Keeping muscle while the weight comes off

Weight loss is not fat loss. Any large, fast weight loss takes some muscle with it, whether the cause is a diet, surgery, or a drug, and the GLP-1s produce weight loss that is both large and fast. The question is how much, and what you can do about it.

What the trials measured

In the STEP 1 trial, a subgroup had body composition scans. Among them, lean mass accounted for roughly 40 percent of the total weight lost on semaglutide. Because fat fell by more than lean mass in proportion, the share of the body that was lean actually rose, which is the number the manufacturers quote. Both statements are true. The practical one is the first: a 20 kg loss can include 7 or 8 kg that was not fat.

Tirzepatide's SURMOUNT-1 composition substudy found a similar pattern: around a quarter of the weight lost was lean mass. Lean mass includes water and organ tissue, not only muscle, so these numbers overstate muscle loss somewhat. They do not overstate it to zero.

Why it matters

Muscle is what moves you, keeps blood sugar in check, and sets the metabolic rate that decides how easy it is to keep the weight off. Losing it makes regain more likely when the drug stops, and makes the regained weight more likely to be fat. In older adults it is also a fall and frailty risk. Every clinician who prescribes these drugs now says the same thing: protein and lifting.

Protein

The common clinical target on a GLP-1 is 1.2 to 1.6 grams of protein per kilogram of body weight per day, higher than the general guideline, because you are in a deficit and eating less of everything. For an 85 kg person that is 100 to 135 grams. On a slowed stomach with no appetite that is hard, which is why it needs to be first on the plate at every meal and tracked rather than guessed. Dairy, eggs, fish, poultry, legumes and a protein powder if you cannot get there with food.

Resistance training

Cardio is good for you and does little for muscle. What preserves muscle in a deficit is loading it: two or three sessions a week of resistance work, bodyweight or weights, with enough effort that the last few repetitions are hard. The combination of adequate protein and resistance training is the only thing shown to meaningfully shift the lean mass share in weight-loss studies. Walking is the floor. Lifting is the lever.

Tracking whether it is working

• Protein, daily. The free food log in Peptide AI shows protein against your target. On a GLP-1 this is the number to watch, more than calories.
• Training, weekly. Log the sessions. Apple Watch workouts import. A week with no resistance sessions is visible.
• Composition, monthly. The scale cannot tell fat from muscle. The body scanner on Premium tracks composition with the phone camera, and a smart scale that reports body fat feeds Apple Health or Health Connect. Either is a rough measure. Rough and monthly beats precise and never.
• Strength. If the weights are going up or holding while the scale goes down, muscle is being kept. That is the cheapest composition test there is.