The cleanest long-term measurement of this effect comes from a 6-year follow-up of the Biggest Loser cohort. Six years after the contestants finished the show, their resting metabolic rate was about 704 kilocalories per day below where it had been at baseline.

Some of that was explained by the smaller body size. The rest, about 499 kilocalories per day, was a downshift the body had made on top of size. The researchers called it persistent metabolic adaptation.

The study didn't involve any GLP-1 peptide. The same pattern shows up after bariatric surgery and after long calorie restriction. The framing "the peptide broke my metabolism" implies a one-way damage event from the peptide; the honest framing is that significant weight loss changes the body's energy economics regardless of method.

People who lose weight fast with low protein and no resistance training tend to end up at a lower resting metabolic rate than people who lose weight slowly with protected lean mass.