Semaglutide and Muscle Loss: What the STEP UP Trial Reveals About Your Thigh Skeletal Muscle

When the scale drops, not everything you lose is fat. This is one of the most important — and most underappreciated — realities of any aggressive weight loss effort, whether you’re running on caloric restriction alone, using a GLP-1 medication like semaglutide, or grinding through a body recomposition phase in the gym. A new secondary analysis from the STEP UP trial, published in 2026 in Diabetes, Obesity & Metabolism, has put some hard numbers on exactly what happens to thigh skeletal muscle during high-dose semaglutide treatment — and the findings are worth understanding carefully if preserving functional strength matters to you.

The study, led by Hjelmesæth, Bhat, Garvey and colleagues, examined body composition and physical function data from participants randomized to semaglutide or placebo as part of the phase 3b STEP UP trial. Among the 55 participants who had valid baseline MRI scans, those receiving semaglutide experienced a statistically significant reduction in overall adipose body tissue volume — a mean treatment difference of 11.1 liters compared to placebo, which is a substantial amount of fat by any measure. Visceral adipose tissue dropped by an estimated 1.5 liters. These are meaningful metabolic wins. But the data also showed reductions in lean body tissue volume of 1.7 liters and a thigh skeletal muscle volume reduction of approximately 1.1 liters, though these muscle-related changes did not reach conventional statistical significance (p = 0.07 and p = 0.16, respectively). What the trial did confirm with statistical significance was a meaningful reduction in thigh muscle fat infiltration — nearly a full percentage point — suggesting that while some muscle volume may be lost, the quality and composition of remaining muscle tissue actually improved.

Perhaps the most clinically reassuring finding was this: despite these compositional changes, sit-to-stand performance did not decline. Participants averaged about 13 sit-to-stand repetitions in 30 seconds at baseline and reached 15 repetitions by week 72 — an improvement that held across both the semaglutide and placebo groups. Functional capacity was preserved, and in some respects enhanced, even as body composition shifted significantly. That’s the nuanced picture the study paints: real changes in muscle volume are occurring, but they don’t necessarily translate into loss of physical function when the intervention includes lifestyle components and the muscle that remains is of higher quality.

Why Muscle Loss During Weight Loss Is a Universal Problem — Not Just a GLP-1 Issue

Before diving into what men on semaglutide should do about this, it’s critical to zoom out and recognize that muscle loss during caloric deficit is not a medication-specific phenomenon. It is a fundamental biological response to energy restriction that applies to every human being who loses weight, regardless of the method. During any significant caloric deficit, your body draws on both fat and lean tissue for fuel. Without specific countermeasures — primarily resistance training and adequate dietary protein — a meaningful portion of weight lost will come from muscle. Research has historically suggested that somewhere between 20 and 40 percent of total weight lost during standard caloric restriction can come from lean mass, depending on the intensity of the deficit, protein intake, training status, and starting body composition.

GLP-1 receptor agonists like semaglutide create unusually large caloric deficits because they profoundly reduce appetite and food intake. This is precisely why they are so effective for weight loss — but it’s also why the muscle-preservation challenge may be amplified compared to more moderate, traditional approaches. When someone eats significantly less food overall, they often eat significantly less protein as well, which removes one of the key signals your body uses to maintain lean tissue. The STEP UP findings are consistent with this framework. The drug worked extraordinarily well at removing fat — 11.1 liters of total adipose tissue is a dramatic result — but that aggressive fat loss came alongside some reduction in muscle volume that, while not statistically significant for the thigh specifically, was directionally consistent and clinically worth taking seriously.

For men who are not on any medication and are simply pursuing fat loss through diet and training, the lesson is the same: the magnitude of your caloric deficit and the quality of your protein intake will largely determine how much muscle you retain. A 700-calorie daily deficit with 180 grams of protein and four days of resistance training per week will yield a very different body composition outcome than a 1,200-calorie daily deficit eating 80 grams of protein and doing only cardio — even if the scale tells a similar story at 12 weeks.

The Case for Resistance Training as Non-Negotiable During Any Weight Loss Phase

The preservation of sit-to-stand performance in the STEP UP trial is encouraging, but it also highlights something important: functional tests like sit-to-stand are not the same as measuring or maintaining muscle mass itself. You can lose meaningful amounts of muscle volume and still perform adequately on low-threshold functional tests, particularly if you were starting from a place of significant obesity where excess body weight was itself the primary limiter. As body weight drops, the mechanical load on your muscles decreases, which can make many everyday tasks feel easier even if absolute muscle mass has declined. This is not a contradiction — it’s simply a reminder that different metrics capture different things, and preserving functional performance is not equivalent to preserving muscle tissue.

This is why resistance training needs to be part of the plan for any man pursuing significant fat loss, whether he’s using semaglutide, intermittent fasting, a ketogenic diet, or a straightforward calorie deficit. The research on this point is remarkably consistent. Progressive resistance exercise is the single most effective intervention for preserving lean mass during caloric restriction. It sends an anabolic signal — through mechanical tension and metabolic stress on muscle fibers — that tells the body this tissue is needed and should not be catabolized. Without that signal, the body has no particular reason to maintain expensive metabolic tissue when energy is scarce.

