Keeping muscle on a GLP-1

Both weight-management labels carry the same sentence about body composition. Tirzepatide lowers body weight with greater fat mass loss than lean mass loss (Zepbound Prescribing Information, revised January 2026), and semaglutide lowers body weight with greater fat mass loss than lean mass loss (Wegovy Prescribing Information, revised February 2026).

Both labels also tie the medication to the rest of the plan. Zepbound is indicated in combination with a reduced-calorie diet and increased physical activity (Zepbound Prescribing Information, January 2026), and the Wegovy label says to administer it in combination with a reduced-calorie diet and increased physical activity (Wegovy Prescribing Information, February 2026). Neither label prints a strength program or a protein number, so those come from the other sources on this page.

Tirzepatide, SURMOUNT-1. In the body-composition analysis of 160 participants scanned by DXA at baseline and week 72, fat mass fell 33.9% on pooled tirzepatide doses against 8.2% on placebo, and lean mass fell 10.9% against 2.6% (Look et al., Diabetes, Obesity and Metabolism, February 2025). Of the body weight lost, approximately 75% was fat mass and 25% was lean mass for both tirzepatide and placebo, and the exact split on tirzepatide was 74% fat and 26% lean (Look et al., 2025). Participants received lifestyle counselling toward a 500 kilocalorie a day deficit and were advised to reach at least 150 minutes a week of physical activity without a specific strength training protocol (Look et al., 2025).

Semaglutide, STEP 1. Two papers print the STEP 1 body-composition result with different figures, so both are here with their owners. The joint advisory reads it as 13.6 kg lost on average, 8.3 kg (62%) fat mass and 5.3 kg (38%) lean body mass, where lean body mass includes muscle and other non-fat tissues (ACLM, ASN, OMA and TOS joint advisory, 2025). The SURMOUNT-1 paper reads it as 10.4 kg fat mass and 6.9 kg lean mass, a split of 60% fat and 40% lean (Look et al., 2025).

Lean mass and muscle. Lean mass is a wider category than muscle. The advisory says muscle mass is about half of lean body mass, which puts muscle at about 20% of the total weight reduction in STEP 1 (joint advisory, 2025). It also cites modeling in which muscle loss runs 10 to 15% of total weight reduction in women and 20 to 25% in men in the absence of structured strength training (joint advisory, 2025). A 2024 review in Diabetes Care puts the lean-mass loss on these medications at about 10%, or about 6 kg, which it calls comparable to a decade or more of aging (Locatelli et al., Diabetes Care, October 2024).

The owners agree on strength training and print different minimums.

• CDC and the federal guidelines: adults need at least 2 days of muscle-strengthening activity each week, working all major muscle groups (legs, hips, back, abdomen, chest, shoulders, and arms), plus 150 minutes of moderate-intensity activity a week (CDC, last reviewed December 20, 2023). NIDDK repeats the same federal figures for people taking weight management medication (NIDDK, last reviewed June 2024).
• The joint advisory: GLP-1s should be prescribed together with a structured exercise program, aiming for regular strength training at least 3 times weekly plus at least 150 minutes of moderate-intensity aerobic exercise weekly, to preserve muscle and bone mass (joint advisory, 2025).
• The ADA: to preserve lean mass, health care professionals should emphasize optimizing protein intake alongside resistance training (ADA Standards of Care, Section 8, 2026).
• The 2024 review: its authors propose tailored resistance exercise training as an adjunct to incretin therapy, and note that supervised resistance training lasting more than 10 weeks can bring large increases in lean mass (about 3 kg) and strength (about 25%) in men and women (Locatelli et al., 2024).

Two days a week is the federal figure and three is the advisory's. If you pick one, keep the owner's name on it.

ACSM's 2026 position stand is the newest summary of how to lift, drawn from 137 systematic reviews covering more than 30,000 healthy adults (ACSM Position Stand, Medicine & Science in Sports & Exercise, April 2026).

• Strength: heavier loads (80% or more of a one-repetition maximum), a complete range of motion, 2 to 3 sets, early in the session, and at least 2 sessions a week (ACSM, 2026).
• Muscle size: higher volumes, at least 10 sets a week (ACSM, 2026).
• What mattered less: training to momentary muscle fatigue, equipment type, exercise complexity, set structure, time under tension, and periodization did not consistently change the outcomes (ACSM, 2026).
• The starting point: ACSM's announcement puts it plainly, saying the most meaningful gains come from moving from no resistance training to any form of resistance training (ACSM, March 17, 2026).

ACSM's earlier weight-loss position stand sets expectations for the scale. Resistance training does not enhance weight loss but may increase fat-free mass and increase loss of fat mass, and it is associated with reductions in health risk (ACSM Position Stand, 2009).

The full protein answer has its own page, linked below. These are the numbers the sources on this page print, each with its owner.

• The recommended daily allowance for the general adult population is 0.8 g per kg of body weight per day, and that value is under review by the National Academies (joint advisory, 2025).
• Higher targets of 1.2 to 1.6 g/kg per day have been proposed during active weight reduction, and 1.2 to 1.7 g/kg per day during strength training (joint advisory, 2025).
• An absolute target of 80 to 120 g a day may be easier to follow while still being adequate (joint advisory, 2025).
• Protein-rich foods can be eaten first in a meal to make sufficient intake more likely when appetite is low (joint advisory, 2025).
• Counsel and regularly monitor people pursuing weight loss to prevent protein insufficiency, and encourage protein supplementation as needed (ADA Standards of Care, 2026).

The advisory adds that it is unclear whether these targets should be based on actual body weight, adjusted body weight, or fat-free mass (joint advisory, 2025). That choice belongs to your clinician or dietitian.

The advisory lists what to check at the start of treatment. It includes muscle strength and function, such as sit-to-stand, stair climb, and timed-up-and-go, with consultation with an exercise physiologist or strength trainer, and it says to consider measuring muscle mass by bioelectrical impedance, air displacement plethysmography, or DXA (joint advisory, 2025). It also notes that these tools measure muscle mass and not muscle health, quality, or function (joint advisory, 2025).

The SURMOUNT-1 split between fat and lean came from DXA scans at baseline and week 72 (Look et al., 2025). Whether you get a scan, a strength test, or neither is a question for your prescriber.

I never write your training program and never set your protein target. Your prescriber, a dietitian, or a trainer does that, and the numbers above belong to their owners. What I do is make sure we get the strength days in every week: we put the days you pick on the calendar, you text me when a session is done or send a screenshot from your watch, and I keep the record. Text me what you eat and I keep the protein count against the target your care team gave you. Before the follow-up, I read back how many sessions happened and how the protein ran, so you walk in with the record.

No clinician reviewed this page. It restates the FDA labels, the trial analyses, and the guidelines linked below, and those are the documents to trust over anything here.