"Both lipids (fat) and amino acids (protein)have been shown to serve as significant energy sources during long runs. Estimates of protein use during exercise suggest that as much as 9% of the total energy expended during a marathon race may be derived from amino acids. Some of the amino acids like alanine may be used by the liver to form glucose- (muscle) does not have the capacity to burn amino acids directly." -Inside Running: Basics of Sports Physiology, David Costill, PhD
MY COMMENTS: I am digging through Costill's old book for some golden running "tidbits"