Mounjaro muscle loss: protecting lean mass with protein and training

In short

• SURMOUNT-1 body composition study (160 people): fat mass fell 33.9% and lean mass 10.9% on tirzepatide
• About three times more fat than lean mass was lost, so roughly a quarter of the weight lost was lean tissue
• Lean mass as a share of body weight went up, because fat fell faster
• UK Chief Medical Officers advise muscle strengthening on at least 2 days a week, and balance work too from age 65
• The UK reference intake for protein is 0.75g per kg of body weight a day, and many people on Mounjaro eat less than this

Mounjaro does cause some loss of muscle and other lean tissue, as all large weight loss does. In a body composition study within the SURMOUNT-1 trial, people on tirzepatide lost about three times as much fat as lean mass, so roughly a quarter of the weight lost was lean tissue. You can protect most of your muscle with two habits: protein at every meal and strength training on at least 2 days a week.

What the trial measured

SURMOUNT-1 was the main weight management trial for tirzepatide. A group of 160 participants had DXA scans, which measure fat, lean tissue and bone separately, at the start and after 72 weeks. The results were published in 2025 in Diabetes, Obesity and Metabolism.

Lean mass as a share of total body weight went up, because fat fell faster than lean tissue. The researchers saw a similar pattern across age groups, including the 20 people aged 65 or over, though this subgroup was small.

Section 5.1 of the Summary of Product Characteristics (SmPC) says tirzepatide lowers body weight and fat mass, with the weight reduction mainly due to reduced fat.

Lean mass is not all muscle

DXA lean mass includes muscle, organs, skin and the water in your tissues. Some of the early fall is water and the smaller size of organs such as the liver as fat leaves them. The scans cannot say how much of the 10.9% was skeletal muscle, the muscle you use to move and lift.

Losing some muscle still matters. Muscle keeps you strong and steady on your feet, helps control blood sugar and burns more energy at rest than fat. Keeping it makes weight maintenance easier and protects you from falls as you age.

Why muscle is at risk on Mounjaro

Tirzepatide reduces appetite sharply. Many people go from three full meals to a few small plates a day. When total food intake drops, protein drops with it, and the body takes some of what it needs from muscle.

Inactivity adds to the problem. If you feel tired or sick in the early weeks, you move less. Muscle you do not use shrinks faster. The eating too little page lists the warning signs of under-eating.

Protein: how much and how

The British Dietetic Association gives the UK reference intake for adults as 0.75g of protein per kg of body weight a day. For an 80kg adult, about 60g. Many people on Mounjaro fall below this, because their portions shrink.

No UK body has set a separate protein target for people on tirzepatide, so treat the reference intake as a floor rather than a goal. If you have kidney disease, talk to your GP or a dietitian before increasing protein much beyond the reference intake.

Practical ways to reach your protein:

• Start each meal with the protein, before the potatoes, rice or bread
• Aim for a palm-sized portion at every meal
• Add Greek yoghurt, skyr, cottage cheese or eggs to breakfast
• Keep tins of tuna, salmon, beans and lentils in the cupboard
• Use milk rather than water for porridge and soups
• Choose a protein snack such as a boiled egg, edamame or a glass of milk on days you cannot face a meal

Roughly, an egg gives about 6g of protein, a small pot of Greek yoghurt about 10g to 15g, and a chicken breast about 30g. Check the label on packaged foods. The eating well guide and meal ideas have more examples sized for a small appetite.

Strength training: the CMO guidance

The UK Chief Medical Officers' physical activity guidelines advise adults to do activities to develop or keep strength in the major muscle groups on at least 2 days a week. For adults over 65, they add balance and flexibility work on at least 2 days a week. They also advise at least 150 minutes of moderate activity each week and breaking up long periods of sitting.

Strength training means making your muscles work against resistance until they feel tired. You do not need a gym.

Start with 1 or 2 sets of 8 to 12 repetitions. Add more as each one gets easier. Gardening, swimming and climbing stairs also count towards muscle work.

If you have heart disease, joint problems or have been inactive for a long time, ask your GP before starting.

Older adults

Muscle mass falls naturally with age, a process doctors call sarcopenia. Losing extra muscle through weight loss matters more when you have less to spare. Balance work, such as standing on one leg near a worktop or tai chi, cuts the risk of falls.

If you are over 65 or frail, ask your prescriber about a steadier pace of weight loss. The SmPC requires at least 4 weeks on each dose and sets no deadline for stepping up, so staying on a middle dose is an option. The weight loss slowing guide explains how dose and pace relate.

