Well it depends on your current weight, but according to this site you shouldn’t go below 1200 calories a day for women or 1800 for men. As a male in the 185-195 weight range I can eat ~1500 calories a day and workout (which uses ~400-500) for many days in a row without any bad effects on my health. Of course I throw a buffet in there every once in a while too though! Make sure you’re getting enough carbohydrates, your brain needs them to function and could be causing your lethargic and stupid feelings.
Actually current studies have back fasting, and so far it is the only thing that has increased the lifespans of mice. Starvation seems to provide some benefit to the longevity of these mice.
Controlled fasting certainly didn’t hurt Ghandi(sp?). Of course it didn’t make him the most attractive person physically.
As the previous post indicated you should take about 1gram of protein per kilo of body weight. So take your weight in pounds say 200 pounds, divide it by 2.2 and you would get about 90.9. grams per day.
Fortunately there are many supplements from Ensure and the like that provide excellent nutrition that are easy to use.