The purpose of this study is to determine wheather the monitored vitamin D (vit D) therapy is safer and more effective than standard therapy in pretrem infants.

Vitamin D (vit D) deficiency is a risk factor of osteopenia of prematurity, which leads to rickets or decreased bone mass mineral density. Recently multiple studies have been published on vit D adjust biological functions. Dosage, safety and effectiveness of vitD supplementation in preterm infants still remains a controversial topic. We hypothesize that monitored supplementation of vit D is more effective and safer than standard therapy 500IU in preterm infants. The study will be carried out in 138 preterm infants, born at 24-32 week of gestational age (GA) at the Princess Anne's Hospital in Warsaw, Poland. We will determine if monitored supplementation of vit D decreases the incidence of vit D deficiency and/or overdosing at 40 week (GA). For secondary objective we shall assess if monitored therapy reduces the incidence of vit D deficiency and/or overdosing at 35, 52 week (GA), prevalence of osteopenia, low bone mass, nephrocalcinosis and nephrolithiasis.

Inclusion Criteria:

• Preterm infants born between 24 and 32 weeks of gestation (estimated by ultrasound)
• In born or admitted to the unit within 48hours from birth.
• Randomization within 7 days from birth.
• Parental consent.
• Mothers willing to return for follow up visits.

Exclusion Criteria:

• Preterm delivery >=33 weeks of gestation or term delivery (estimated by ultrasound).
• Major congenital abnormalities.
• Participation in another trial.
• Severe illness at birth deemed incompatible with survival.
• Congenital HIV infection.
• Total parenteral nutrition > 14 days.
• Cholestasis