Mechanism of early tumor anorexia.

Whether in tumor-bearing rats a temporal relationship exists between an increase in plasma free tryptophan (FTRP), an increase in brain serotonin (5-HT), and onset of anorexia was studied. Rats were assigned to three groups: tumor-bearing (TB), pair fed (PF), and controls. Food intake was recorded daily. In TB rats anorexia developed on Day 18 and thereafter food intake decreased progressively until end of study. After tumor inoculation, tumor became palpable on Day 10 and continued to grow exponentially until end of study. Rats were killed on Days 6, 10, 16, 18, 22, and 26 to determine plasma FTRP, FTRP/LNAA, and brain 5HT and compared to PF and controls. On Day 6, before tumors became detectable, FTRP and FTRP/LNAA were increased (P < 0.05) in TB rats vs controls. Both continued to increase so that by Day 18 when food intake had started to decrease (P < 0.05), brain 5-HT increased and correlated with the onset of anorexia (R2 = 0.6, P < 0.05). Increases in plasma FTRP the precursor to brain 5-HT occurred in TB rats before physical appearance of tumor and increased until an increase in brain 5-HT occurred, leading to anorexia.