Mounjaro and Your Bum: What Actually Happens to Fat and Shape

What the SURMOUNT trials tell us about fat loss and body distribution

The SURMOUNT-1 trial, published in the New England Journal of Medicine, randomised 2,539 adults with obesity and followed them for 72 weeks. Participants on tirzepatide 15mg lost around 20–21% of body weight on average. That figure matters here because it was total body weight, not fat from any selected area.

The body does not respond to a GLP-1 or dual GIP/GLP-1 medicine by choosing where to shed fat first. Reduction happens broadly, driven by a sustained calorie deficit as appetite falls and gastric emptying slows. The buttocks, which hold a meaningful proportion of subcutaneous fat in many people, will reduce alongside fat stores elsewhere. In larger studies the pattern tends to follow the individual's own fat distribution, areas that carry more fat generally lose more in absolute terms.

One thing worth setting aside gently: the idea that a medicine can preserve fat in one area while burning it elsewhere. That is not how tirzepatide works. The SURMOUNT programme, and the clinical pharmacology behind it, show whole-body metabolic change, not topical reshaping.

For the full evidence picture on tirzepatide's mechanism and trial results, our tirzepatide overview covers the dual-agonist pharmacology in more depth.

Muscle loss, the 'Mounjaro bum' concern, and what you can do about it

A common question (one our prescribers hear regularly) is whether Mounjaro causes the bum to lose muscle rather than just fat, resulting in a flat or deflated look. If you are specifically concerned about how Mounjaro affects bum loss and what drives the change in shape, that concern is legitimate and worth understanding properly. Rapid weight loss of any kind, including from calorie restriction alone, can lead to some lean-tissue loss alongside fat loss. This is not unique to tirzepatide.

The practical response is straightforward: adequate dietary protein and resistance-based exercise help preserve muscle during treatment. NHS guidance on weight management consistently emphasises physical activity alongside any medicine-supported programme, partly for this reason. A prescriber or dietitian can advise on specific targets based on your starting point and how treatment is going.

If you want to understand what happens to weight more broadly as you move through the dose schedule, information on the 5mg dose stage explains what that phase of titration typically involves.

Lower-GI symptoms, rectal bleeding, and when to seek help

Mounjaro's most common side effects are gastrointestinal: nausea, changes in bowel habit, constipation, diarrhoea, and in some cases reflux or abdominal discomfort. These are documented in the NHS tirzepatide medicine page and generally peak in the days after starting or after a dose increase, then settle.

Constipation in particular can lead to straining, and straining can cause or aggravate conditions such as haemorrhoids, which may produce rectal bleeding. That is a separate issue from the medicine's pharmacological action. If you are experiencing bleeding from the back passage while taking Mounjaro, that page covers the possible causes and what warrants prompt medical attention.

The MHRA confirmed acute pancreatitis as a known but infrequent risk with GLP-1 medicines in early 2026. Severe stomach pain that radiates to the back, with or without vomiting, needs urgent medical assessment. You can report any suspected side effect through the MHRA Yellow Card scheme.

Who is assessed for Mounjaro, and what the consultation involves

Mounjaro is licensed for adults with a BMI of 30 or above, or 27 and above where a weight-related condition is present, such as high blood pressure, type 2 diabetes, or obstructive sleep apnoea. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance. A BMI figure alone is never the whole picture; the prescriber reviews your health history, current medicines, and any contraindications before recommending treatment.

At nume, that review is carried out by a GPhC-registered Independent Prescriber on the day your consultation is submitted. There is no algorithm making that decision. For anyone considering whether treatment is appropriate, our Mounjaro treatment page explains the full eligibility picture and what the process looks like from the first step.

Understanding the cost of treatment before you commit is reasonable. Background on how UK Mounjaro pricing has changed puts the market context in plain terms. When you are ready to talk to a prescriber, checking your eligibility is the natural next step.