I've done a few different fasting types and different carb splits and cycles. Alternate day fasting, one meal a day and up to 4 days with water only. Now that I know a little more I think that keto and extended fasting are not optimal for maintaining a fit athletic physique. It works great for fatties because cutting their sugar onslaught allows their high Leptin hormone levels do their job of reducing hunger while releasing fat to be burned by a high BMR with no hormonal setbacks or muscle loss. Thin people have less Leptin and it drops exponentially as you cut down to low body fat levels. Let's say you have a total of 30 lbs of fat on your body. If you lose just 10 lbs you possibly cut your Leptin levels by 33%. I say possibly because the scaling of Leptin production and fat cell size may not be linear. That may be an exponential curve too. When Leptin is low, hunger is cranked up to mind melting levels while systems are being put in standby mode. Energy levels become low, hormones drop, muscle is wasted and insulin secretion is on a hair trigger whether it be carbs or fat intake. Unfortunately for us most of the science is going towards finding the easiest way to convince fatties to reduce their over indulgence. Not much is being pursued for being competition shredded year-round while maintaining maximum strength and endurance. If anything could do this simply all in one it would be Leptin with it being the big boss over hormones in the body whether it be for energy, hunger or muscle tone. Human Leptin is controlled and expensive as fuck so it would be a poor choice to cheaper alternatives if going the PED route. There are some studies linking dietary intake to natural Leptin increases and some of them are linked here.  Edit: Long fasts once or twice a year is not bad for the autophagy though.