Re: Breaking 8.5-day fast tomorrow - Refeeding thoughts.
I think that each fast improves your ability to digest and convert food in to muscle mass and subcutaneous fat.
There are several reasons many people do not gain healthy weight, in your case we know it is cancer. Cancerous tumors tend to burn a lot of calories. I hope this has improved as well with another fast.
Other reasons are over active thyroid, diabetes type 1, Celiac, eating disorders and some infections.
Gaining weight is directly related to intake of calories and expending calories. Get yourself an app called "MyFintnessPal" to help you count how many calories per day you consume. IMO you need to consume 5000 calories a day to gain weight. Eat energy-dense foods like nuts, dry fruits, good oils, Ghee, Avocados, etc.
Do some weight lifting everyday and increase weights slowly. Also take Creatine supplement in order to gain muscle mass.
What is your height and current weight?
Archus