40 days? I think it'll be a 30 day fast.
This is not me questioning your resolve. This is me thinking you're gonna die by day 30 and I am not being flip about this. It is an unbelievably stupid way to lose weight.
Sorry if you don't wanna hear that. And don't tell me you can't lose any other way -- I am living proof that you can (and don't say I don't know about major weight loss. I've lost 145 pounds).
Tell your doctor, who will tell you the same thing I am, that this is insane, and ask your doctor what you should be doing.
I am sorry this is harsh and I realize you are desperate, but you are driving a car without brakes on a crowded highway. This is a way to do serious damage to yourself. It is incredibly foolish.