I've been listening to a number of podcasts by Dr. Ted Naiman and recall him saying that after a year's time, Peter Attia had lost lean tissue from fasting and TRE, which is one reason he's changed his mind on doing it. In Attia's podcast interview he mentioned Dexa scans as the only way to determine where weight loss has come from, and presumably, that's how he knew.

Ted Naiman also said that calorie restriction for longevity was garbage, as based on mice-based studies only.

But then he also said he's been wrong about a lot of stuff before, for decades, and wonders which half he's wrong about now. Good question! We can only figure out what works for each of us, I guess.