Keeping muscle while the scales drop

The scales do not know the difference between fat and muscle. Your body does, and so does your metabolism once the medicine stops.

By Sian Anderson. Published 21 October 2026. 6 minute read. Information, not medical advice.

Sian Anderson, Writer and editor, MyGLP. Sources checked 21 October 2026.

What the trials measured

In a sub-study of the STEP 1 semaglutide trial, participants had body composition scans. About 40% of the weight they lost was lean mass and 60% was fat. In SURMOUNT-1 with tirzepatide, the split was about 25% lean to 75% fat. Lean mass includes water, organs and connective tissue as well as muscle, so those figures overstate the muscle loss. Still, muscle is in there, and the proportion is similar to what dieting alone produces.

Neither trial asked people to lift weights or eat extra protein. That is the gap you can close.

Why it matters

Muscle burns calories at rest. Lose it and your daily needs fall, so keeping weight off after the medicine gets harder.
Muscle is what gets you up from a chair at 75. Women start with less of it and lose it faster after the menopause.
Muscle takes up glucose from the blood. Less of it means worse blood sugar control.
Strength protects joints, balance and bone. See the bone guide.

The two things with evidence

First, protein. Weight loss studies that gave 1.2 to 1.6g per kg a day kept more lean mass than those at the standard 0.8g. The protein guide turns that into meals.

Second, resistance exercise. Muscle that is used is muscle the body keeps. The UK Chief Medical Officers recommend strength activities on at least two days a week for every adult, and the recommendation applies with more force when you are losing weight fast. Two sessions of 20 to 30 minutes a week is enough to hold most of what you have.

What counts as resistance exercise

The test is whether the last few repetitions are hard. If you could keep going for another ten, add weight or slow it down.
Activity	Kit	Counts?
Squats, lunges, press-ups, glute bridges, rows with a band	None, or a resistance band	Yes
Dumbbell or kettlebell work at home	A pair of weights, from about £20	Yes
Gym machines or free weights	Membership	Yes
Pilates, yoga with holds, reformer	Class or mat	Yes, for the upper body and core in particular
Carrying shopping, gardening, stairs	Life	Partly. Good, but not enough on its own
Walking, swimming, cycling	Shoes, pool, bike	No. Good for heart and mood, not a muscle stimulus

The home strength guide gives a 20-minute routine with no kit that covers the whole body.

How much loss is too much

You cannot measure muscle on bathroom scales. Smart scales that claim to show body fat are rough at best. Better markers: can you do as many press-ups against the kitchen counter as you could three months ago? Does the shopping feel heavier? Is your grip weaker? If strength is going backwards while weight comes off fast, protein is low, training is missing, or both.

Slowing the weight loss helps too. A loss of 0.5 to 1kg a week, after the first month, costs less muscle than 2kg a week. If the medicine is taking you faster than that, the prescriber can hold you at a step for longer.

After the medicine

The weight people regain after stopping is largely fat, and the muscle lost during the loss does not come back on its own. Starting strength training during the loss, rather than after, is the difference between finishing lighter and stronger and finishing lighter and weaker. Two sessions a week, started this week, is the whole prescription.

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Sources

Once-weekly semaglutide in adults with overweight or obesity (STEP 1) (Wilding et al, New England Journal of Medicine, 2021)
Tirzepatide once weekly for the treatment of obesity (SURMOUNT-1) (Jastreboff et al, New England Journal of Medicine, 2022)
Protein requirements during weight loss (review) (Leidy et al, American Journal of Clinical Nutrition, 2015)
UK Chief Medical Officers' physical activity guidelines (Department of Health and Social Care)
Physical activity guidelines for adults aged 19 to 64 (NHS)

Sian Anderson

Sian writes the health and nutrition guides on MyGLP. Every clinical claim links to the NHS, NICE, a medical royal college, a regulator or a peer-reviewed paper, and the pages carry no medical advice.

Questions

Do weight loss injections cause muscle loss?

Fast weight loss by any method takes some muscle. In the trials, about a quarter (tirzepatide) to two fifths (semaglutide) of the weight lost was lean tissue, which includes water and organs as well as muscle. Protein and strength training reduce the muscle share.

How do I keep muscle on Mounjaro?

Eat 1.2 to 1.6g of protein per kg of body weight a day and do resistance exercise on two days a week. Both have trial evidence. Walking is good for you but does not protect muscle on its own.

How much strength training do I need?

The UK Chief Medical Officers recommend strength activities on at least two days a week. Twenty to thirty minutes a session, with the last few repetitions hard, is enough to hold most of your muscle.

Should I slow my weight loss down?

After the first month, 0.5 to 1kg a week costs less muscle than 2kg a week. If you are losing faster and strength is dropping, ask the prescriber to hold your dose at the current step for longer.