The GLP-1 Side Effect Doctors Don't Talk About Enough

8/5/2026

By Kim Porterfield, CN, CFS | Institute of Eating Management | Estimated read time: 8 minutes

A client of mine went on a GLP-1 medication and lost hard-earned lean muscle and gained unwanted fat in a month. She wasn't doing anything wrong. She was following her prescription exactly as directed. And she's not alone.

I've spent over twenty years measuring body composition, not just weight, for clients. What I've seen with people on GLP-1s has made me genuinely cautious about these medications. Not because they don't work, but because of what they're taking along with the fat.

This isn't a post telling you to stop your medication. It's what I wish every person heard before they started, and what I think everyone currently on one needs to hear now.

What the Scale Doesn't Show

A client was recommended retatrutide, one of the newest medications in this category, to support a health concern unrelated to her weight. In fact, she wasn't trying to lose more weight because she only weighs in the low 120s.

I saw her a month after she had started the medication. When I measured her, I was perplexed because she had lost 5.87 pounds of lean body mass and she had gained 5.67 pounds of fat since seeing her 4 weeks before. This shocked me because we had been working on building muscle for months. I went through a list of things that often cause muscle loss, one of which is the use of GLP-1 drugs. I almost did not mention it because I never in a million years would have suspected that was the cause for her. But it was.

Less muscle, more fat, in a month. Nothing about her weight on a bathroom scale would have told her that. She was shocked too.

I've seen this pattern in more than one client.

Another client, who was already a healthy weight, took semaglutide for four weeks after seeing friends have success with it. She came to be measured, and we found that she had lost 13.21 pounds of lean body mass and gained 8.11 pounds of fat in that single month. She was shocked as well. The only surprise for me was that she ever took it in the first place or that a doctor would even prescribe it to her. After all, she was not overweight and was not diabetic.

Neither of these women "failed" at anything. They followed their prescriptions. The problem is what these medications can do to your body composition when food intake drops too low to support your muscle.

The Dose Makes the Difference

The starting dose and the escalated dose turned out to be almost two different experiences, and the difference showed up clearly in the numbers.

I tracked one client's body composition over an entire year while she was on Wegovy, and the pattern that emerged was shocking.

In her first three months, on the starting dose:
She lost roughly 11 pounds of fat.
Her lean muscle mass actually went up slightly, about 2 pounds.

She was working with a trainer, eating enough to support her training, and her body was responding exactly the way you'd want: fat coming down, muscle holding or building. This gave me hope for these medications.

Then her dose was increased.

Over the following nine months, at the higher dose:
She lost less than 6 pounds of fat.
She also lost roughly 43 pounds of lean muscle mass.

She had the same trainer and put in the same effort in the gym. The only thing that changed was the dose. Once it went up, her appetite dropped to the point where she couldn't eat enough to protect the muscle she'd built.

Let me clarify my role here: I wasn't managing her nutrition. She came to me specifically to track her body composition while her prescribing doctor managed her medication. When I flagged what was happening, shockingly her doctor kept her on the higher dose anyway.

This is the pattern I now watch for in every client on one of these medications: the starting dose is often where people do best. But the moment the dose goes up and they physically can no longer eat a reasonable amount of food, muscle loss accelerates, often much faster and greater than fat loss.

I currently have several clients that are thriving on a starting dose (tirzepatide specifically). They are losing the body fat that they need to lose while holding muscle, simply because they're using it to quiet food noise while they still eat enough to support their body. I've also talked some clients into going back down to a lower dose after a dose increase caused this same pattern and gotten them losing fat again instead of muscle. But interestingly, those clients that did not need to lose weight (like the two I mentioned previously) did not fare as well even on the starter dose aka a micro dose. They discontinued the medication and resumed losing body fat and/or gaining muscle with a personalized and balanced plan.

Why "Eat More Protein" Isn't Enough

Almost every GLP-1 provider tells patients to eat more protein and lift weights. Both are good advice. Neither is the whole picture.

Muscle protein synthesis, the process your body uses to build and preserve muscle, needs both protein and carbohydrate working together. Carbohydrate spares protein from being burned for energy and supports the insulin response your muscles need to use that protein. When someone's appetite is suppressed to the point where they can barely eat protein, let alone protein and carbohydrate in adequate amounts, the "eat more protein" advice becomes almost impossible to follow, regardless of how hard they're trying or how much they're lifting.

