Fasting is very healthy. I question the validity of five pounds a week. You may lose that much the first week due to excess water weight and stored intestinal waste. But to consistently lose that much weight per week over a longer duration is neither healthy nor practical.
It takes roughly a deficit of 3,500 calories to lose one pound a week. If you do the math, the only way to lose five pounds a week (in fat) would be to eat nothing at all. And that's based on a 2,500 calorie daily intake. Obviously, that's not healthy since your body would then begin consuming lean muscle tissue.
I've found that 1.5 to 2 pounds a week is very doable, yet it takes a good bit of discipline.
JB