Twelve percent of American adults now take GLP-1 drugs for weight loss. More than ten million people. The obesity GLP-1 market is projected to reach roughly $67 billion by 2035. These are drugs that work: patients lose 15 to 25 percent of their body weight. The question nobody marketed is what they lose it from.
## The Third
Semaglutide and tirzepatide suppress appetite systemically. They do not distinguish between the calories that sustain adipose tissue and the calories that sustain muscle. In Regeneron's COURAGE trial, 33 percent of semaglutide-induced weight loss was lean mass. Across the broader literature, the range is 15 to 40 percent. A patient who loses 30 pounds on Wegovy loses roughly ten pounds of lean tissue.
The clinical trials that won FDA approval enrolled older adults in numbers too small to characterize body composition changes in patients over 65. Dosing protocols were calibrated in younger populations. In 2026, prescribing guidance caught up: start at the lowest effective dose, delay escalation if weight loss exceeds 1.5 kilograms per week, track muscle mass routinely, require 1.2 to 1.6 grams of protein per kilogram daily. The behavioral fix arrived. The pharmaceutical one is racing to join it.
## The Stack
Regeneron's Phase 2 COURAGE trial combined semaglutide with trevogrumab, an antibody that blocks myostatin, the protein that signals muscle to stop growing. At 26 weeks, lean mass loss dropped from 6.5 percent to 3.3 percent. When they added garetosmab, which blocks activin A, the triplet preserved 81 percent of the lean mass that semaglutide alone would have destroyed. But the triplet's discontinuation rate was substantially higher.
Eli Lilly paid up to $1.925 billion for Versanis Bio in 2023 to acquire bimagrumab, an antibody targeting activin type II receptors. The BELIEVE study, published in Nature Medicine with 507 participants over 48 weeks, combined bimagrumab with semaglutide and cut the lean mass fraction of total weight loss from 21 percent to 7 percent. Then Lilly killed the bimagrumab trial in patients with Type 2 diabetes.
Scholar Rock launched EMBRAZE, a Phase 2 trial of apitegromab in obese patients already on GLP-1 therapy. Its next-generation candidate SRK-439, a selective myostatin inhibitor optimized for cardiometabolic disease, remains preclinical.
## The Debate
At the American Diabetes Association's Scientific Sessions in June, Samuel Klein argued that GLP-1 weight loss is not disproportionately muscle and that no data link these drugs to frailty or sarcopenia. The SEMALEAN study tracked 106 patients and found lean mass declined by three kilograms at seven months but then stabilized. Handgrip strength improved by 4.5 kilograms at twelve months.
Eric Ravussin presented the counterpoint: the absence of evidence is not evidence of absence, particularly for older patients whose clinical endpoint is falls and fractures, not body composition. Nature Reviews Endocrinology published a review of sarcopenia causes in GLP-1 patients the same year. The prescribing guidance would not have changed if the question were settled.
## The Second Prescription
If muscle preservation becomes standard of care, every GLP-1 prescription generates a second prescription. The add-on market bootstraps off an installed base of more than ten million Americans, growing at double digits annually. Regeneron's 52-week COURAGE data is expected later this year. Scholar Rock needs to prove apitegromab works outside spinal muscular atrophy. Lilly holds the molecule, the sunk cost, and a terminated trial.
The biggest drug category in a generation may need a second drug to complete the prescription. Three companies are spending billions to answer a question the science has not yet resolved: does the muscle loss matter? The market has decided the question is worth buying before the answer arrives.