The effects of dieting on plasma tryptophan concentration and food intake in healthy women

Although many people diet, relatively few dieters are successful in maintaining weight loss. The extent to which dieting behavior might dampen satiety responses normally mediated by the neurotransmitter serotonin remains uncertain. This study tested the hypothesis that dieting behavior decreases the availability of plasma tryptophan (TRP) and the ratio of TRP to other branched-chain amino acids (BCAA) that compete for entry into the central nervous system (CNS). This effect could diminish the CNS concentration of TRP, the amino acid precursor for serotonin synthesis, thus interfering with serotonin-mediated influences on food intake. Using a fixed-order design, 15 healthy, normal-weight women were studied longitudinally during an ad lib dietary intake phase and subsequent reduced-calorie diet phase. Physiological and behavioral measures were collected at baseline, at the end of the ad lib-intake phase, and at the end of the 4-week study diet phase. Food intake was measured by a single-item test meal. Plasma TRP and TRP: sigma BCAA significantly decreased following the study diet compared to baseline (p < 0.05). Change in TRP and TRP: sigma BCAA significantly correlated with decrease in body weight (p < 0.01). No significant relationship was observed between postdiet change in TRP or TRP: sigma BCAA ratio and postdiet change in test meal food intake, with covariation for weight loss. The observed decreases in plasma TRP and TRP: sigma BCAA extend previous reports suggesting that dieting behavior may diminish central serotonin function through a reduction in precursor availability.