When people try to lose weight, the goal is usually to lose body fat. In reality, the number on the scales reflects more than fat alone.
Some of the weight lost during dieting can come from fat-free mass, which includes muscle as well as water, organs and other non-fat tissues. That does not mean successful weight loss inevitably causes serious muscle wasting, but preserving strength and muscle is worth thinking about — particularly during substantial or rapid weight loss.
The encouraging part is that muscle loss is not entirely outside your control. Resistance training, adequate protein and avoiding unnecessarily extreme calorie restriction can all help.
This is particularly relevant for people using weight-loss medicines such as Mounjaro or Wegovy. These treatments can produce much larger weight changes than many traditional diets, so protecting nutrition and physical function becomes increasingly important.
This guide explains why muscle can be lost during weight loss, what the research actually shows and what you can do to lose weight without sacrificing more strength and lean tissue than necessary.
Why Can You Lose Muscle While Losing Weight?
Weight loss happens when the body uses more energy than it receives from food and drink.
It then has to draw on stored energy. Most of that ideally comes from body fat, but the body does not exclusively burn fat while perfectly preserving every gram of lean tissue.
Muscle becomes more vulnerable when calorie intake is very low, protein intake is inadequate or the muscles are no longer being challenged physically.
This is one reason the fastest possible rate of weight loss is not necessarily the best target.
The NHS currently advises people trying to lose weight to aim for a gradual loss of around 0.5 to 1kg per week. This is a general target rather than a strict limit, and people receiving medically supervised obesity treatment may sometimes lose more quickly.
The broader principle is more useful than the exact number: weight loss should generally be sustainable enough that you can continue eating adequately, remain physically active and preserve normal function.
If your progress slows, continually cutting calories is not always the answer. Our guide to weight-loss plateaus explains why weight loss commonly slows over time and what to consider next.
How Much Muscle Do People Normally Lose?
There is no universal percentage.
You may see claims that “25% of weight loss is muscle”, but this needs some context.
Studies usually measure lean mass or fat-free mass, not skeletal muscle alone. Lean mass measurements include body water and several tissues other than muscle, so a fall in lean mass does not mean the same amount of actual muscle tissue has disappeared.
The proportion also varies according to age, starting body composition, the size of the calorie deficit, protein intake and exercise.
Someone who performs regular resistance training while eating enough protein may retain considerably more lean tissue than someone following a severe diet while becoming less active.
This is why focusing only on total kilograms lost can be misleading. Two people can lose the same amount of body weight while experiencing quite different changes in body composition and strength.
Resistance Training Makes a Meaningful Difference
If you want the body to retain muscle during weight loss, it helps to keep giving that muscle a reason to stay.
Resistance training provides exactly that signal.
A 2025 systematic review and meta-analysis examined resistance exercise during dietary weight loss in people living with overweight or obesity. Adding resistance training did not substantially change total body-weight loss, but it reduced the loss of fat-free mass, increased fat-mass loss and improved muscular strength compared with dieting alone.
That is an important distinction. The scales may move at a similar rate, while what is happening underneath can be quite different.
You do not need an elaborate bodybuilding programme to benefit.
Weights, gym machines, resistance bands and challenging body-weight exercises can all provide useful resistance. What matters is that the muscles are working hard enough to adapt.
The NHS recommends muscle-strengthening activity involving the major muscle groups on at least two days per week. Its examples include exercises using weights, resistance bands or your own body weight.
Walking, cycling and other cardiovascular exercise remain very valuable for general health, but they do not completely replace resistance exercise when preserving muscle is the goal.
Protein Becomes More Important When You Eat Less
Muscle needs amino acids from dietary protein for maintenance and repair.
When total food intake falls substantially, it becomes easier for protein intake to fall as well.
This is particularly noticeable in people whose appetite has been strongly reduced by dieting or weight-loss medication. Someone may feel that they are “eating healthily” but simply be eating too little overall to obtain much protein.
There is no single protein target that suits every person losing weight.
However, a recent British Dietetic Association review of nutritional recommendations for people taking obesity medicines found that 1.2–1.5g of protein per kilogram per day was the most common recommendation in the literature it examined. The BDA also highlighted resistance exercise together with adequate protein as important for reducing muscle and lean-mass loss.
