Background: Vitamin D has been shown to be an important immune system regulator. Vitamin D insufficiency during the winter may cause increased susceptibility to infections, particularly viral upper respiratory tract infections (URIs).

Objective: To determine whether vitamin D supplementation during the winter season prevents or decreases URI symptoms in adults.

Methods: A randomized, double-blind, placebo-controlled trial was conducted in 162 adults. Participants were randomized to receive vitamin D3 50 mcg (2000 IU) daily or matching placebo for 12 weeks from December 2006 to March 2007. A biweekly questionnaire was used to record the incidence and severity of URI symptoms

Inclusion Criteria:

• ambulatory adult age 18-80 and stable medical condition with no change in medications for 6 months prior to study entry.

Exclusion Criteria:

• morbid obesity (body mass index > 35 kg/m2)
• current tobacco use
• history of hypercalcemia, nephrolithiasis or sarcoidosis
• pregnancy
• recent hospitalization
• current liver or kidney disorders, malignancy and malabsorption
• use of immunosuppressants or medications that interfere with vitamin D metabolism such as phenytoin and carbamazepine.