Water weight is a big one, and is part of your FFM. I lost 10lb of water weight in my first 24 hours on tirzepatide.

Some of it is likely bone density as well. You can prevent the bone density and muscle loss with proper diet and exercise, though.

Googling it, 70% of FFM is water.

Yeah, I've swung 10lbs in 24 hours just going from well fed to fasted without water. And it certainly wasn't fat I lost, just water and I'm surely any mass in my... various tracts.