That does not mean every person should automatically multiply their current body weight by 1.5 and treat the answer as a compulsory target. Very high body weight, kidney disease, age and other medical factors can change what is appropriate.
For many people, a simpler approach is to make sure each main meal contains a meaningful protein source.
That could be eggs, fish, lean meat, Greek-style yoghurt, cottage cheese, milk, tofu, lentils, beans or other protein-rich foods.
If appetite is poor, smaller protein-rich meals may be easier than trying to eat one very large meal later in the day.
Our guide to what to eat on Mounjaro or Wegovy covers protein, fibre, fluids and managing reduced appetite in more detail.
What About Muscle Loss on Mounjaro?
Weight-loss injections have created particular concern about muscle loss because they can produce relatively large reductions in body weight.
Tirzepatide, sold as Mounjaro for weight management in the UK, causes both fat mass and lean mass to fall as weight decreases.
A body-composition substudy of the SURMOUNT-1 trial used DXA scans to examine people receiving tirzepatide. After 72 weeks, the average body-weight reduction in the tirzepatide group was around 21%.
Of the weight lost, approximately 75% was fat mass and 25% was lean mass. Interestingly, the approximate proportion was similar in the placebo group, despite the tirzepatide group losing much more total weight.
This is important because it does not support the simplistic claim that Mounjaro uniquely causes the body to “eat muscle”.
Some loss of lean tissue occurs during many forms of substantial weight loss.
However, because Mounjaro can produce large absolute weight losses, the absolute amount of lean tissue lost can still be meaningful. That makes resistance exercise and adequate nutrition important rather than optional extras.
If you are considering medication, our guide to Mounjaro vs Wegovy compares the two main treatments, while our broader UK weight-loss injection guide covers eligibility, risks and what to expect.
Does Wegovy Cause Muscle Loss Too?
Semaglutide, the active ingredient in Wegovy, also causes reductions in lean mass alongside fat mass when substantial weight is lost.
This is not unexpected. Losing a smaller body also means the body eventually needs less supporting tissue, and changes in glycogen and water can contribute to measurements of reduced lean mass.
The practical question is therefore not whether lean mass changes at all, but whether useful skeletal muscle and physical function are being preserved as well as reasonably possible.
Someone who continues resistance exercise, eats adequately and remains strong may be in a very different situation from someone whose appetite has become so poor that they are barely eating and are becoming progressively weaker.
If a weight-loss medicine is making it difficult to eat enough nutritious food, that is something to discuss with the prescriber rather than simply accepting because the scales are moving quickly.
More Aggressive Weight Loss Is Not Necessarily Better Weight Loss
It can be tempting to see a larger weekly loss as evidence that a diet is working better.
But pushing calorie intake lower and lower has disadvantages.
Very restrictive diets can make it harder to consume enough protein, vitamins and minerals. Energy levels may fall, training quality can deteriorate and the diet can become increasingly difficult to sustain.
The NHS recommends gradual weight loss for most people and warns against approaches that rely on overly restrictive eating.
There are situations where rapid weight loss is medically appropriate. Structured very-low-calorie programmes, bariatric surgery and obesity medicines can all produce substantial losses under professional care.
The important distinction is between medically planned rapid weight loss and trying to achieve the lowest possible calorie intake on your own.
If you are losing weight so quickly that you feel persistently weak, cannot maintain normal activities or have stopped being able to train, it is worth reviewing the plan rather than assuming that faster is automatically better.
How Can You Tell Whether You Are Losing Too Much Muscle?
Unfortunately, a smart bathroom scale cannot give you a reliable answer.
Consumer body-composition scales usually estimate muscle and body fat using bioelectrical impedance. The result can change significantly according to hydration, recent food intake, exercise and stored carbohydrate.
A sudden change in the scale's “muscle mass” measurement may therefore reflect water rather than actual loss of muscle tissue.
Function is often more useful.
Ask whether you can still perform normal daily activities comfortably. Are your gym lifts broadly stable? Can you still climb stairs, carry shopping and get out of a chair easily?
A small decline in training performance while dieting is not necessarily alarming, particularly after substantial weight loss. A steady and obvious loss of physical capability is more important.
Waist circumference, strength, fitness, clothing fit and the long-term trend in body weight can together provide a more useful picture than one body-fat percentage on a bathroom scale.
