it's mostly a myth though there's studies that suggest long-term effects of a calorie deficit and metabolic damage. if you're in a calorie deficit, you will be in a slightly catabolic state most of the day (calories are typically anabolic). it'll have minimal/no effect on muscle loss for most people unless if you fast for several days at a time.
if you are fasting on a regular cadence, brain fog usually goes away after practice. if you're allowed to, a little bit of black coffee may help though and keep most benefits of your fast (black coffee will probably disrupt your fast) and no it isn't just for weight loss though it is an effective tool for weight loss (your opportunities to eat in a feeding window are lower and mostly it removes snacking).
in addition to weight loss, one of the primary health benefits is that your body will reap senescent cells that are not dead but not productive anymore, which I believe is associated with radical mutations that could manifest itself as cancer. there are other benefits but i forget them off-hand.
. I’m too fat to do a shirtless selfie.
