Weight-Loss Drugs Also Cost Muscle and Bone, and Diet and Exercise May Help

Incretin-based medications have transformed obesity care. Obesity affects about 650 million people worldwide, and drugs such as liraglutide, semaglutide and tirzepatide now deliver weight loss that rivals bariatric surgery. That raises an important question for anyone interested in healthspan: how much of the weight lost is metabolically protective fat, and how much is muscle and bone that people need for strength, mobility and independence as they age?

This narrative review by Italian endocrinology and exercise-medicine groups searched PubMed from February to October 2025, limited to English-language papers from the past ten years. Two authors independently assessed eligibility. Eligible work included systematic reviews, meta-analyses, randomized trials, observational studies and guidelines in adults with obesity, and animal studies and pediatric populations were excluded. The authors then organized the evidence into trial efficacy, body composition, bariatric surgery parallels, and nutritional, exercise and adjunctive strategies.

The efficacy data are striking. Liraglutide 3.0 mg produced 8.4 kg of loss at 56 weeks versus 2.8 kg with placebo. Semaglutide 2.4 mg yielded 14.9-17.4% loss in the STEP program and beat liraglutide in STEP-8 (-15.8% vs -6.4%). Tirzepatide reached -20.9% at 72 weeks in SURMOUNT-1 versus -3.1% with placebo, and SURMOUNT-5 favored it over semaglutide (20.2% vs 13.7%). Benefits extend to cardiovascular events, heart failure with preserved ejection fraction, knee osteoarthritis, sleep apnea and MASH.

Body composition is the concern. A meta-analysis of randomized trials found about 25% of total weight loss was lean mass. In a STEP-1 DXA subset, 40% of loss (6.92 of 17.32 kg) was lean mass, or 13.2% of baseline lean mass. In a SURMOUNT-1 DXA sub-study of 160 participants, tirzepatide cut fat mass 33.9% and lean mass 10.9%, versus 8.2% and 2.6% with placebo. By contrast, a 94-patient real-world semaglutide cohort lost 1.43 kg of lean mass (31% of total loss). In SEMALEAN, lean mass declined and then stabilized while handgrip strength improved.

Bone is often overlooked. Liraglutide raised the bone formation marker P1NP by 16% versus a 2% fall with placebo, while semaglutide 1.0 mg raised the resorption marker CTX and was linked to lower BMD at the lumbar spine, hip and tibia. Because age-related muscle loss averages about 0.37-0.47% per year, the authors note that drug-induced losses could equal roughly a decade of aging. Based on the abstract, they recommend protein- and fiber-rich hypocaloric diets, amino acid, vitamin and mineral supplementation, and supervised aerobic, resistance and strength training. They also point to emerging muscle-sparing drugs and microbiota-targeted approaches.

Several caveats apply. This is a narrative review without formal quality grading, and it relies on a single database. Many body-composition data come from small DXA sub-studies, and DXA cannot separate muscle from other lean tissue. Muscle function data are sparse, and trial results on lean mass are inconsistent. The supplied full text ends partway through the bariatric surgery section, so the detailed diet, supplement and exercise evidence is summarized here from the abstract alone.