Is fasting helpful? How many people have noticed changes with this?

In context, I have noticed everything I consume including beef, chicken, rice, bread, and supplements like d3, creatine, ashwagandha cause immediate changes in my body. They can either increase my heart rate, libido, I get boners, or they can make me crash. If I am so sensitive to everything I consume, is it best to water fast for a week or more? I have been dealing with PFS for 4 months now and have all physical and cognitive symptoms especially insomnia which is driving me nuts. Any advice is appreciated.