Metformin, a widely used treatment for type 2 diabetes (T2D), is known to reduce blood glucose levels and suppress appetite. This study reports a significant elevation in the appetite-suppressing metabolite N-lactoyl phenylalanine (Lac-Phe) in the blood of individuals treated with metformin across seven observational and interventional studies. Lac-Phe levels were found to rise in response to acute metformin administration and post-prandially in patients with T2D or metabolically healthy volunteers. The study collected serum samples from 33 volunteers at Brigham and Women's Hospital, including lean non-T2D, lean pre-T2D, obese non-T2D, obese pre-T2D, and obese T2D individuals. Untargeted metabolomic profiling using UPLC–MS/MS quantified 1,168 serum metabolites, revealing a marked increase in all measured N-lactoyl amino acids in obese T2D individuals compared to obese non-T2D individuals. This increase was not related to body mass index (BMI) but rather to T2D status. The study also found a strong correlation between Lac-Phe levels and metformin concentrations among individuals with T2D. Interventions studies further demonstrated that metformin treatment increases Lac-Phe levels, both in healthy individuals and those with T2D. Additionally, Lac-Phe levels increased post-prandially, suggesting a role in appetite regulation. These findings suggest that targeting Lac-Phe pharmacologically could lead to stronger appetite-suppressing effects and potentially new treatments for obesity.Metformin, a widely used treatment for type 2 diabetes (T2D), is known to reduce blood glucose levels and suppress appetite. This study reports a significant elevation in the appetite-suppressing metabolite N-lactoyl phenylalanine (Lac-Phe) in the blood of individuals treated with metformin across seven observational and interventional studies. Lac-Phe levels were found to rise in response to acute metformin administration and post-prandially in patients with T2D or metabolically healthy volunteers. The study collected serum samples from 33 volunteers at Brigham and Women's Hospital, including lean non-T2D, lean pre-T2D, obese non-T2D, obese pre-T2D, and obese T2D individuals. Untargeted metabolomic profiling using UPLC–MS/MS quantified 1,168 serum metabolites, revealing a marked increase in all measured N-lactoyl amino acids in obese T2D individuals compared to obese non-T2D individuals. This increase was not related to body mass index (BMI) but rather to T2D status. The study also found a strong correlation between Lac-Phe levels and metformin concentrations among individuals with T2D. Interventions studies further demonstrated that metformin treatment increases Lac-Phe levels, both in healthy individuals and those with T2D. Additionally, Lac-Phe levels increased post-prandially, suggesting a role in appetite regulation. These findings suggest that targeting Lac-Phe pharmacologically could lead to stronger appetite-suppressing effects and potentially new treatments for obesity.