It's both personal experience and what I've read. I've been doing low carb for years and about 6 years ago I got into long fasting for reasons other than weight loss. I did a 15 day fast and it was great. I documented all this if you want to look back in my posting history. Since then I've read many books on fasting, diet and health.
These books are the ones I can recommend for a deep dive:
This is the book I started with and is great for longer fasts:
Good You-tubers that I mostly trust on Keto/Fasting:
I do the Alternate Day Fasting method - http://www.amazon.com/The-Alternate-Day-Diet-Skinny-HealthierLife/dp/0399534903
500 calories on Mon, Wed, Fri and 1800 on tues, thurs, sat and Sun