The main purpose of this study is to determine whether weekly oral vitamin D supplementation is effective to prevent acute respiratory tract infections in children. The hypothesis of the study is that vitamin D supplementation reduces the incidence and severity of acute respiratory tract infections in children.

Acute respiratory tract infections (ARTIs) are associated with high levels of morbidity and socioeconomic impact, particularly affecting young children. Observational studies have shown an association between vitamin D (VD) deficiency and higher incidence and severity of respiratory infections. Chile has a high prevalence of VD deficiency, particularly in the southern regions of the country. The objective of this study is to evaluate the safety and efficacy of oral VD supplementation to decrease the incidence of ARTIs in 275 preschool children. A randomized, placebo-controlled, double-blind study will be performed in Santiago, Coyhaique and Punta Arenas. Children aged 18 to 36 months will be given weekly oral 5600 IU of vitamin D3 (VD3), 11200 IU of VD3, or placebo doses during 6 months.

Inclusion Criteria:

• Children aged 18 to 36 months attending daycare in Santiago, Coyhaique or Punta Arenas.

Exclusion Criteria:

• History of chronic illness requiring immunosuppression
• History of metabolic bone disease
• Use of vitamin D supplementation greater than 400 IU daily, either by milk formula or vitamin supplements in the last 3 months.
• Use of fish oil supplements in the last 3 months.
• Immunodeficiency
• Planned trip to sunny climate during study period.