‘King Kong' of fat jabs ‘helps you TONE up' – as scientists discover surprising ‘fitness boost'

The Irish Sun

07-07-2025

A NEW study challenges the idea that weight loss jabs decrease muscle mass, alongside fat.
Scientists say one injection - known as the 'King Kong' of weight loss jabs - may swerve this side effect.
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The weight loss jab Mounjaro leads to no more muscle loss than you'd get from losing weight without the medication
Credit: Getty
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Scientists also found that jab may help remove fat from within muscles, boosting exercise performance
Credit: Getty
In fact, it could help users tone up, researchers from the University of Glasgow said.
Their study analysed the body composition of more than 200 people and found they lost no more muscle from the weight loss jab
It suggested muscle loss from shedding weight will be similar with or without the injections.
But researchers also noticed that Mounjaro-users also lost fat from inside their muscles - much more than they would have done from losing weight without the medicines.
Getting rid of the fat marbling muscles may improve how they work and boost people's ability to exercise, scientists said.
That's because the fat that gets into muscles and surrounds vital organs - which is different from the fat layer just beneath the skin - has been linked with muscle weakness and poorer physical performance.
The study's lead author Naveed Sattar, a professor of metabolic medicine at Glasgow University's Institute of Cardiovascular and Medical Sciences, told
'Having less muscle fat usually means the remaining fat muscle cells are more efficient in generating
power
, which could explain the boost to people's exercise abilities.
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"These results are hugely encouraging as less muscle fat is also linked to a reduced risk of cardiovascular disease and
'However, we don't yet know how weight-loss injections remove additional muscle fat, and more evidence is needed to understand the mechanisms involved.'
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Muscle is important for health. Not only does it support the body to move efficiently - and therefore support healthy ageing - but it plays a role in blood sugar regulation, metabolism and bone health.
It also shapes the body, and people who use the jabs may find their body is lacking tone or definition as a result, compared to other slim people.
When someone is overweight or not very active, they're more likely to get streaks of fat in their muscles. It's possible to reduce it by doing aerobic exercise such as jogging, walking and cycling.
But for people who have difficulty exercising due to their weight or other conditions, using weight loss jabs to get rid of muscle fat holds promise.
The study, published in
It assessed 246 people with type 2 diabetes who were either given varying doses of Mounjaro - 5
mg
, 10 mg, or 15 mg - once a week, or daily insulin injections.
Researchers used MRI scans to measure amount of muscle and fat each person had in their thighs before the treatment, and a year after starting it.
They compared muscle loss in tirzepatide users to typical muscle loss in people of the same age and sex, who lost the same amount of weight without the jabs - using the data of 3,000 people from the UK Biobank study.
Both groups lost a similar amount of muscle.
Everything you need to know about fat jabs
Weight loss jabs are all the rage as studies and patient stories reveal they help people shed flab at almost unbelievable rates, as well as appearing to reduce the risk of serious diseases.
Wegovy – a modified version of type 2 diabetes drug Ozempic – and Mounjaro are the leading weight loss injections used in the UK.
Wegovy, real name semaglutide, has been used on the NHS for years while Mounjaro (tirzepatide) is a newer and more powerful addition to the market.
Mounjaro accounts for most private prescriptions for weight loss and is set to join Wegovy as an NHS staple this year.
How do they work?
The jabs work by suppressing your appetite, making you eat less so your body burns fat for energy instead and you lose weight.
They do this my mimicking a hormone called GLP-1, which signals to the brain when the stomach is full, so the drugs are officially called GLP-1 receptor agonists.
They slow down digestion and increase insulin production, lowering blood sugar, which is why they were first developed to treat type 2 diabetes in which patients' sugar levels are too high.
Can I get them?
NHS prescriptions of weight loss drugs, mainly Wegovy and an older version called Saxenda (chemical name liraglutide), are controlled through specialist weight loss clinics.
Typically a patient will have to have a body mass index (BMI) of 30 or higher, classifying them as medically obese, and also have a weight-related health condition such as high blood pressure.
GPs generally do not prescribe the drugs for weight loss.
Private prescribers offer the jabs, most commonly Mounjaro, to anyone who is obese (BMI of 30+) or overweight (BMI 25-30) with a weight-related health risk.
Private pharmacies have been rapped for handing them out too easily and video calls or face-to-face appointments are now mandatory to check a patient is being truthful about their size and health.
Are there any risks?
Yes – side effects are common but most are relatively mild.
Around half of people taking the drug experience gut issues, including sickness, bloating, acid reflux, constipation and diarrhoea.
Dr Sarah Jarvis, GP and clinical consultant at patient.info, said: 'One of the more uncommon side effects is severe acute pancreatitis, which is extremely painful and happens to one in 500 people.'
Other uncommon side effects include altered taste, kidney problems, allergic reactions, gallbladder problems and hypoglycemia.
Evidence has so far been inconclusive about whether the injections are damaging to patients' mental health.
Figures obtained by The Sun show that, up to January 2025, 85 patient deaths in the UK were suspected to be linked to the medicines.
But Mounjaro-takers lost 0.5 per cent more fat from within their muscles.
Researchers didn't assess participants' ability to exercise after taking the jabs.
"As the study was not specifically designed to evaluate muscle-related changes, assessments of strength, mobility, and physical performance are not available," they wrote.
They also said other factors may have lead to loss of fat within muscles.
"There were no specific instructions in our study about dietary changes or exercise training beyond the standard of care at each site and region, and we cannot exclude the influence that individual differences in lifestyle management might have had on the results," scientists wrote.
"It has been reported that resistance exercise training or dietary supplements might exert a positive effect on muscle composition, and are recommended when substantial weight reduction is achieved or expected (eg. after bariatric surgery)."
The findings indicate that Mounjaro gets rid of fat from different parts of the body better than conventional weight loss, scientists said, as well as dimming hunger pangs.
That could help to explain evidence suggesting weight-loss injections may reduce the risk of diabetes and many other chronic diseases.
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Researchers called for more studies "into long-term changes in muscle strength, mobility, and physical performance in people treated with tirzepatide".
Other studies have shown that weight loss jabs cause a drop in muscle mass. For example,
Britain will be 'fat free' within a decade
The Health Secretary told The Sun he is on a mission to slash levies by giving more people access to drugs such as Ozempic and Mounjaro.
Wes Streeting said: 'The jabs are a route not just to lower weight, but lower taxes.'
Obesity-related illnesses cost the NHS a whopping £6billion every year.
Currently people with a BMI of 35 or more - or 30 but with a health condition - can be prescribed jabs on the NHS through specialist weight-management services.
Read the full story here and find out whether you are likely to be eligible.