected the COR-paired flavor to a greater extent than control rats. We conclude that animals deficient in the essential amino acid, THR, show a learned preference for the flavor paired with repletion.