DELAYED PLASMA-CLEARANCE OF PHENYLALANINE AND TYROSINE IN ELDERLY MEN

The plasma levels of 17 amino acids were measured in three groups of men: healthy young men, healthy elderly men, and demented tube-fed elderly men living in a nursing home, prior to, and again 2, 4, and 6 hours after the consumption of a standard protein-containing meal. The standard meal provided per kilogram of body weight 8.3 calories, 0.33 grams of protein, 0.90 grams of carbohydrate, and 0.37 grams of fat. The concentrations of all amino acids, except tryptophan, rose significantly at 2 hours and returned to baseline by 6 hours. Plasma phenylalanine and tyrosine were significantly (P < 0.05) higher at 2 hours in the two groups of elderly men than in the young healthy men. In other respects the pre- and post-prandial amino acid profiles were not influenced by age. The ratio, tyrosine/other large neutral amino acids (tyr/LNAA), was significantly higher in both elderly groups than in the young men at nearly all time points before and after the test meal. It was generally higher in the demented older men than in the healthy older men. The data demonstrated a delayed plasma clearance of phenylalanine and tyrosine in old age. The elevated plasma tyr/LNAA ratio in the elderly men may have tended to augment an entry of tyrosine, the precursor of norepinephrine and dopamine, into their brains.