What Most People Aren't Told Going In

Research on GLP-1 discontinuation is becoming harder to ignore. In the STEP 1 extension trial, participants who stopped semaglutide after a year of treatment regained about two-thirds of their lost weight within the following year, along with a reversal of most of the cardiometabolic improvements they'd gained. A more recent systematic review and meta-regression published in eClinicalMedicine found a similar pattern: roughly 60% of lost weight returns within a year of stopping.

What this means in practice: many people aren't being told they're likely signing up for indefinite use, not a short-term intervention. I met with a new client today that said she had considered a GLP-1 but did the math. She said she could not stomach paying "$600 a month (or more) to not be able eat" for the rest of her life. Imagine paying almost $7200 a year for 20-30+ years. Many could not keep up with that expense and would be forced to stop for that reason alone.

And for those who do stop, blood work and weight shift back toward where they started. And many will be left less healthy than before they started due to malnutrition. This is a reason to have a plan for both scenarios, staying on it long-term or eventually coming off it, before you're in the middle of either one.

Unfortunately there is more. I cannot in good conscience post this blog without also mentioning something many have no idea about: as of August 2026, more than 4,100 individual federal lawsuits have been filed against GLP-1 drug makers like Novo Nordisk and Eli Lilly (Drugwatch, August 2026). Breakdown of these federal lawsuits at the time of posting:

Gastroparesis and GI Injury: 3,928 pending cases involving severe stomach paralysis, bowel obstructions, and gastrointestinal complications.
Vision Loss / NAION: Roughly 200 pending cases involving claims of permanent optic nerve damage.

If you are going to take these drugs to lose weight, you must be well-Informed and consider the risks carefully. I doubt these plaintiffs thought anything bad would happen to them, but their lives have not been changed for the better.

FAQs

Does this happen to everyone on a GLP-1?
The clients I've seen do well are typically on a lower, stable dose, still eating enough to support their body, and having their body composition measured, not just their weight. The pattern of significant muscle loss tends to show up most with dose increases and appetite suppression severe enough to limit real food intake.

Can't I just use one of those body fat scales at home or at the gym?
I wouldn't rely on it. Those scales use bioelectrical impedance, sending a small electrical current through your body and estimating your composition from how that current moves. The problem is that reading shifts with things that have nothing to do with your actual muscle or fat: how hydrated you are, how much carbohydrate you've eaten recently, even which parts of your body are touching the scale. Studies have found these devices can be off by a significant margin.

I had a client I was measuring weekly with skinfold calipers, the same method I use with all my clients. Her caliper readings were dropping, and she was training and eating more consistently than ever. Her home bioelectrical scale told her a different story entirely, one that didn't match what was physically happening in front of us. That's the gap between a scale estimating your composition and someone measuring it.

Can you rebuild muscle after stopping a GLP-1?
Often, yes, with the right nutrition and training approach. It takes time and consistency, and the sooner you start rebuilding, the better, but it's not a lost cause.

Should I stop taking my GLP-1?
That's a conversation between you and your prescribing doctor. I'm not here to tell you to stop a medication. What I can help with is protecting your muscle and metabolism while you're on it or rebuilding after you're off it.

How do I know if I'm losing muscle instead of fat?
The scale can't tell you. Body composition tracking, the kind I do with clients, is the only way to see what's changing underneath the number.

What if I can barely eat right now because of my dose?
That's the hardest window to work in, but it's still worth reaching out. We may talk about working with your doctor on the dose itself, alongside whatever nutrition structure your intake can currently support.

What To Do Next

If you're currently on a GLP-1, whether you're doing well on it or starting to notice you can't eat like you used to, this is exactly the kind of situation my one-on-one counseling is built for. I track body composition, not just weight, so we can see what's actually happening to your muscle and fat and build a plan around it before a dose change makes it harder.

And if you've already come off a GLP-1, or you're planning to eventually, it's absolutely possible with the right structure behind it.

Ready to get started?
Read about our private counseling by clicking the link below. You can fill out the contact form linked on that page and I'll follow up personally with an email explaining pricing, scheduling and how to get started ASAP.

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Client results shared throughout this post are individual and vary based on medication type, dose, training, and nutrition. They're shared anonymously and only as a pattern I see clinically, not a guarantee of what anyone else will experience. A quick note on terms: "GLP-1" has become the catch-all name for this entire category, even though the newer medications work through additional pathways beyond GLP-1 alone. What follows applies to all of them. The mechanism is becoming more aggressive with each new generation, but the pattern I'm about to walk you through doesn't change.

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