The supply and utilisation of nutrients are of greater biological relevance during early childhood than during any other period of life. The nutrient supply must cover maintenance requirements and the needs for physical activity. Children need large additional energy and substrate intakes for body growth. Healthy new-born infants double their body extremely rapidly in only 4–5 months after birth, and in preterm infants even in only about 6 weeks, which requires a very high substrate supply per kg bodyweight. The quantity and quality of nutrient supply during early life modulates the differentiation of tissues and organs and has short- and long-term consequences for health. The rapid growth of infants and children, who double their body weight within only 6 weeks in utero and within 4–5 months after birth, respectively, depends on very large nutrient supplies per kg body weight. Healthy young infants need about 3 times more energy per kg body weight than adults, primarily due to the added metabolic requirements for growth. Premature infants who grow at rates similar to normal intrauterine growth have even greater metabolic needs.