The side effect of drastic muscle loss isn't quickly reversible though.

Because muscle is about 3x as expensive to maintain as fat, unless you are on some major strength training to maintain, muscle is the very first thing that your body gets rid of when losing weight.

So now you are off the GLP-1. But you also have to eat fewer calories to maintain a calorie deficit because you no longer have all that calorie burning muscle.

* And depending on when you start your GLP-1 journey, men when they hit 40 naturally lose between 5% and 8% of their total muscle mass ++a year++ a decade (women, probably earlier). In addition to the muscle loss due to weight loss.

* And it takes approximately 3 times longer in your 50s to create a pound of muscle that it does in your 20s.

* And you can't train like you did in your 20s or you will hurt yourself.

* And as you age, your body can't absorb the protein it needs to build muscle as efficiently as it did when you are young. So you need to eat even more food to compensate.

Ok. This is now more of a rant about getting older than it is about GLP-1s.

-- Edited to reflect that muscle loss % is decade, per the corrections in replies.

> men when they hit 40 naturally lose between 5% and 8% of their total muscle mass a year

I think your numbers might be per decade, not per year?

If I lost 5% of my muscle mass every year since I was forty I would only have 32% of my muscle mass left. At 8% I’d be down to 16% left.

Are these factual claims? Seems a little fuzzy to me.

It doesn’t have to be a foregone conclusion that GLP-1 causes muscle loss though. The primary reason for the muscle loss is the reduction in calories and protein intake. If you want to keep most of your muscle mass you can just make sure you keep up with your daily caloric and protein needs.