High-Calorie Diet Consumption Induces Lac-Phe Changes in the Brain in a Time-of-Day Manner Independent of Exercise

<b>Background/Objectives:</b> N-lactoyl-phenylalanine (Lac-Phe), an exercise-induced metabolite, has been shown to reduce food intake, decrease body weight and adiposity, and improve glucose homeostasis without affecting energy expenditure. Until now, Lac-Phe has mainly been investigated in blood plasma, showing its appetite-suppressing effects. Interestingly, these beneficial effects were caused by a temporary increase in Lac-Phe levels after exercise. Second, despite the central role of the central nervous system in the homeostatic control of energy metabolism, little is known about the presence and function of Lac-Phe in the brain. The goal of this study is to investigate how Lac-Phe concentrations in the brain change during the 24 h light/dark cycle. <b>Methods</b>: We conducted an experiment in rats in which time-restricted running was combined with time-restricted feeding (TRF) of a high-calorie diet, after which Lac-Phe levels were measured in the hypothalamus and cortex using stable isotope dilution LC-MS/MS. Microglia were isolated from rat brains to study Lac-Phe-related gene expression. <b>Results</b>: We found that Lac-Phe levels changed over time within the 24 h light/dark cycle in the hypothalamus and/or cortex, even without exercise. Our study indicates that brain Lac-Phe is not only induced by exercise but also by high-calorie diet intake independent of exercise. Finally, we showed that microglial cells are cytosolic nonspecific dipeptidase 2 (<i>CNDP2</i>) positive and therefore able to produce Lac-Phe. Hereby, we identified <i>SLC16A1</i> in microglia as a possible key mediator of Lac-Phe production. <b>Conclusions</b>: We conclude that high-calorie diet consumption induces Lac-Phe changes in the brain in a time-of-day manner independent of exercise. This study provides new knowledge on the presence and production of Lac-Phe in the brain. Further research is needed to elucidate the potential mechanism by which Lac-Phe reduces food intake and body weight by targeting appetite-suppressing neurons.