Somethings I want to clarify from the manga:
The drop in metabolism is only around 120 at most. And that "starvation mode" only hits if you're literally starving to death. I mean concentration camp style. Lookup the Minnesota Study/Experiment, it's where the data's from. Check the pictures, cause that tells you how far you'd have to go to get starvation mode to kick in.
Intermittent Fasting, where you fast (no food) a min of 16 hours has advantages of triggering HGH secretion. Combined with cyclic keto diet, you can increase testosterone in your body (males only, dunno about females) for increased muscle gain. Ketogenic diets tend to have very, very low glycemic index foods, to avoid triggering insulin production. But cycling off it can supercharge your glucose stores in muscle, to allow for growth and high-intensity training. Combined with Intermittent Fasting, you're basically tricking your body to produce the hormones you want for body building, when you want it.
That being said, Cyclical Keto diet works better if you're 18 or around 15% bodyfat already.