The goal of this clinical trial is to learn about the effective dosage of enteral vitamin D supplementation in preterm infants with gestational age fom 28 to 34 weeks. The main questions it aims to answer is:

- Does higher dose of enteral vitamin D supplementation (800IU/day) increase vitamin D level better than standard recommended dose (400IU/day) to achieve sufficient level of vitamin D?

Participants will:

• Take the enteral vitamin D supplementation every fay for 28 days
• Have their blood level of vitamin D examined before and after supplementation for 28 days

The study conducted in 2 hospitals in Jakarta region, RSCM and RSUD Koja. Preterm with 28 to 34 weeks of gestational age and fulfill the inclusion criteria will be enrolled in the study after parents signed the informed consent.

Blood sample will be collected to get the baseline of vitamin D serum. The study subject will be given enteral vitamin D supplementation for 28 days (4 weeks). The dosage of vitamin D is randomized with block randomized in sealed envelope.

Subjects then will be followed up to 28 days of supplementation. After 28 days of supplementation, the subjects' blood will be collected to get the vitamin D serum level. Data taken will be analyzed using IBM Statistical Product and Service Solutions (SPSS).

Inclusion Criteria:

• neonates with gestational age 28-34 weeks
• tolerated enteral feedings more than 30ml/kg

Exclusion Criteria:

• infants with major congenital anomaly
• infants with gastrointestinal obstruction
• infants with congenital renal failure
• infants with cholestassis