A phase III multicenter randomized double blind clinical trial will be conducted. After obtaining written consent the infant will be randomized, during the first two weeks of life, to a study group to receive either 400 IU or 1,000 IU / day of vitamin D to the year of age.

Baseline and all follow up visits (2, 6, and 12 months of life) will include anthropometric measurements and a questionnaire about health issues. A blood sample will be obtained at baseline for analysis of 25OH vitamin D, and at 6 and 12 months for analysis of 25 OH vitamin D, and calcium.

Healthy term born infants of appropriate size for gestational age will be included. We will need to include 359 children in each group.

The primary objective of the study is to decrease the proportion of infants with acute bronchitis during the first year of life by supplementation of 1,000 IU/day vitamin D. Secondary otcomes are: To check that the administration of 1,000 IU/day vitamin D decreases the proportion of infants with upper respiratory tract infections, the proportion of children under one year of age hospitalized for acute bronchiolitis, and the demand on the healthcare system due to respiratory infections and absences from work for parents and achieves a higher proportion of children with adequate blood levels 25 OH vitamin D.

Inclusion Criteria:

• Healthy term born infants of appropriate size for gestational age
• Parents provide informed consent to participate

Exclusion Criteria:

• Infants with small size for gestational age
• Infants with gestational age < 37 weeks
• Infants with congenital anomalies
• Infants with chronic gastrointestinal tract, liver, kidney, heart, or neurological diseases
• Infants with disorders of vitamin D or calcium metabolism or others inborn errors of metabolism