Habituation to Dietary Proteins with Higher Tryptophan Levels Increases Fasted and Postprandial Concentrations of Tryptophan and Its Metabolites in Brain Regions of the Growing Pig.

BACKGROUND

The source of dietary protein with different tryptophan concentrations can influence brain tryptophan and its metabolite concentrations. However, little is known about changes in tryptophan metabolites after a single meal.

OBJECTIVES

To determine how fasting and postprandial brain concentrations of tryptophan and its metabolites vary in the growing pig fed different dietary proteins.

METHODS

After 7 d of meal adaptation, 9-wk-old fasted male pigs (n = 180) consumed an isocaloric meal without protein or with protein sources (45 g) differing in their tryptophan content (0.1 mg/g, 3.7 mg/g, 6.6 mg/g, and 10 mg/g diet for zein, casein, whey protein isolate, and α-lactalbumin, respectively) before being killed at 0 (fasted values, 12 h from previous meal), 1 h, 2 h, 3 h, 4 h, and 6 h postprandially (n = 6/dietary and time point combination). Blood plasma tryptophan, and tryptophan, serotonin, melatonin, kynurenine, and 3-hydroxykynurenine concentrations in hippocampus, striatum, prefrontal cortex, and pineal gland were determined. Analysis of variance and analysis of covariance models were used to analyze the baseline values (0 h) and the complete dataset (0‒6 h), respectively. Light intensity in the crates (23‒256 lux) was included in the models as a covariate.

RESULTS

In all brain regions, fasting concentrations of tryptophan and its metabolites (except serotonin) generally mirrored (P ≤ 0.05 or tended to follow P < 0.1) the tryptophan content of the protein sources consumed during the adaptation period. Postprandially, the pigs given α-lactalbumin had the highest (P < 0.001) increase in plasma and brain tryptophan concentrations compared to the other meals. Conversely, postprandial brain serotonin increased (or tended to increase) for all dietary treatments (P ≤ 0.05), whereas brain kynurenine and 3-hydroxykynurenine did not increase over time. Postprandial pineal gland melatonin varied (P < 0.001) across dietary treatments.

CONCLUSIONS

Fasting and postprandial brain tryptophan concentrations and tryptophan metabolite concentrations can be modulated through dietary intervention.