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Obesity is an epidemic that represents a public health challenge, according to a scientific article published in the medical science journal The Lancet. And the situation is expected to worsen. According to the World Obesity Atlas 2025, the total number of adults living with obesity is expected to increase by more than 115% between 2010 and 2030, rising from 524 million to 1.13 billion. The scenario is even more concerning because most countries worldwide lack sufficient plans and policies to address rising obesity levels, as pointed out in one of the main conclusions of the document prepared by the World Obesity Federation.
It is in this context that the so-called “weight-loss pens” are gaining increasing traction. These are injectable medications that mimic the action of the intestinal hormone GLP-1 (and, in some cases, also GIP), increasing satiety and improving glycemic control. Initially developed for the treatment of type 2 diabetes, these drugs are also now among the main approved therapies for obesity treatment.
How weight-loss pens work
GLP-1 agonists are medications that mimic the action of the body’s natural hormone called GLP-1, which plays a role in blood sugar control. They bind to receptors on cells—like fitting into a “lock”—and activate these receptors to produce effects similar to those the hormone itself would cause. This is the case with semaglutide.
Dual agonists work similarly but act on two different receptors simultaneously: in addition to GLP-1, they also act on GIP. This is the case with tirzepatide, which acts on the brain and gastrointestinal tract to reduce appetite, increase satiety, and slow gastric emptying.
Although effective for weight loss, the rapid weight reduction promoted by these medications is not limited to the loss of body fat. Part of the weight lost can also come from lean mass, which is composed of muscles, bones, and organs.
A literature review on the effect of semaglutide on lean mass in overweight or obese patients, regardless of whether they have type 2 diabetes, published in 2024, analyzed different databases in search of randomized controlled trials (RCTs) or observational studies. Out of six studies involving over 1,500 adults, weight loss was primarily due to fat mass reduction. However, cases of notable lean mass reductions raise an alert, with up to 40% of the total weight reduction, especially in trials with a larger number of participants.
A sub-study from the first major phase 3 clinical trial evaluating tirzepatide (SURMOUNT-1) shows that approximately 75% of the weight lost was fat and the remaining 25% was lean mass. The study, conducted with 160 participants followed for 72 weeks with DXA scans (low-power X-rays to measure bone density and body composition), showed that treatment with the tirzepatide weight-loss pen reduced fat mass by an average of 33.9% and lean mass by 10.9% compared to reductions of 8.2% and 2.6%, respectively, in the placebo group. In other words, although most weight loss occurs through fat reduction, the study confirms that there is also significant loss of lean mass during the use of the medication, even in a proportion similar to that observed in other weight-loss therapies.
The results justify the alert from the Brazilian Society of Endocrinology and Metabolism (SBEM) regarding muscle loss during the use of GLP-1 agonists as one of the main reasons why treatment should be accompanied by a multidisciplinary team. Losing too much muscle can compromise strength, mobility, and energy metabolism, making long-term weight maintenance difficult and increasing the risk of sarcopenia, which is the progressive and generalized loss of skeletal muscle mass, strength, and performance, especially in the elderly.
How to reduce lean mass loss during the use of weight-loss pens
1) Sufficient protein distributed throughout the day to preserve lean mass
Adequate protein intake helps preserve muscle during weight loss, as distributing protein across all meals favors muscle protein synthesis. According to a study by the American Medical Directors Association, for adults over 65 years old, international consensus recommends around 1 to 1.2 g of protein per kilogram of body weight per day (g/kg/day) as a baseline, potentially reaching or exceeding this value. When exercise or illness is present, the need tends to increase.
In an article published on the MyMinerva Foods portal, Raphael Einsfeld, coordinator of medicine at the São Camilo University Center in São Paulo, and Marcus Quaresma, a nutritionist specializing in exercise physiology applied to clinical practice, explain that, depending on specific clinical conditions, the protein intake range for sedentary adults and the elderly can be flexible between 0.6 and 1.0 g/kg/day. “However, in physically active individuals, particularly those who engage in strength training, these needs are increased. For physically active adults, an intake between 1.6 and 2.2 g/kg/day is recommended, while for the elderly, the recommended values range between 1.2 and 1.6 g/kg/day.”
In the guidelines of the European Society for Clinical Nutrition and Metabolism (ESPEN) for geriatrics, it is recommended that all elderly individuals, especially those at nutritional risk, be assured a minimum consumption of 1.0 g/kg/day, with individual adjustments according to their clinical status.
2) Resistance training (weightlifting)
Randomized trials, published in the New England Journal of Medicine, show that combining diet with strength exercises, ideally along with aerobics, reduces fat while preserving lean mass and improves functional capacity, including in obese elderly individuals.
It is worth noting that beyond the physical benefits, which generate quality of life by promoting more energy and well-being, exercising positively impacts mental health, potentially reducing stress, anxiety, and depression. According to a publication by the Pan American Health Organization, one in four adults, about 1.4 billion people worldwide, do not engage in the 150 weekly minutes of moderate-intensity physical activity recommended by the WHO.
3) Gradual weight loss and monitoring during the use of pens
Adjusting doses and goals, during the use of weight-loss pens, with the healthcare team, checking symptoms, and, when possible, monitoring body composition help the healthcare professional guide the patient on decisions regarding protein intake, training, and treatment progression. Monitoring can be done through DXA scans or bioimpedance—which uses electrodes on the patient’s hands and feet to measure the electrical resistance of different body tissues, estimating the percentage of fat, lean mass (muscles), total body water, and bone mass.
Obesity is today a global epidemic as serious as malnutrition once was—or still is in many parts of the world, according to an article published in the science journal The Lancet. Both reveal distinct faces of the same challenge: malnutrition, whether due to lack or excess. In this scenario, weight-loss pens represent an important scientific advancement, especially given the difficulty in controlling weight and the increase in associated comorbidities. But the discussion goes beyond pharmacology. In an era of haste and aesthetic pressure, where thin and aesthetically “perfect” bodies are valued on social media, the risk is transforming medical treatment into a promise of instant results—the desired magic formula.
Therefore, the debate about tirzepatide, semaglutide, and other similar therapies must be conducted responsibly. Studies demonstrate efficacy and safety when weight-loss pens are used under professional guidance, but they also point to significant losses of lean mass and potential risks when their use is indiscriminate. Ultimately, there are no safe shortcuts: the most sustainable path to health remains medical follow-up, a balanced diet, care for the body and mind—and the awareness that weight is just one of many measures of well-being.
Reference sources:
1. Aerobic or Resistance Exercise, or Both, in Dieting Obese Older Adults
2. A systematic review of the effect of semaglutide on lean mass: insights from clinical trials
3. Body composition changes during weight reduction with tirzepatide in the SURMOUNT-1 study of adults with obesity or overweight
4. Obesity epidemic
5. Proteína para que te quero: a relação entre ingestão proteica, força e massa muscular
6. Evidence-based recommendations for optimal dietary protein intake in older people: a position paper from the PROT-AGE Study Group
7. The majority of countries in the world do not have sufficient plans and policies in place to deal with rising obesity levels, researchers at the World Obesity Federation have warned
8. Worldwide trends in underweight and obesity from 1990 to 2022: a pooled analysis of 3663 population-representative studies with 222 million children, adolescents, and adults
