A) Concentrations of ketone bodies (acetone, β-hydroxybutyric acid, acetoacetic acid) and plasma free fatty acids (FFA) during 40 days of fasting in humans. Note the more than three orders of magnitude change in β-hydroxybutyrate and the doubling of FFA; B) Brain substrate utilization in three fasting obese volunteers after several weeks of food deprivation. Many studies suggest that human brain cells can survive with little to no glucose, but this has not been clearly demonstrated (Redrawn from: ()). C) Emperor penguins can fast for periods lasting for over 5 months. The picture shows Emperor penguins and their chicks a few weeks before fledging (courtesy of Yvone le Maho). The parents go back and forth between the open sea and their colony on sea ice, next to a glacier, which offers protection against wind, to regurgitate food conserved in their stomach to feed their chicks while they are themselves fasting. Fasting penguins undergo 3 phases (; ; ). The first phase (phase I) represents a transition between the fed state and starvation, during which the penguin stops utilizing diet-derived energy. This phase, which lasts between several hours and several days, is characterized by a rapid decrease in protein loss. The following phase (phase II), is a ketotic phase associated with protein sparing which can last for several days in rats to several months in obese geese, king penguin chicks, bears, and seals (; ; ; ; ; ; ; ; ; ; ; ). Phase III is brief, since the high protein loss leads to death. During phase III glucose and total plasma protein levels are reduced, and uric acid increases while ketone bodies values remain low. Wild animals that fast for long periods are efficient at sparing proteins during long periods of fasting, with only 2–10% of total energy coming from proteins versus the 20–40% in species less adapted to fasting.