Respiratory tract infection involves a large number of infectious diseases in the respiratory tract. They are split into upper respiratory tract infections (URTI) or lower respiratory tract infections (LRTI). LRTI, such as pneumonia, tend to be far more serious conditions than URTI, such as the common cold.
Typical symptoms of URTI include tonsillitis, sore throat, laryngitis, sinusitis, ear infections, and the common cold. Symptoms of URTI can include cough, sore throat, runny nose, nasal congestion, headache, low grade fever, facial pain or pressure and sneezing.
LRTI are much more serious and are the leading cause of death among all infectious diseases. The two most common types are bronchitis and pneumonia. Cough, chest pain and fever are the common symptoms. Influenza infections affects both the upper and lower respiratory tracts, but the much more dangerous strains, such as H1N1, infects deep into the lower lungs causing severe symptoms. Besides cough and fever, prostration (having to lie down) is a prominent symptom of a true influenza infection. An H1N1 influenza strain killed up to 50,000,000 people in 1918 and 1919, and, for unknown reasons, was especially fatal for young people in the prime of life.
Recently, Dr Jaykaran Charan and colleagues of the Indian Institute of Public Health published a meta-analysis of all randomized controlled trials of vitamin D and respiratory tract infections.
They found five studies that met their criteria, but their paper went to the reviewers before publications of the two most recent randomized controlled trials, one of which was positive (4,000 IU/day for a year) and one of which was negative (100,000 IU/month for 18 months).
Dr Charan combined the five randomized controlled trials he found that were conducted before the above two studies and found that the combined relative risk for a respiratory infection was about half for those taking vitamin D (relative risk = .58). The dose of vitamin D in the five trials ranged from 400 IU/day to 2,000 IU/day, with one using a single dose of 100,000 IU. The length of the trials ranged from 3 months to three years.
The authors concluded,
“On the basis of this study, we conclude that vitamin D is useful in prevention of respiratory tract infections.”
However, many questions remain. What is the optimal dose of vitamin D? Is the dose different for URTI and LRTI? What is the 25(OH)D threshold for an anti-infective effect? How long does it take for vitamin D to become effective? Are daily doses required or will weekly or monthly doses prevent infections? Which viruses and bacteria are susceptible to vitamin D? Will vitamin D help prevent the common cold?
Until those questions are answered, and it will take decades for scientists to do so, I believe you should keep your 25(OH)D in natural ranges, around 50 ng/ml.