I have not been doing full on fasting, but more of a light fast for the past month or so. My diet will consist of juices (spinach, kale, beets, celery, cucumber, etc.), I typically have 1-2 juices in the morning then fast throughout the remainder of the day until the evening. If I have anything to eat during the day then it's just a piece of chocolate, it's a known appetite suppressor. Then, in the evening, I'll have a small meal, such as a salad. You will be very hungry, but your body eventually adjust after you've fasted in the fashion that I'm speaking of for awhile. Now, a full on fast, no eating, that's rough and I haven't done one of those in awhile. I only went a for a few days before stopping.
So, I can't tell you what to expect with a 40 day fast. I can tell you my approach makes me very hungry and you can tend to become moody, to say the least.
Oh yeah, and weight lost of course, you're body will trim down and your face will thin out. Stay hydrated.
What inspired you to fast 40 days?