What GLP-1 Drugs Actually Do (and Don't) to Your Body

GLP-1 drugs (semaglutide, liraglutide, tirzepatide) mimic GLP-1 (glucagon-like peptide-1), a gut hormone that slows digestion and signals fullness to the brain[35], cutting appetite far more than the body's own version of it does. The results are real: roughly 15% body weight loss over 68 weeks with semaglutide[36] and over 20% with tirzepatide[37] — well beyond what diet alone usually achieves. What they don't do is change how your body splits that weight loss between fat and muscle. Studies show a meaningful chunk of it — commonly a quarter to two-fifths — is lean mass, not fat[38]. That's the same problem any aggressive deficit causes (Section 1); it's just arrived at through a smaller appetite instead of a smaller plate.

The fix is the same one this guide already gives: protein (Section 2) and resistance training, both more important here, not less, since appetite suppression often produces a bigger deficit than someone would otherwise choose. The catch is practical — a smaller appetite leaves less room to hit a protein target, so put protein on the plate first and treat everything else as what's left over. Nausea and reduced appetite for large meals are common, especially early on[36][37], and can squeeze fibre and micronutrients (Sections 3 and 6) the same way they squeeze protein — worth watching, not ignoring.