Transitional formulas (22 kcal/oz) are recommended for infants less than 35 weeks gestation at birth. However, few data are available related to follow-up of infants receiving these formulas who were 28-34 weeks gestation at birth.

Primary hypothesis: Provision of supplemental vitamin D to a transitional formula will lead to higher serum 25-hydroxyvitamin D (25-OHD) levels and no infant with a serum 25-OHD less than 20 ng/mL when assessed at approximately 52 weeks post-menstrual age (PMA).

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.)