The evolution of large human brain size has had important implications for the biology of our species. Humans achieve their large brain sizes through a growth pattern that is distinct from that of other primates. In humans, very rapid brain growth that is typical of the fetal period is extended through the first year of postnatal life. The rapid growth in brain size contributes to very high daily energy requirements during infancy and early childhood. Recent studies of human brain growth and energy needs show that over half of a newborn's resting energy budget is allocated to the brain. Energy demands of the brain peak at about age five years (65% of basal energy expenditure), and decline to 20–25 percent of basal metabolism in adulthood. To accommodate these high metabolic demands, human infants are born with high levels of body fat and continue to gain fat during the first year of life. In addition, breast milk is a rich source of energy and fatty acids that are essential for brain development. Regulation of body growth during childhood also helps to compensate for the high energy demands of the brain.