I’ve done fasting before. Even with water. Now I don’t feel thirst unless I exercise and I can suppress my hunger for a long period of time too. But Dave, you do lose muscle from being in a deficit. Your metabolism DOES slow down from being in a deficit if you don’t jumpstart your body by exercising like a mofo. That’s not broscience at all, I can tell you first hand (I lost 16 lbs in 4 days once when I was 15 and probably around 7% or 8% bf, never doing that again though).
And here’s why you lose muscle: your body will use up all the food you’ve last eaten, right? Then it will start to use up all the glycogen stores in the muscles themselves. Then it will start burning fat. We all know this. But what people don’t know is that muscle requires maintenance, once it is built it does not just stay there indefinitely. It needs to be maintained. You working out in a deficit now means that your body doesn’t have all the necessary amino acids for muscle growth and maintenance. ‘Okay then you just supplement with a LITTLE bit of protein Double, just make sure that you’re still in a deficit while doing it’. But what stupid people don’t realize is that their heart muscle is much more important than their skeletal muscle. Your body already knows this though. So in order to keep your heart functioning, your body will use the protein, break it down, and convert it into fuel for your body to use as energy. Almost none of it will go to your muscles.