Part 2: The second part of the study will occur when the infant has reached full enteral feedings (minimum of 100 cc/kg/day) and TPN has been discontinued. This part of the study will include stable isotopes. Infants should be receiving a steady state volume and concentration of feedings per kg for at least two (2) days and throughout the first 24 hours of the sample collection period. The time recorded that the infant receives the first isotope will mark the beginning of the sample collection period. Infants may be receiving either breast milk (fortified or unfortified) or infant formula. If the infant is receiving breast milk, a 10mL sample of breast milk will be obtained to determine zinc and copper content. Zinc and copper contents of infant formula will be determined from published tables by the manufacturer. The infant will receive 1 mg Zn67 and 80 mcg Cu65 stable isotope orally mixed into one of the infant's morning feedings on the study day. In addition, the infant will receive 0.1 mg Zn70 intravenous stable isotope. Stool will be collected from the ostomy bag for 72 hours. A urine bag will be placed on the infant's skin to collect all the urine for 24 hours, followed by a spot urine sample collection twice a day for three (3) days after the isotope infusion. The final urine collection should occur at approximately 96 hours after the isotope infusion. A small amount of blood (2 mL) will also be taken at the time of the intravenous isotope infusion. This blood will be tested for zinc, copper, and ceruloplasmin levels. Fractional zinc absorption and endogenous fecal zinc excretion will be measured using stable isotopes. Urinary excretion will be measured from the urine collection. Zinc balance will be calculated from the difference in absorbed zinc (intake x fractional absorption) and zinc losses (urinary excretion and endogenous fecal zinc excretion). Fractional copper absorption will be measured from the difference in intake of Cu65 and the losses of Cu65 (in the ostomy and urine). Copper balance will be measured from the difference in total copper intake and total copper losses (in the ostomy and urine).