e newborn’s lungs fill with air during the first breath, and gas exchange across the pulmonary circulation abruptly replaces placental transfer. Similarly, birth triggers a major transformation in nutritional pathways and metabolism of the newborn. At a time when nutritional needs are high (per unit body weight higher than at any other time after birth) the newborn must abruptly begin to swallow, digest, and absorb a variety of complex proteins, lipids, and carbohydrates. No longer are readily available simple substrates provided by the mother via the placenta. The newborn begins to convert substrates-protein to glucose, glucose to fat, and must adapt to a discontinuous supply of nutrients. Glycogen stored during feedings must be converted to glucose to support energy needs between feedings. Adding further to the vulnerability of the newborn period, many of the digestive and absorptive functions of the newborn are incompletely developed (Schmitz, 1991).To ease the transition through this turbulent period, the newborn needs to be provided with easily digestible and bioavailable nutrition in the form of breast milk or infant formula. The infant depends on a diet of closely regulated composition and does not achieve nutritional independence until weaning. Here I will focus on the physical growth during infancy. Physical growth is more rapid during the first few months postpartum than at any other time after birth. In the first three months, a healthy infant’s birth length increases by 20% and, by the end of the first year, by 50%. By the end of infancy (2 year old) the average child has already achieved half the ultimate adult height. Although the rate of growth is remarkably high during infancy, it is actually decelerating from the extremely high rates achieved during growth in utero. ( ) A healthy infant gains about 25 cm during the first year, but growth rate slows during the second year. From age 1 to 2 years...
