Prolactin stimulation by protein is mediated by amino acids in humans.

In normal humans, ingestion of protein stimulates PRL secretion. I investigated the mechanism of this effect by feeding free amino acids, both singly and in combination, to normal subjects. The serum PRL response to a high protein liquid mixed meal was duplicated by ingestion of an equivalent free amino acid mixture, indicating that intact protein or peptides are not required. The time course of the response and the presence of normal responses in two vagotomized subjects suggest that neither traditional gut hormones nor vagus nerve activity is involved in this response. Of the single amino acids tested, phenylalanine and tyrosine were the most potent stimulators of PRL secretion and can account for most, if not all, of the PRL-releasing activity of the mixed meal. D-Phenylalanine, the biologically inactive optical isomer, was nearly ineffective in releasing PRL. Administration of naloxone, phentolamine, or propranolol did not alter the PRL response to the various test meals, indicating that neither opioid, alpha-adrenergic, nor beta-adrenergic stimulation is involved in meal-induced PRL secretion.