Evidence-graded answer
Does GLP-1 work without diet changes?
Some weight loss happens regardless — appetite is reduced enough that calories drop naturally. But the trial-magnitude weight loss (10-22%) was achieved with structured diet and counseling. Without any nutrition support, expected weight loss is closer to 5-10%.
STEP-1, SURMOUNT-1, and SELECT all included reduced-calorie diet + physical activity counseling as part of the protocol. The trial mean weight loss reflects drug + lifestyle, not drug alone.
Real-world cohort studies (without structured lifestyle support) typically show smaller losses: 5-10% body weight at 1 year for semaglutide, 8-12% for tirzepatide.
Protein intake (1.2-1.6 g/kg ideal body weight) and resistance training are the two highest-yield additions that affect both magnitude of loss and lean-mass preservation.