Beyond exercise and appetite: The expanding biology and therapeutic potential of N-lactoyl-phenylalanine.

N-lactoyl-phenylalanine (Lac-Phe) has emerged as a signaling metabolite connecting cellular metabolism to systemic physiology. Synthesized through carnosine dipeptidase 2-mediated conjugation of lactate and phenylalanine, Lac-Phe increases acutely in response to exercise and feeding, the primary drivers of its elevation under physiologic conditions. In preclinical models, Lac-Phe acts as a potent regulator of energy balance. Its administration suppresses appetite and reduces body weight in obesity, whereas pharmacologic interventions such as metformin elevate circulating Lac-Phe to produce similar anorexigenic effects. Converging evidence implicates central mechanisms, including inhibition of orexigenic agouti-related peptide neurons, positioning Lac-Phe as a mediator linking peripheral metabolic signals to appetite control. The first human Lac-Phe clinical trial in individuals with obesity began dosing in 2025, evaluating appetite suppression and glucose-lowering effects. Beyond metabolism, Lac-Phe promotes anti-inflammatory macrophage polarization, conferring protection in murine models of colitis and spinal cord injury. Circulating Lac-Phe also rises in conditions such as mitochondrial dysfunction, sepsis, and phenylketonuria, suggesting broader associations with perturbed energy metabolism and systemic stress responses. This review integrates current knowledge spanning molecular mechanisms, physiological regulation, and clinical translation. We examine Lac-Phe biosynthesis, tissue distribution, and regulatory patterns across physiological and disease states, and highlight emerging mechanisms of action in metabolic and inflammatory signaling. Finally, we discuss key knowledge gaps, highlighting the need to define targets, transporters, and tissue sources to shape the next phase of discovery. Collectively, these advances position Lac-Phe at the forefront of exerkine biology and as a promising molecular link between metabolism, immunity, and therapeutic innovation. SIGNIFICANCE STATEMENT: Evidence across molecular, physiological, and translational domains positions Lac-Phe as a promising therapeutic target. This review frames our understanding of Lac-Phe biology-from its biosynthesis to its roles in energy balance and outlines the key questions that will define ongoing discovery.