During the transition into extrauterine life, the newborn goes through a fasting period (presuckling) that starts from the cessation of the transplacental supply of nutrients to the beginning of lactation. During this presuckling period, the newborn depends on its own reserves in a unique period of stress and vulnerability. The energy expenditure needed by the neurons is so high that under these circumstances the gluconeogenesis is not capable of reestablishing the blood glucose levels. Consequently, during the presuckling period there must be a different energy source and lipogenic substrates that help to maintain the synthesis of neurotransmitters.