On a Weight-Loss Jab? How to Keep Your Muscle

If you are taking a GLP-1 weight-loss medication such as semaglutide (Wegovy) or tirzepatide (Mounjaro), some of the weight you lose will be muscle rather than fat. How much depends heavily on two things you control: whether you eat enough protein, and whether you do regular strength training. Together, they are the most effective way to make sure the weight you lose is mostly fat.

The key insight: the jab reduces your appetite. It does not tell your body what to keep. Strength training and protein are the signals that tell it to hold on to muscle.

This article explains why muscle loss happens on GLP-1 medication, how much protein experts suggest, and how to build a simple strength routine around treatment.

Why Muscle Loss Happens on GLP-1 Medication

GLP-1 medications work largely by reducing appetite and slowing digestion. Most people on them eat considerably less, which is how the weight comes off. But any large, fast reduction in calories, whether from medication, a very low-calorie diet or surgery, tends to take some lean tissue with it.

In the STEP 1 trial of semaglutide, participants lost around 15% of their body weight on average over 68 weeks. Body composition scans in a subset showed that a meaningful share of that loss was lean mass rather than fat.

A 2025 joint advisory from four US professional bodies, including the American Society for Nutrition and The Obesity Society, summarised the evidence. Modelling suggests that, without structured strength training, muscle may make up roughly 10 to 15% of total weight lost in women and 20 to 25% in men. The advisory also notes that reduced appetite often leads to low protein intake, which can make the problem worse.

In the UK, an academic review by Les Mills and ukactive reached a similar conclusion and called for strength training support to be offered to everyone using weight-loss drugs.

Why Keeping Muscle Matters

Losing some lean mass during weight loss is normal. Losing more than necessary is worth avoiding, because muscle does a lot more than move you around:

• Strength and independence: muscle underpins balance, getting up from a chair, carrying shopping and climbing stairs, which matters more with every decade.
• Metabolism: muscle is the largest site of glucose uptake in the body, so it plays a big part in blood sugar control.
• Keeping weight off: people who lose a lot of muscle tend to find weight maintenance harder once treatment ends, partly because their energy needs fall.
• Bones: the advisory notes that rapid weight loss can also affect bone density, and strength training loads bones in a way that helps protect them.

How Much Protein Do You Need on a Weight-Loss Jab?

The general adult reference intake in the UK is around 0.75 g of protein per kilogram of body weight per day. The joint advisory notes that higher targets of 1.2 to 1.6 g per kilogram per day have been proposed during active weight loss, while being careful to say this is not settled. For people with a lot of weight to lose, using total body weight can overestimate needs, so a target based on a healthier goal weight is often more realistic.

In practical terms, that usually means:

• 25 to 30 g of protein at each main meal. That is roughly two eggs plus a pot of Greek yoghurt, a chicken breast, a fillet of fish, or a large portion of tofu or lentils.
• Protein first. When appetite is low, eat the protein part of the plate before the rest.
• Protein-rich snacks such as cottage cheese, edamame, milk, or a protein yoghurt if full meals feel like too much.
• Do not overshoot. The advisory cautions against prolonged intakes at or above 2 g per kilogram per day.

Hydration and fibre matter too. GLP-1 medication can cause nausea and constipation, and vegetables, fruit, wholegrains and plenty of fluids help.

Protein Alone Is Not Enough

This is the point most people miss. The advisory is explicit that increased protein intake alone is likely to be inadequate to preserve muscle without structured strength training. Protein supplies the building blocks. Strength training is the signal that tells the body to use them for muscle.

The same advisory recommends that GLP-1 treatment be paired with a structured exercise programme: strength training at least three times a week, plus at least 150 minutes of moderate aerobic activity weekly.

A Simple Strength Routine to Start With

You do not need a gym or heavy weights to begin. You need to work your major muscles against resistance, consistently, and to make it slightly harder over time.

