Body composition refers to the proportion of fat, muscle, bone, and water that makes up your body weight. When you lose weight on Mounjaro, ideally you want to lose primarily fat mass while preserving lean muscle mass. This distinction matters because muscle is metabolically active tissue that helps maintain your metabolism, supports daily function, and protects long-term health.
How Mounjaro Affects Body Composition
Clinical data from the SURMOUNT trials showed that approximately two-thirds of the weight lost on tirzepatide was fat mass, with the remaining third being lean mass. This ratio is fairly typical for weight loss interventions, but it highlights the importance of taking active steps to minimise muscle loss.
The significant calorie reduction caused by Mounjaro's appetite-suppressing effects means your body may break down muscle tissue for energy if you are not careful. This is particularly relevant for older patients or those who were already relatively inactive before starting treatment.
Strategies to Protect Your Muscle Mass
Prioritise Protein
Protein is the most important dietary factor for muscle preservation during weight loss. Current evidence suggests aiming for:
- 1.2-1.6 g of protein per kg of body weight daily as a minimum
- Distribute protein evenly across 3-4 meals (e.g., 25-40 g per meal)
- Good UK sources include chicken, fish, eggs, Greek yoghurt, beans, lentils, and cottage cheese
Resistance Training Is Non-Negotiable
Lifting weights or performing resistance exercises sends a powerful signal to your body to retain muscle. Even two sessions per week can make a significant difference. If you are new to strength training, many local councils offer affordable gym access through leisure centre memberships, often as little as £25-35 per month.
Do Not Slash Calories Too Aggressively
Mounjaro already reduces your appetite substantially. Avoid combining the medication with very low-calorie diets (below 1,000 kcal per day) unless medically supervised. Eating too little accelerates muscle loss and can leave you feeling exhausted.
How to Measure Body Composition
- DEXA scan: The gold standard, available privately in the UK for around £100-150. Provides accurate fat and lean mass measurements.
- Bioimpedance scales: Less accurate than DEXA but useful for tracking trends over time. Available from £25-100.
- Waist-to-hip ratio: A simple tape measure can give you a good indication of fat distribution changes. The NHS considers a waist circumference above 94 cm for men and 80 cm for women to indicate increased health risk.
The Bigger Picture
Focusing solely on scale weight can be misleading. A patient who loses 10 kg of fat and gains 2 kg of muscle shows only 8 kg of weight loss on the scale, but their health transformation is remarkable. Discuss body composition monitoring with your prescriber, especially if you feel your scale progress does not match how you look and feel.