For men using semaglutide specifically, integrating resistance training is especially important because the medication-driven appetite suppression makes it easy to eat very little without feeling hungry — and very little food often means very little protein. A practical approach is to treat your protein target as a non-negotiable daily minimum, set before anything else in your nutrition plan. For most men in a fat loss phase, a target of 0.7 to 1.0 grams of protein per pound of body weight is a reasonable and evidence-informed range. If appetite suppression makes hitting that target difficult, leaner protein sources — chicken breast, egg whites, Greek yogurt, protein shakes — offer high protein density with relatively low caloric volume, making it easier to reach your target even when overall food intake is reduced.

Training frequency and volume should be organized around at least three full-body or split resistance sessions per week, with an emphasis on compound movements that load the large muscle groups of the lower body — squats, Romanian deadlifts, leg press, split squats — as well as the upper body and back. The thigh skeletal muscle findings from STEP UP make lower-body training particularly relevant. The quadriceps, hamstrings, and glutes are among the largest muscle groups in the body and represent an enormous portion of your total lean mass. Prioritizing their preservation during a fat loss phase pays dividends not just aesthetically, but metabolically — more muscle mass means a higher resting metabolic rate and better long-term weight management outcomes.

Muscle Fat Infiltration: The Overlooked Quality Metric That Actually Improved

One of the more clinically interesting findings from the STEP UP secondary analysis is the significant reduction in thigh muscle fat infiltration — a nearly one percentage point decrease (−0.92 percentage points, p = 0.001) in men on semaglutide compared to placebo. This metric, sometimes called intermuscular or intramuscular adipose tissue, refers to fat that has accumulated within and between muscle fibers. It is distinct from subcutaneous fat under the skin or visceral fat around the organs. High levels of muscle fat infiltration are associated with insulin resistance, reduced muscle strength relative to size, and impaired metabolic function. It is, in a sense, a marker of muscle quality — and it tends to be elevated in individuals with obesity, type 2 diabetes, or metabolic syndrome.

The fact that semaglutide treatment significantly reduced this form of fat deposition within the muscle itself is a finding that doesn’t get nearly enough attention. It suggests that the metabolic improvements associated with the medication extend beyond simple reductions in body weight or visceral fat — they include a qualitative improvement in the tissue composition of the muscle itself. Muscle with less fat infiltration contracts more effectively, responds better to insulin, and likely generates more force per unit of size. In this respect, some of what appears as a quantitative loss of muscle volume may actually represent a purification of the remaining tissue — removing the fatty infiltrate that was embedded within it.

This concept has practical implications for men across all approaches to fat loss. Excess body fat, particularly visceral and ectopic fat (fat stored in organs and tissues where it doesn’t belong), impairs muscle quality over time. Men who are carrying significant amounts of excess weight — whether they are using medication or not — likely have elevated levels of intramuscular fat that is degrading the performance and metabolic function of their muscle tissue. Meaningful fat loss, achieved through any sustainable method, tends to improve this metric. That’s worth knowing, because it reframes the relationship between scale weight and actual body composition in ways that simple weight measurements cannot capture.

The practical takeaway here is that tracking weight alone is a poor proxy for what’s actually happening inside your body. If you have access to body composition measurements — DEXA scans, MRI, or even a reliable bioelectrical impedance device used consistently — tracking lean mass alongside total weight gives you a far more meaningful picture of whether your fat loss strategy is preserving the tissue that matters most. For most men, a successful fat loss phase should result in a fat mass to lean mass ratio that clearly favors fat loss: losing significantly more pounds of fat than pounds of muscle, ideally in a 3-to-1 or better ratio.

What This Means For You

The STEP UP secondary analysis does not suggest that semaglutide is uniquely harmful to muscle. What it shows is that large, rapid weight loss — enabled by a highly effective pharmacological intervention — produces the same body composition trade-offs that any aggressive fat loss approach produces, with the encouraging caveat that muscle quality may actually improve even as volume decreases. The drug works. It removes fat aggressively. And the functional performance data suggests that, when lifestyle interventions are included, physical capacity can be maintained or improved.

But the research also makes clear that passive weight loss — whether through medication or any other method — is not enough to fully protect your muscle. The men who will come out of a fat loss phase with the best body composition are those who prioritize protein, who show up to lift heavy things consistently, and who treat muscle preservation as an active goal rather than an afterthought. That principle does not change based on whether you’re using semaglutide, eating in a calorie deficit the old-fashioned way, or somewhere in between.

Regardless of the tools you use to get leaner, the fundamentals remain constant: eat enough protein every single day, perform progressive resistance training at least three times per week with an emphasis on large compound movements, monitor your body composition rather than just your body weight, and understand that quality of muscle tissue is as important as quantity. The thigh skeletal muscle findings from STEP UP are a valuable reminder that what’s happening below the skin matters — and that taking an active role in preserving and building muscle will always be the difference between a successful transformation and simply becoming a smaller version of your former metabolically compromised self.