The effect in humans of ingesting different dietary proteins on plasma tryptophan, tyrosine, and the amino acid ratios that predict their brain uptake

Overnight‐fasted healthy males (n=6) with venous catheters ingested on four mornings 12oz V‐8 Juice containing 40 g of α‐lactalbumin (LAC), wheat gluten (GLUT), zein or starch. Blood was taken before ingestion and at 30 min intervals for 4 hr. Tryptophan (TRP), tyrosine (TYR) and other large neutral amino acids (LNAA) were assayed by HPLC. Predictors of brain TRP and TYR uptake (plasma TRP/∑LNAA, TYR/∑LNAA) were calculated. 0‐Time plasma TRP was ≈50 nmol/ml; TRP/∑LNAA was ≈0.010. LAC increased plasma TRP (3‐fold) and TRP/∑LNAA (50%) within 2 hr; starch did not change either TRP variable, while GLUT caused modest (25%) and zein larger reductions (50%) in both variables. The peak difference in plasma TRP/∑LNAA between LAC and zein was ≈3‐fold. Changes in TYR/∑LNAA were minimal. Plasma TRP/∑LNAA predicts brain TRP uptake and serotonin (5HT) synthesis from TRP in rats. The results thus suggest that in humans, LAC ingestion may raise and zein ingestion lower 5HT synthesis in/release by brain neurons, and perhaps produce 5HT‐mediated changes in mood and behavior.