USE OF A DIETARY MANIPULATION TO DEPLETE PLASMA TYROSINE AND PHENYLALANINE IN HEALTHY-SUBJECTS

The aim of the present study was to lower plasma concentrations of tyr osine, the amino acid precursor of noradrenaline and to determine whet her this manipulation impaired noradrenergic function as measured by t he evening rise in concentrations of plasma melatonin. Eight healthy v olunteers received three drinks: (i) an essential amino acid load with tyrosine, (ii) the same load without tyrosine and its precursor, phen ylalanine and (iii) tap water. The tyrosine- and phenylalanine-deficie nt drink lowered plasma tyrosine by approximately 50% over 5 h. Howeve r, this did not alter the evening plasma melatonin levels compared to the other two drinks. The results suggest that amino acid loading prod uces a modest decline in plasma tyrosine levels but this does not lowe r noradrenergic neurotransmission in the pineal gland.