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)
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, suntan lotion, clothing, or dark skin to effectively generate vitamin D in your skin. In most parts of 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 the 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 optimize your vitamin D levels to reduce your risk of colds and flu this winter make sure you keep up your cod liver oil.