Tirzepatide did not impact metabolic adaptation in people with obesity, but increased fat oxidation.

Tirzepatide, a glucose-dependent insulinotropic polypeptide and glucagon-like peptide-1 receptor agonist, promoted significant body weight reduction in the phase 3 clinical trials. We conducted a preclinical study and a phase 1 clinical trial (NCT04081337) to understand potential mechanisms mediating tirzepatide-induced weight loss in mice and people with obesity. In calorie-restricted, obese mice, chronic treatment with tirzepatide reduced the drop in energy expenditure that occurred in vehicle-treated and pair-fed mice, indicating that tirzepatide attenuated metabolic adaptation. Respiratory exchange ratio also decreased in tirzepatide-treated mice, indicating increased fat oxidation. In the clinical trial, tirzepatide appeared to have no impact on metabolic adaptation but led to increased fat oxidation and reductions in appetite and calorie intake during an ad libitum test meal (vs. placebo). This is the first study to provide insights into the mechanisms of action of tirzepatide on weight loss with respect to calorie intake, energy expenditure, and macronutrient utilization.