Elevation of plasma glucose and glucagon after tryptophan ingestion in man

To evaluate the effect of tryptophan on blood sugar in man, we have orally administered 10 g of this amino acid to 14 normal subjects and determined their plasma levels of glucose, insulin, glucagon, and growth hormone for 4 hr after the load. Seven of the subjects also received a placebo. Tryptophan intake was followed by a slight but significant elevation of glycemia (maximum increment: 11% above basal values at 180 min, p = 0.02). This elevation of plasma glucose was accompanied by a clear rise of glucagon levels (peak: 60% at 140 min, p = 0.0007) and by increased concentrations of circulating insulin and growth hormone. Placebo administration did not significantly modify blood glucose or any of the hormones measured. In contrast to the reported hypoglycemic effect of tryptophan in rats, our data indicate that this amino acid increases plasma glucose in man. Given that tryptophan appears to possess the capability of eliciting glucagon secretion, its effect on blood glucose can be reasonably attributed to an enhanced glycogenolysis and/or gluconeogenesis provoked by its release of this hormone.