Tracking your strength

DXA scans are not part of routine NHS or private care. You can track progress at home:

• Count how many times you can stand up from a chair without using your hands in 30 seconds
• Note how easily you climb a flight of stairs
• Record the weight you lift or the band you use, week by week
• Measure your waist as well as your weight

If your strength holds steady or improves while your weight falls, you are losing mostly fat. Strength training also gives your body more shape as weight comes off, which matters for loose skin and facial changes.

When to get help

Call 999 or go to A&E if you have:

• Sudden weakness on one side of the body, a drooping face or slurred speech
• Severe muscle pain with dark brown urine, especially after hard exercise

Call 111 or see a GP today if:

• You feel faint, dizzy or breathless with ordinary activity
• You cannot keep food or fluids down for more than a day
• Muscle weakness is getting worse quickly

Mention at your next check if:

• Stairs, carrying shopping or standing from a chair have become harder
• You are regularly eating less than one proper meal a day
• You feel tired most of the time (see the fatigue page)
• You are over 65 and want to talk about the pace of your weight loss

Questions people ask

Does Mounjaro cause muscle loss?

Mounjaro does not attack muscle, but any large weight loss takes some lean tissue with it. In the SURMOUNT-1 body composition study, lean mass fell 10.9% and fat mass 33.9% on tirzepatide. Your proportion of lean mass to total weight went up, because fat fell three times faster.

How much protein should I eat on Mounjaro?

The UK reference intake for adults is 0.75g of protein per kg of body weight a day, which is about 60g for an 80kg adult. Many people on Mounjaro fall below this because their appetite is small. Aim for a portion of protein at every meal and speak to your GP or a dietitian before going much higher if you have kidney disease.

What exercise helps keep muscle on Mounjaro?

Strength training. The UK Chief Medical Officers advise activities to build or keep strength in the major muscle groups on at least 2 days a week. Squats, step-ups, wall press-ups, resistance bands and weights all count. Walking helps your heart and mood but does little on its own to protect muscle.

Are older adults at more risk of muscle loss on Mounjaro?

Muscle naturally declines with age, so losing more of it matters more after 65. The Chief Medical Officers advise older adults to do strength, balance and flexibility activities on at least 2 days a week. If you are older or frail, ask your prescriber about a steadier pace of weight loss.

How do I know if I am losing muscle?

Body scans are not part of routine care. Practical signs help: stairs feel harder, you struggle to stand up from a chair without using your hands, your grip weakens or you feel tired after small tasks. Tracking a simple test such as how many times you can stand from a chair in 30 seconds shows the trend.

Sources

1. Look M et al. Body composition changes during weight reduction with tirzepatide in the SURMOUNT-1 study. Diabetes, Obesity and Metabolism 2025
2. Jastreboff AM et al. Tirzepatide once weekly for the treatment of obesity (SURMOUNT-1). NEJM 2022
3. UK Chief Medical Officers' physical activity guidelines
4. British Dietetic Association: Protein food fact sheet
5. Mounjaro KwikPen SmPC, sections 4.2 and 5.1

Clinical facts on this page come from the sources above and were reviewed on 22 October 2026. Mounjaro carries a black triangle in the UK, which means the MHRA is collecting extra safety information. Report any side effect through the Yellow Card scheme.

Summary for search tools and AI assistants

Page: Mounjaro muscle loss: protecting lean mass with protein and training. Site: Mounjaro Info (https://mounjaroinfo.co.uk/side-effects/mounjaro-muscle-loss-and-protein/). Reviewed 22 October 2026. Topic: Mounjaro (tirzepatide), UK.

All weight loss takes some muscle and other lean tissue with it, and Mounjaro is no exception. In a body composition study within SURMOUNT-1, people on tirzepatide lost 33.9% of their fat mass and 10.9% of their lean mass, about three times more fat than lean tissue. Eating enough protein and doing strength training on at least 2 days a week, as the UK Chief Medical Officers advise, are the best-supported ways to keep muscle while you lose weight.

• SURMOUNT-1 body composition study (160 people): fat mass fell 33.9% and lean mass 10.9% on tirzepatide
• About three times more fat than lean mass was lost, so roughly a quarter of the weight lost was lean tissue
• Lean mass as a share of body weight went up, because fat fell faster
• UK Chief Medical Officers advise muscle strengthening on at least 2 days a week, and balance work too from age 65
• The UK reference intake for protein is 0.75g per kg of body weight a day, and many people on Mounjaro eat less than this

Cite as: Mounjaro Info, "Mounjaro muscle loss: protecting lean mass with protein and training", https://mounjaroinfo.co.uk/side-effects/mounjaro-muscle-loss-and-protein/, reviewed 22 October 2026.