DEXA scanning provides a better estimate of body composition than most consumer devices, but even DEXA measures lean mass rather than pure skeletal muscle. Most people do not need repeated scans simply to manage ordinary weight loss.
Muscle Preservation Matters Even More With Age
Age changes the calculation.
Adults naturally tend to lose muscle mass and strength as they get older. If significant weight loss is added to that process, preserving muscle becomes particularly important.
An older person living with obesity may still gain major health benefits from losing weight, including improvements in mobility, diabetes and cardiovascular risk.
But success should not be defined only by reaching a lower number on the scales.
Being able to stand from a chair, climb stairs, carry shopping, maintain balance and recover from illness can matter much more than losing an extra few kilograms.
This is where resistance exercise and adequate nutrition become particularly valuable.
People who are older, frail or struggling to eat enough during substantial weight loss may benefit from individual advice from a registered dietitian. Our guide to private dietitian costs for weight loss explains typical UK fees and what appointments involve.
What Does a Sensible Muscle-Preserving Weight-Loss Plan Look Like?
You do not need to optimise every gram of protein or spend six days a week in the gym.
A good starting point is much simpler.
Lose weight at a pace you can sustain, eat enough protein-containing food, continue being physically active and perform some form of resistance exercise at least twice a week.
If you are using Mounjaro, Wegovy or another obesity treatment, do not let appetite suppression turn into chronically inadequate nutrition.
The BDA specifically emphasises high-quality protein foods, nutrient-dense meals, adequate fluids and resistance exercise for people using obesity medicines.
Sleep and recovery matter too. Training is the stimulus for maintaining muscle; recovery is when your body adapts to that stimulus.
And remember that preserving absolutely every gram of lean tissue is neither realistic nor necessary. A smaller body normally carries somewhat less lean mass.
The more useful goal is to emerge from weight loss healthier, capable and strong — rather than simply lighter.
Our guide to how weight loss really works explains the relationship between calories, protein, activity and habits, while how to keep weight off after losing it looks at the longer-term challenge after the active weight-loss phase ends.
Frequently Asked Questions
Do you always lose muscle when losing weight?
Some reduction in fat-free mass is common during weight loss, but it is not all skeletal muscle. Resistance exercise and adequate protein can help reduce the amount lost.
Is 25% of weight loss normally muscle?
Not exactly. Around one-quarter of weight loss has been reported as lean or fat-free mass in some studies, including the SURMOUNT-1 tirzepatide substudy. Lean mass includes much more than skeletal muscle, so it is inaccurate to describe the whole amount as muscle.
What exercise is best for preserving muscle while dieting?
Resistance exercise has particularly good evidence. A 2025 systematic review found that adding resistance training to dietary weight loss helped preserve fat-free mass and improve strength.
How often should I strength train?
The NHS recommends strengthening the major muscle groups on at least two days each week.
How much protein should I eat while losing weight?
There is no single target for everybody. In a recent review of nutrition advice for people taking obesity medicines, the BDA found that 1.2–1.5g/kg/day was the most commonly suggested range. Individual needs can differ, particularly with kidney disease or very high body weight.
Do I need protein shakes?
No. Protein shakes are a convenient food rather than a requirement. Protein can come from ordinary foods such as meat, fish, eggs, dairy, tofu, beans and lentils.
Does Mounjaro make you lose muscle?
Tirzepatide causes some lean-mass loss as part of overall weight reduction. In the SURMOUNT-1 body-composition substudy, approximately 75% of weight lost was fat mass and about 25% was lean mass.
Is walking enough?
Walking is excellent for health and helps maintain activity during weight loss, but dedicated resistance exercise provides a stronger stimulus for maintaining muscle.
Can you gain muscle while losing weight?
Yes, particularly if you are new to resistance training, returning after a break or have a higher starting body-fat level. For experienced trainees, maintaining muscle while losing fat may be a more realistic target.
Are body-fat scales accurate for muscle loss?
They are useful for rough trends but not precise measurements. Hydration can change their estimates considerably from one day to the next.
When should muscle loss be medically assessed?
Speak to a healthcare professional if weight loss is accompanied by significant weakness, repeated falls, difficulty standing from a chair, worsening mobility, severe fatigue or weight loss that you were not trying to achieve.
