Autophagy kicks in around 15-17 hours in fasting. I would suggest searching autophagy and cancer. I cannot remember the names of the doctors who go over fasting. There are many ways to fast. Some are just water, or bone broth, etc. A big thing is to cut sugar/carbs/processed foods from your food intake forever. It is a life change, not a "diet". Dr Eric Westman @ Duke University has been helping people lose weight (doctors send their patients to him to lose weight so they can undergo surgery). He puts his patients on prescription strength low carb eating. No more than 20g of carbs per day.