Vitamin D and immunity

Vitamin D is technically a hormone, produced in the skin through UVB sunlight exposure and converted in the liver and kidneys into its active form, the active form of vitamin D. Almost every cell in the immune system has a receptor for the active form of vitamin D, making it one of the most broadly acting immune regulators known. In the first-line immune defence, vitamin D stimulates the production of natural chemical defences against bacteria and viruses in your airways and on your skin. In the targeted immune system, vitamin D is involved in immune balance, regulatory immune cells and the body’s normal antibody response.

What is happening

Vitamin D deficiency is defined as your vitamin D level in the blood below 50 nanomoles per litre. By this measure, an estimated 40 percent of the general European population is deficient, rising to over 70 percent in northern European countries during winter months. In populations with limited outdoor time, including hospitality workers predominantly working evening and night shifts, deficiency rates can approach 80 to 90 percent during winter. The skin only produces vitamin D from UVB sunlight when the sun is above approximately 45 degrees elevation, which at northern European latitudes only occurs between approximately April and September. For the rest of the year, sun exposure at northern latitudes produces essentially no vitamin D regardless of duration.

The respiratory tract is the area where vitamin D's immune role is most clinically relevant for most people. A combined review of multiple studies of 25 controlled studies published in the British Medical Journal found that vitamin D supplementation reduced the risk of respiratory infection by 12 percent overall, with the effect strongest in people who were deficient at baseline. The dose used in these trials varied widely, but consistent evidence supports a daily supplement of 1000 to 2000 IU as maintaining adequate levels in most adults during winter months, with deficient individuals often requiring higher doses to restore levels before a maintenance dose is relevant.

What to do next

Food sources of vitamin D are limited and insufficient as a sole source. Oily fish is the richest dietary source, with a portion of salmon providing around 400 to 600 IU. Egg yolks, mushrooms exposed to UV light and fortified dairy products provide smaller amounts. The gap between what food provides and what the body requires, particularly during winter at northern latitudes, makes supplementation the most reliable approach for most Europeans from October through April. Testing your vitamin D level in the blood in late autumn, before the winter deficit period, gives you a clear picture of your baseline and allows supplementation to be calibrated to your actual level rather than a population average estimate.

Sources: WHO healthy diet guidance, EFSA water guidance, NCBI sleep stages, WHO stress guidance.

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