Those things are fun, I did a lot of abstinence experiments myself when I was young. I learned that I don't want any constraints for food ever.

Eating only powdered protein low quality and fake stuff will mess you up and your kidneys. So I once choose coma patient food. It comes in many varieties for all ages and is cheap in bulk. It is complete, tastes medium awful and you only feel horrible for the first few days. Then just drained, zombified and mildly happy until you can't take this stuff anymore. High fibre content is important.