Your ability to avoid colds and flu is related to vitamin D. Vitamin D levels in your blood fall to their lowest point during winter due to lack of sunlight. Vitamin D is essential for the body to produce its own antibiotics so a person with a low vitamin D blood level is more vulnerable to contracting colds, influenza, and other respiratory infections. Studies show that children with rickets, a vitamin D-deficient skeletal disorder, suffer from frequent respiratory infections, and children exposed to sunlight are less likely to get a cold. The increased number of deaths that occur in winter, largely from pneumonia and cardiovascular diseases, are most likely due to vitamin D deficiency.
So can't I just eat more vitamin D rich foods to avoid colds this winter?
It is very difficult to get adequate amounts of vitamin D from your diet unless you are eating large amounts of offal, oily fish and seafood. Some estimates have been made on food composition with daily intakes of vitamin D between 2-3 μg /day (80-120 IU) for adults (Baghurst & Record 2002). This is well short of the 4000IU that we'd recommend for adults to achieve optimal levels of vitamin D (see here for how we justify this dose)
Hang on - I thought I could still get my vitamin D dose by getting outside in the sun?
Sun exposure on the skin is the other option for boosting your vitamin D levels. Vitamin D is produced by your skin in response to exposure to ultraviolet B rays from the sun. But these rays cannot penetrate glass, sun tan lotion, clothing, or dark skin to effectively generate vitamin D in your skin. In most parts of New Zealand and Australia a "vitamin D winter" occurs for around 4-6 months in which weather conditions make it impossible to produce enough vitamin D from sunlight.
The largest study of its kind (with over 19,000 subjects) from the Archives of Internal Medicine (2009) gave a summary:
"The findings of our study support an important role for vitamin D in prevention of common respiratory infections, such as colds and the flu. Individuals with common lung diseases, such as asthma or emphysema, may be particularly susceptible to respiratory infections from vitamin D deficiency."
If you're looking to optimise your vitamin D levels to reduce your risk of colds and flus this winter make sure you keep up your cod liver oil.