Exercise and GLP-1 agonists: lose weight without losing muscle

GLP-1 agonists — semaglutide and tirzepatide — have changed the treatment of obesity and type 2 diabetes. They lead to consistent weight loss and bring real health benefits. But part of the weight lost is not fat: it is muscle. And muscle that is lost and not regained has consequences.

At Clínica Myalgia we have created a supervised exercise programme, designed by Sports Medicine doctors and physiotherapists, for people on this treatment: to preserve and build strength and muscle mass while the weight comes down — and to keep the results afterwards.

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The benefits of GLP-1 treatment

GLP-1 agonists act on appetite and satiety and reduce calorie intake. Clinical trials show average weight loss of around 15% with semaglutide and over 20% with tirzepatide after about a year and a half — results that, until recently, only bariatric surgery could achieve.

Beyond weight, the benefits are broad and well documented: better control of blood glucose and HbA1c; lower blood pressure and triglycerides; fewer major cardiovascular events in people with established cardiovascular disease; improvement in sleep apnoea and fatty liver; and less pain and better function in knee osteoarthritis, largely thanks to reduced joint load.

It is therefore a treatment that makes sense for many people — when prescribed and monitored by their own doctor. What follows is not an argument against the treatment. It is an argument for doing it well.

The less-discussed side: loss of muscle mass

When the body loses weight quickly, with little protein intake and little mechanical stimulus, it doesn't lose only fat. In studies measuring body composition (DXA), a significant part of the weight lost with GLP-1 agonists — in some trials between a quarter and about 40% — is lean mass, which includes skeletal muscle.

Losing some lean tissue is normal with any weight loss. The problem arises when that loss is large, rapid and not compensated: that is when we talk about sarcopenia — the loss of muscle mass, strength and function. In a person aged 55 or 65, who is already naturally losing muscle mass every year, the treatment can speed up in months a process that would otherwise take a decade.

There are three reasons why this often happens during treatment:

• Less appetite almost always means less protein — and muscle needs it to be maintained.
• Rapid weight loss puts the body into a marked negative energy balance, in which muscle is also used as a source of energy.
• Without strength training, there is no signal telling the body to keep its muscle.

The consequences of sarcopenia

Muscle isn't just for lifting weights. It is the body's largest metabolic organ and what keeps us independent as we age. Losing it has real consequences:

• Slower metabolism and rebound weight gain. Less muscle means fewer calories burned at rest. When treatment ends or the dose is reduced, weight tends to come back — mostly as fat, leaving the person with a worse body composition than at the start.
• Poorer insulin sensitivity. Muscle is the main destination for glucose after meals. Losing it works against one of the goals of the treatment.
• Weakness, fatigue and reduced function. Difficulty climbing stairs, getting up from a chair or carrying bags; lower tolerance for everyday exertion.
• More pain and more injuries. Less strength in the muscles that stabilise the knee, hip and spine means more load on joints and tendons — one of the most common causes of pain we see in clinic.
• Loss of bone mass. Rapid weight loss and less muscle pull on bone increase the risk of osteopenia and, over time, fractures.
• Falls and frailty in older people, and a worse starting point if illness or surgery comes along.

What the evidence recommends

International recommendations for managing obesity with medication are consistent: treatment should be accompanied by structured strength training, at least twice a week, and by adequate protein intake (around 1.2 to 1.6 g per kg of body weight per day, adjusted case by case). Studies that combined exercise with drug treatment show better preservation of lean mass, better function and — above all — better weight maintenance after stopping the medication.

Strength training is the most effective stimulus for getting the body to maintain and build muscle while it loses fat. It isn't optional. It is part of the treatment.