Tirzepatide induces weight reduction but the underlying mechanisms are unknown. This 6-week phase 1 study investigated early effects of tirzepatide on energy intake. Male and female adults without diabetes (n = 114) and a body mass index from 27 to 50 kg per m2 were randomized 1:1:1 to blinded once-weekly tirzepatide or placebo, or open-label once-daily liraglutide. The primary outcome was change from baseline to week 3 in energy intake during an ad libitum lunch with tirzepatide versus placebo. Secondary outcomes assessed self-reported ingestive behavior and blood-oxygenation-level-dependent functional magnetic resonance imaging with food photos. Tirzepatide reduced energy intake versus placebo at week 3 (estimated treatment difference -524.6 kcal (95% confidence interval -648.1 to -401.0), P < 0.0001). With regard to secondary outcomes versus placebo, tirzepatide decreased overall appetite, food cravings, tendency to overeat, perceived hunger and reactivity to foods in the environment but did not impact volitional restriction of dietary intake. At week 3 versus placebo, tirzepatide did not statistically significantly impact blood-oxygenation-level-dependent activation to highly palatable food photos (aggregated category of high-fat, high-sugar foods and high-fat, high-carbohydrate foods) but decreased activation to high-fat, high-sugar food photos in the medial frontal and cingulate gyri, orbitofrontal cortex and hippocampus. Our results suggest tirzepatide reduces food intake, potentially by impacting ingestive behavior. ClinicalTrials.gov registration: NCT04311411 .
© 2025. The Author(s).