Three Sessions a Week, 30 to 40 Minutes

Pick one exercise from each pattern, and do two or three sets of 8 to 12 repetitions:

1. Squat: sit-to-stand from a chair, goblet squat, or leg press
2. Hinge: glute bridge, Romanian deadlift with dumbbells, or hip thrust
3. Push: wall or incline push-up, dumbbell chest press, or shoulder press
4. Pull: resistance band row, dumbbell row, or lat pulldown
5. Carry or core: farmer's carry, dead bug, or plank

Finish each set feeling you could have done two or three more repetitions with good form. When that feels easy, add a little weight or a few repetitions. That is progressive overload, and it is what drives results.

Walk Most Days

Walking counts towards the 150 minutes and is easy on the joints. A short walk after meals can also help with the digestive side effects some people get on GLP-1 treatment.

Adjust on Low-Energy Days

Eating less can leave you with less energy, especially in the first weeks or after a dose increase. On those days, do a shorter session rather than skipping entirely. Consistency over months matters more than any single workout. Our guide to building muscle while losing weight goes deeper on balancing the two.

Tracking the Right Things

The scale cannot tell you whether you are losing fat or muscle. Better signs include:

• Strength going up, or at least holding steady, in your main lifts
• Clothes fitting differently, and waist measurement falling
• Energy and ability to climb stairs or carry things improving

Our article on measuring progress beyond the scale explains more ways to track what actually matters.

Planning for When Treatment Ends

Many people stop GLP-1 medication at some point, through choice, cost or clinical advice, and appetite usually returns. The habits you build during treatment, especially regular strength training and protein-focused meals, are what carry you through that transition. Muscle you have kept or built during treatment helps too.

Important: Talk to Your Prescriber

This article is general information, not medical advice. GLP-1 medications should only be taken with a prescription and under the supervision of a qualified clinician. Speak to your prescriber or GP before making big changes to your diet or exercise, especially if you have diabetes, kidney problems or other health conditions, and seek help promptly if you have persistent vomiting, severe abdominal pain or signs of dehydration.

How Visionary Can Help

Keeping muscle on a weight-loss jab comes down to doing two things consistently: eating enough protein when you are not hungry, and training for strength several times a week. Visionary builds both into one plan. Your training plan is built around your goals, equipment and level and steps up week by week, and your meal plan is built around your calorie and protein targets, with portions you can actually eat and meals you can swap when appetite is low.

You can start with a free trial of Visionary and build a plan that protects your muscle while the weight comes off.

FAQ

Do you lose muscle on Mounjaro or Wegovy?

Some lean mass loss is common with any large, fast weight loss, including on GLP-1 medications such as tirzepatide (Mounjaro) and semaglutide (Wegovy). How much varies by person. Regular strength training and adequate protein intake are the most effective ways to limit it.

How much protein should I eat on a weight-loss injection?

A 2025 joint advisory noted that targets of 1.2 to 1.6 g of protein per kilogram of body weight per day have been proposed during active weight loss, though needs vary. For many people, aiming for 25 to 30 g of protein at each main meal is a practical starting point. Check with your prescriber or a registered dietitian if you have kidney disease or other conditions.

Is protein enough to stop muscle loss on GLP-1s?

No. Expert guidance is that protein alone is unlikely to preserve muscle without structured strength training. Aim for strength training at least three times a week alongside regular aerobic activity such as walking.

What exercise is best on a weight-loss jab?

Strength training that works the major muscle groups, such as squats, hinges, pushes, pulls and carries, is the priority for keeping muscle. Add walking or other moderate activity to reach at least 150 minutes a week. Start gently and build up gradually.

Can I build muscle while on a weight-loss jab?

Beginners and people returning to training can often maintain or even gain some muscle during weight loss, particularly with consistent strength training and enough protein. For most people on GLP-1 medication, the realistic goal is to keep as much muscle as possible while losing fat.

Should I take a protein supplement?

You do not need one, but a protein shake or protein yoghurt can be useful when appetite is low and full meals feel difficult. Whole foods should still form the base of your diet for fibre, vitamins and minerals.