Effect of Vitamin D in Diets of Preterm Infants

Inclusion Criteria: * Born at 28 0/7 to 34 6/7 weeks PMA and 1000-2250 g birth weight. * Currently 34 0/7 to 38 6/7 weeks post-menstrual age at time of consent. * Born at Texas Children's (including Pavilion for Women) or Methodist campus hospitals or transferred within 48 hours of birth. * Care expected to be provided at one of these institutions until discharge home. * Any initial feeding will be permitted, but expected to transition to primarily (80% of feeds or up to 2 breast milk feeds per day) infant formula by 38 6/7 weeks PMA or hospital discharge, whichever comes first. * Able to tolerate 22 kcal/oz transitional formula and receive a volume of at least 130 mL/kg/day total feeding volume. * No longer receiving any form of mechanical ventilation or diuretics. Low flow nasal cannula (\< ¼ LPM) will be permitted if it is anticipated this will be discontinued before hospital discharge. Exclusion Criteria: * Bronchopulmonary dysplasia (BPD) requiring daily use of diuretics beyond 38 6/7 weeks PMA (or hospital discharge, whichever comes first) and \> 22 kcal/oz concentration formula beyond 38 6/7 weeks PMA. * Major congenital anomalies, history of proven Stage 2 or above NEC, or severe feeding intolerance. * Caloric density greater than 22 kcal/oz. * Higher order multiples (However, twins are acceptable. Twins will be randomized together. Only data from 1 twin picked at random will be used in the final analyses.)