You don't have to look to far to understand the effects long-term fasting has on the body. It has been studied endlessly in hunger-strikers.

http://www.livescience.com/28984-hunger-strike.html

"After the third day of a hunger strike, the body starts to use muscle protein to make glucose, a sugar that's needed for cell metabolism. Levels of important electrolytes, such as potassium, fall to dangerous levels. The body also loses fat and muscle mass."

So, you're probably fine up until 3-days in. Remember how we talk about the various problems with LC flu and electrolytes? Well, it is also going to be an issue with fasting. Your body will have to tear down bone to get potassium and probably muscle to get other electrolytes.

"Low levels of thiamine (vitamin B1) become a real risk after two or three weeks and can result in severe neurological problems, including cognitive impairment, vision loss and lack of motor skills."