I've spent 25 years in operating rooms, watching bodies change. Bypasses, sleeves, revisions — thousands of patients, thousands of before-and-afters. And if there's one thing that experience taught me, it's this: the number on the scale is one of the least useful pieces of information I have about a patient's health.

I think about that a lot now, watching the GLP-1 era unfold.

The number everyone quotes

Semaglutide and tirzepatide are remarkable drugs. The weight loss numbers from the major trials are real, and for a lot of patients, life-changing. But there's a statistic that doesn't make it into the marketing, and it should: in patients who don't have structured support around it, roughly 25–40% of the weight lost on a GLP-1 is lean mass — muscle, not fat.

That number comes from body composition data in the STEP and SURMOUNT trial substudies, and it tracks with what I've seen for decades in surgical weight loss, too. Rapid weight loss, on its own, does not discriminate much between fat and muscle. Your body doesn't know the difference between a bypass and a injectable — it just knows it's in a caloric deficit, and it will burn whatever's easiest to burn unless you give it a reason not to.

Why this actually matters

Here's the part that gets lost: a patient who loses 30 pounds and loses 10 of them as muscle has not achieved the same outcome as a patient who loses 30 pounds and keeps their muscle intact. Even though the scale says the same thing.

Muscle is metabolically active tissue. It's what keeps your resting metabolic rate from collapsing. It's what protects you from the "yo-yo" pattern I've watched play out in patients for two and a half decades — lose weight, regain weight, and each cycle leaves you with a little less muscle and a little more fat than before, even if your weight ends up right back where it started.

I've operated on patients in that cycle. I've seen what it does to their metabolic health over time, independent of what the scale says. It's one of the most under-discussed risks in weight management, surgical or pharmacological.

What I tell patients now

This isn't a reason to avoid GLP-1 therapy. It's a reason to stop treating it as the whole plan.

The patients who do best — the ones I'd point to as real success stories, not just scale success stories — are the ones who pair the medication with something to protect muscle while the weight comes off: adequate protein intake, resistance training, and in some cases, additional support for lean mass preservation during the deficit.

None of that is complicated. But it has to be intentional, because your body will not do it by default.

The question I'd ask instead of "how much weight will I lose"

If you're considering a GLP-1, or you're already on one, the more useful question isn't "how much weight can I lose." It's "how much of what I lose will actually be fat."

That's a harder question to answer from a prescription pad alone. It's why weight loss, done well, has always been less about the drug or the surgery and more about everything built around it. Twenty-five years in, that's the one thing I haven't seen change.

Have questions about muscle preservation during GLP-1 therapy? [Reach out / subscribe / etc. — placeholder for your CTA.]