Recently we have shown that surplus dietary tryptophan (TRP) reduced the plasma concentrations of cortisol and noradrenaline in pigs. Stress hormones are known to affect insulin sensitivity and metabolism. We now investigated the long-term effects of surplus dietary TRP on 1) plasma and urinary stress hormone kinetics, 2) insulin sensitivity for glucose and amino acid clearance, and 3) whole body nitrogen balance. Pigs were fed for 3weeks a high (13.2%) vs normal (3.4%) TRP to large neutral amino acids (LNAA) diet, leading to reduced fasting (14 h) plasma cortisol (17.1+/-3.0 vs 28.9+/-4.3 ng/mL, p<0.05) and noradrenaline (138+/-14 vs 225+/-21 pg/mL, p<0.005) concentrations, lower daily urinary noradrenaline (313+/-32 vs 674+/-102 ng/kg day, p<0.001) and adrenaline (124+/-13 vs 297+/-42 ng/kg day, p<0.001) but higher dopamine (5.8+/-0.5 vs 1.5+/-0.2 microg/kg day, p<0.001) excretions, respectively. Insulin sensitivities for both glucose and amino acid clearance, (as measured by the intraportal hyperinsulinaemic (1 mU/kg min) euglycaemic euaminoacidaemic clamp technique), were lower by 22% in pigs on the high vs normal TRP/LNAA diet (14.8+/-1.4 vs 18.9+/-0.9, p<0.05 and 69.7+/-4.3 vs 89.7+/-6.8 mL/kg min, p<0.05, respectively) without affecting urinary nitrogen excretion (35.5+/-1.0 vs 36.6+/-1.0% of dietary nitrogen intake, p=ns). In conclusion, long-term feeding of surplus dietary TRP inhibits both baseline adrenocortical and sympathetic nervous system activity, it induces insulin resistance for both glucose and amino acid clearance but it does not affect whole body protein catabolism. This indicates that the bioactive amino acid TRP contributes to homeostasis in neuroendocrinology and insulin action and that low baseline adrenocortical and sympatho-adrenal axis activity are associated with insulin resistance.