The two hormones Mounjaro mimics
Mounjaro contains the active ingredient tirzepatide, a synthetic peptide designed to activate two natural human gut hormone receptors at the same time: GLP-1 (glucagon-like peptide 1) and GIP (glucose-dependent insulinotropic polypeptide). Both hormones are released after eating and play a role in appetite regulation, gastric emptying and the body's post-meal insulin response.
What activating the GLP-1 receptor does
- Slows the rate at which food leaves the stomach.
- Increases the feeling of fullness after eating.
- Reduces appetite signalling in the brain.
- Improves glucose-dependent insulin secretion — insulin is released in response to a meal, not at random.
What activating the GIP receptor adds
GIP works alongside GLP-1 in the body's natural incretin response. Activating GIP appears to enhance the appetite-suppressing effect of GLP-1, improve how the body handles fats and lipids, and may reduce some of the gastrointestinal side effects that come with high-dose GLP-1-only treatment. The dual receptor activation is what makes tirzepatide pharmacologically distinct from semaglutide.
Why this matters in practice
For most patients, the combined effect of GLP-1 and GIP activation is a meaningful reduction in calorie intake without conscious dieting, better post-meal blood sugar control, and a degree of satiety that lifestyle change alone is rarely able to produce.
Mounjaro is not a metabolism booster, not a fat burner and not a cosmetic medicine. It is a pharmacological tool that makes sustainable dietary change easier to achieve and easier to maintain. The long-term outcomes depend on what happens during treatment, not only on the medicine itself.
What it does not do
- It does not directly burn fat.
- It does not change your basal metabolic rate.
- It does not treat the psychological drivers of eating.
- It does not maintain weight loss after it is stopped — appetite tends to return to its pre-treatment level.
Sources: SmPC for Mounjaro (eMC); NICE TA1026; BNF tirzepatide monograph; Frias et al. SURPASS-2 trial publication; Jastreboff et al. SURMOUNT-1 trial publication.