GLP-1 muscle loss: where running and lifting fit

Make movement part of the plan from the start

At First Gear, we want you to have a clear next step when you decide to move more. During GLP-1 treatment, that step belongs inside a wider plan for strength, nutrition, and activity. A 2025 joint advisory from four medical and nutrition organizations calls for exercise and nutrition support during treatment. ^[1]

GLP-1-based medications can produce substantial weight loss and health benefits, while reduced appetite can make adequate nutrition harder. ^[1] Give yourself something to practice alongside what you track: a strength session, a walk, a regular place for movement in your week.

We built First Gear to coach the running and walking part on Apple Watch. Here is how strength training, food, and aerobic activity fit together, and where our coaching gives you a clear next step.

Do semaglutide and tirzepatide cause muscle loss?

Start by separating muscle from lean mass. Trials show reductions in lean mass alongside larger reductions in fat mass. Lean mass is not the same as skeletal muscle: DXA body scans also count water and other nonfat soft tissues. A lower lean-mass reading alone does not establish loss of strength or a muscle disease. ^[2][3]

In a 140-person exploratory STEP 1 substudy, semaglutide treatment over 68 weeks reduced fat mass by 19.3 percent and lean body mass by 9.7 percent. The share of body weight made up of lean mass increased. These findings were reported in a conference abstract. ^[2]

In the 160-person SURMOUNT-1 body-composition substudy, tirzepatide treatment over 72 weeks reduced fat mass by 33.9 percent and lean mass by 10.9 percent. About 75 percent of the weight lost was fat and 25 percent was lean mass, similar proportions to placebo. ^[3]

The tissue percentages describe changes from each tissue's starting amount. They are not percentages of starting body weight. These separate studies also cannot tell you which drug preserves muscle better. ^[2][3]

Where does retatrutide fit?

Semaglutide acts at the GLP-1 receptor. Tirzepatide acts at GIP and GLP-1 receptors. ^[1][3] Retatrutide adds activity at the glucagon receptor, making it a triple agonist. As of October 3, 2026, retatrutide remains investigational and is not FDA approved. ^[4]

Retatrutide already has early body-composition research. A phase 2 substudy in adults with type 2 diabetes found substantial fat loss, with a proportion of weight loss from lean mass similar to other obesity treatments. It does not establish that running or lifting prevents muscle loss during retatrutide treatment. ^[5]

Replace the 'skinny fat' worry with a practical plan

We would rather help you plan your next activity than give you another appearance label to worry about. 'Skinny fat' is an imprecise phrase for a concern worth discussing: getting lighter without paying attention to muscle or physical capacity. It is not a diagnosis, and appearance cannot tell you how strong someone is.

The semaglutide and tirzepatide findings above do not show an inevitable slide toward a higher body-fat percentage. Fat fell more than lean tissue, and body composition improved by those measures. Lean tissue still deserves attention. ^[2][3]

Put a useful question on the table with your care team: where do strength training, enough nourishment, and aerobic activity fit in my week? That gives you decisions to make and actions to take.

Give strength training its own place

Our run/walk coaching has a specific job. Your strength sessions have another: resistance training makes muscles work against a load, using weights, machines, bands, or your own body. It provides a direct strength stimulus and also loads bones. ^[9]

The distinction shows up in weight-loss research. In a six-month trial of 160 older adults with obesity, lean mass fell by about 2 percent with resistance training, 3 percent with combined aerobic and resistance training, and 5 percent with aerobic training alone. Strength improved more in the resistance and combined groups. This was dietary weight loss, not a GLP-1 trial. ^[6]

The joint advisory recommends adequate protein together with structured strength training during GLP-1 treatment. Protein alone is unlikely to do the whole job. Ask your clinician or a registered dietitian to tailor nutrition to your health and intake. ^[1]

GLP-1 weight loss and bone density: what is known?

