Where did you pluck that number from? The longest recorded medically supervised Water Fast is actually 382 days!! With the correct medical supervision and supplements a person can fast for the length o time it takes to deplete all their fat stores! A person who is clinically obese can Water Fast for a lot longer than 40 days with the correct supervision. It depends entirely on your fat stores and how much you have conserved fats for the body to burn for energy!