The protein-sparing effect is a process that allows one's body to derive energy from fatty tissue, dietary carbohydrates and fats, or sources other than muscle, according to Reference.com. The process helps to conserve muscle tissue.
Under normal circumstances, the body metabolizes amino acids to produce energy. This process requires digestion of protein, but with the protein-sparing effect, bodybuilders hope to preserve muscle tissue. To accomplish their goal, they increase the protein in the diet. Increasing protein consumption does not increase efficiency of the body in digesting protein. The dietary change results in wasting additional protein; however, ingesting a greater amount of protein means the body retains more protein due to the increase in volume. The bodybuilder hopes to use this process to remain ahead of the body's metabolizing protein for amino acids needed for energy, according to Reference.com.
According to Wikipedia, some proponents of water fasting claim the body adapts to a lack of incoming protein during the fast and begins drawing upon stored fat for energy. If one is involved in an extended fast, however, the metabolism slows, requiring the individual to minimize any physical or mental activity for his own safety. Reference.com reports proponents of water fasting claim their body enters this mode about two days after the last meal.