Bone health deserves a place in the conversation, particularly when fracture risk is already elevated. In a 52-week trial of 64 adults at increased fracture risk, mostly postmenopausal women, semaglutide 1.0 mg was associated with lower hip and spine bone mineral density than placebo. Bone density was a secondary outcome, and the authors noted that accompanying weight loss could help explain increased bone resorption. ^[7]

That small trial does not establish that every GLP-1-based medication causes osteoporosis or fractures. It also does not predict the effect for a younger adult, another dose, or another drug. ^[7]

NIH guidance includes weight-bearing activity, resistance training, and balance work for bone health. Brisk walking and running both provide weight-bearing movement. Running is one option within that mix. ^[9]

Does exercise protect bone during GLP-1 treatment?

A 2024 analysis of a randomized trial offers encouraging, specific evidence. After an initial low-calorie diet, 195 adults with obesity were assigned to exercise, liraglutide, both, or placebo for a year. The combined group had greater weight loss while hip and spine bone density did not differ significantly from placebo. ^[8]

Liraglutide alone produced lower bone density than exercise alone. However, the direct comparison between combination treatment and liraglutide alone was not statistically significant for bone density. The exercise program mixed cycling, circuit strength training, and individually chosen activities. This was not a trial of running alone, semaglutide, tirzepatide, or retatrutide. ^[8]

Use that evidence to support a conversation about a combined exercise plan. It cannot promise that jogging will cancel out medication-associated bone loss.

Use running and walking for the aerobic part

This is the part we build First Gear around: a familiar route, time set aside, and clear cues for walking and jogging. Brisk walking counts as aerobic activity, and a session can include both. CDC guidance includes aerobic activity and muscle-strengthening work as separate parts of an active week. ^[10]

Aerobic training develops cardiorespiratory fitness. In the older-adult weight-loss trial, aerobic and combined training improved peak oxygen consumption more than resistance training alone. The combined program delivered both fitness and strength improvements. The aerobic sessions used walking, cycling, and stair climbing, so this was not evidence for a particular running plan. ^[6]

Running also supplies weight-bearing loading. That makes it a useful companion to strength work when impact is appropriate, but it does not replace training the major muscle groups. ^[9][10]

Choose the version that fits today. Our running and walking guide explains the tradeoffs. If run/walk sessions fit your health advice, First Gear gives you a voice to guide the changes while you set the pace.

Choose the first session, then make room for the rest

General adult guidance calls for at least 150 minutes of moderate aerobic activity or 75 minutes of vigorous activity per week, or an equivalent mix, plus muscle-strengthening work on at least two days. Those are weekly goals to work toward, not a first-session requirement. Some activity is better than none. ^[10]

We favor a start you can picture: the path, the time, and the next session in your week. You can begin below those weekly totals. Keep the strength and nutrition parts of your plan visible, too.

• Choose your first walk. Pick a familiar sidewalk or path and leave room to turn back. Add short jogs only when they fit your current ability and health advice.
• Make space for strength. Include the legs, hips, back, abdomen, chest, shoulders, and arms. Ask for help choosing exercises and resistance you can manage. ^[10]
• Plan nutrition alongside movement. Discuss adequate energy, protein, calcium, vitamin D, and fluids with your care team, especially if appetite suppression makes eating difficult. ^[1]
• Bring persistent nausea, vomiting, dizziness, or unusual weakness to your prescriber. Do not force a workout when you cannot eat or hydrate adequately. ^[1]

Let us coach the run/walk part

When running and walking fit the plan you have made with your care team, put First Gear on your Apple Watch. Set aside twenty minutes and start with a walk. In standard range coaching, our coach follows your heart-rate readings to cue a jog or a walking break. You decide how to respond, and you can keep walking or stop.

Pick Lana or Cal as your coach, choose your session days, and give the next walk a place in your week. You can try the first session free, with no subscription. We take care of the run/walk cues so you have a clear next step during the session.

Use First Gear alongside the strength and nutrition plan you build with your care team. We do not assess muscle loss, bone density, medication effects, or readiness for impact. The studies cited here do not establish a health outcome from using First Gear.

See your first run/walk session