How can weight loss injections change your body composition?

When people think about weight loss, they often focus on the number on the scale. But weight loss injections (WLIs) can actually help improve your overall health, including your body composition.

They target fat loss, particularly the type that harms your health most, while helping you hold on to muscle. And they can bring about some physical changes too, like less joint pain or a more defined face.

Medically reviewed by

Alexandra Cristina Cowell, Writer & Clinical Content Reviewer

This guide breaks down exactly how WLIs change your body composition, what that means for your health, and how to get the best results.

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Medically reviewed by

Dr Alexandra Cristina Cowell

Writer & Clinical Content Reviewer

on Jul 24, 2025.

Meet Alexandra Cristina

Last updated on Oct 17, 2025.

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WLIs can help you lose fat while preserving muscle, improving your overall body composition.
They reduce visceral fat – the type that increases your risk of heart disease and diabetes.
Some lean mass is often lost, but your proportion of muscle mass to fat should increase.
You may notice unexpected benefits like better posture, less joint pain and reduced bloating.
You can protect your muscles further by eating more protein and doing resistance training.

How can GLP-1s affect internal changes to your body?

There’s a lot of ways that GLP-1 receptor agonists (the active ingredients in many weight loss injections) can affect the internal changes in your body, as well as what you see in the mirror.
Because these medications help you stay in a calorie deficit, they naturally lead to weight loss. Studies show that this weight loss actually tends to come more from fat than from muscle, meaning your body composition shifts in a healthier direction.

GLP-1s can also improve how your body stores and uses energy. They influence the hormones linked to hunger, fat storage and your metabolism, helping you burn more fat while keeping more of your muscle. This can cause your metabolism to improve too, especially if you have insulin resistance or high levels of fat around your organs.

This image shows data taken from the SURMOUNT study.

Types of fat loss

Visceral fat

Visceral fat is the fat that wraps around your organs – the most dangerous type. High levels of visceral fat are linked to heart disease, type 2 diabetes, and even some cancers.

GLP-1s are particularly effective at reducing visceral fat. In fact, studies show they reduce it more significantly than diet alone. In one large analysis, people taking GLP-1s saw substantial drops in visceral fat, regardless of whether they had diabetes or fatty liver disease.

This makes visceral fat loss one of the best health benefits of weight loss injections. Less visceral fat means less strain on your internal organs and a lower risk of developing long-term health problems.

Subcutaneous fat

Subcutaneous fat sits just under your skin. It’s the kind you can pinch around your belly or hips. It’s less harmful than visceral fat but still contributes to your overall weight and body shape.

GLP-1s also reduce this kind of fat. In an analysis of weight loss injection studies, experts found that semaglutide users lost an average of over 2kg of subcutaneous fat, along with reductions in waist and hip measurements. So even while your priority might be your internal health, you’re likely to notice visible changes too, like a slimmer face or looser clothing.

What is the importance of muscle preservation during weight loss?

Losing weight can lead to some loss of muscle as well as fat. This happens with any weight loss approach, if there isn’t care taken to protect and strengthen muscles.

Preserving muscle is important because it helps with your strength, mobility, and also keeps your metabolism running efficiently. Studies show that you burn more energy – even at rest – when you have more muscle.

In studies of GLP-1-based weight loss injections, around 75% of the weight lost was fat, and only 25% was lean mass (which includes muscle and water). This suggests that most of the weight you lose on GLP-1s comes from fat, rather than lean tissue. These results are supported by a healthy and nutritious diet, so it’s important to make sure you’re still eating nutritiously, even with a reduced caloric intake.

And the quality of the weight loss matters. Real-world studies show that although some lean mass is lost with GLP-1 treatment, the proportion of lean mass actually goes up – because more fat is being lost overall. That means your body composition is shifting in a direction of less fat with relatively more muscle.

Muscle preservation also helps improve insulin sensitivity and blood sugar regulation, which are also key benefits of weight loss injections.

Ways to prevent muscle loss

While GLP-1s can help protect your muscle mass during weight loss, you can reinforce this effect by doing a few key things:

Get enough protein. Eating plenty of protein gives your body the resources to maintain muscle during a calorie deficit.
Do resistance or strength training. Lifting weights, doing bodyweight exercises, or using resistance bands can all help signal your body to hold onto muscle tissue.
Keep active. Even light movement like walking or yoga can help reduce muscle breakdown if done regularly.

By combining weight loss injections with strength training and good nutrition, you’ll lose fat, but also be stronger, healthier, and more toned too.

Unexpected physical changes

Reduced inflammatory swelling

Fat tissue stores energy, but it also releases inflammatory molecules that can cause fluid retention and swelling. When you lose fat, especially visceral fat, inflammation tends to go down too.

You might notice that your rings fit more loosely, your ankles feel less puffy, or your shoes slip on more easily. These small changes are signs of reduced systemic inflammation, and can make daily movement feel more comfortable.

Changes to your face

Fat loss can show up in your face too.

You might notice a sharper jawline, more visible cheekbones, or a more defined neck area. These changes can be especially noticeable if you’re losing a lot of subcutaneous fat, which tends to collect around the lower face and chin.

Joint relief

When you carry less weight, your joints don’t have to work as hard – particularly your knees, hips and lower back.

One study found that for every pound lost, four pounds of pressure are taken off the knees. That means losing just 10 pounds could relieve 40 pounds of force from your joints.

In our user feedback data, over 40% of people using WLIs said they experienced less back pain after weight loss treatment.

Better posture

As your body becomes lighter, your core muscles don’t have to work as hard to stabilize you, so you may find it easier to stand taller and move more freely.

Losing abdominal fat can also take strain off your lower back, helping to correct your posture and reduce slouching.

That said, none of this happens overnight. To get the most out of weight loss injections – and to protect your muscles and joints – you’ll still need to follow a balanced diet, move your body regularly, and prioritize sleep. These habits can help turn your results into a long-term lifestyle change.

What weight loss injection could help me reach my goal body composition?

Weight loss injections can help reshape your body, reduce inflammation, and preserve muscle. Though what works best for you might be different from what works best for someone else.

If you’re not already using a GLP-1, we can help you explore your options and find the right treatment for your goals.

Sourcing guidelines:

When we present you with stats, data, opinion or a consensus, we’ll tell you where this came from. And we’ll only present data as clinically reliable if it’s come from a reputable source, such as a state or government-funded health body, a peer-reviewed medical journal, or a recognised analytics or data body. Read more in our editorial policy.

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Disclaimer: The information provided on this page is not a substitute for professional medical advice, diagnosis, or treatment. If you have any questions or concerns about your health, please talk to a doctor.