The term Lucy comes from the song by the Beatles Lucy in the Sky with Diamonds which is commonly thought to be about LSD. Lucy is often used as a code word for LSD when referring to the drug in a public setting or over the phone where one does not want to openly discuss illegal activities. Lucy is used like a person's name in such discussions. Like saying Lucy took me on a trip last night to indicate you tripped on LSD the previous night. I tripped balls with Lucy last weekend!
The capacity of your body to store muscle and liver glycogen, however, is limited to approximately 1,800 to 2,000 calories worth of energy, or enough fuel for 90 to 120 minutes of continuous, vigorous activity.
Glycogen is a stored form of glucose. When glycogen has also run out the body breaks down fat to produce another chemical called ketones, and ketones are used as a source of energy. When the body is making and using ketones we say that the body is in a state of ketosis. Ketosis begins after about 48 hours (2 days) or water fast for women, or about 72 hours (3 days) of water fast for men.