I'm planning on fasting

Why do you believe you should fast for 40 days?  Yes, something bad very well can happen to you.  Anemia, or depleted iron in your body, can result as well as low blood sugar, and irregular heartbeat.  You will lose muscle mass, grow weak and fatigue, you will not feel well at all.  It could do damage to your heart since the heart is a muscle, too.  It is not advisable to go that long without food.  I hope you reconsider doing so.  You don't want to do irreversible damage to your body.  As far as water go, you should be drinking at least 64 ounces per day.

When you say fasting,am sure u don't mean fasting for 40days straight?you can fast every morning and that will be ok but 40days? Datz like walking on top of the sea