Hello Roman;
The idea that fasting slows down metabolism at all is not scientifically proven. Nor is it proven that it speeds up metabolism. What fasting does is it takes away all of the fuel your body has until it begins burning body fat. Your body still needs energy to run, so it won’t just conserve energy and not move food or glucose stores.
However, if you stay at a caloric deficit all of the time, your body is too smart for that. It will realize you are starving it, causing your body to actually hold on to body fat so it doesn’t run out of energy. That is where you need 1 or 2 refeed days to help trick your body (or your metabolism if you want to call it that) into not knowing what you’re doing.
If you take Saturday and eat at maintenance while still fasting, or if you just don’t fast for one of the days, then your body will get confused; your body will say, “You were in a routine of eating less and not eating at certain times, why have you changed things all of a sudden?”
So don’t be too worried about metabolism. If you eat right, move a lot, and stay in a caloric deficit, your body will change. IF is just a tool to help with the third part of that equation.
Hope this helps!
