Your Winter Guide to Vitamin D and Immune Function
Dr. Don Colbert
Dec 5, 2018

It’s estimated that 25-50% of adults in the United States are deficient in Vitamin D.

Even when it’s a sunny winter day, you may not get enough of the “sunshine vitamin.”

In fact, it’s estimated that 25-50% of adults in the United States are deficient in Vitamin D (1).

And, it seems to be more problematic in wintertime months, when we come down with more colds and flu.

What’s the Vitamin D and immune function connection?

Here’s all about Vitamin D, the research regarding immunity, risk factors for deficiency, and how to get enough.


Vitamin D is a fat-soluble hormone that is both synthesized naturally by the body, and absorbed in the digestive tract.

There are two forms of Vitamin D, animal-synthesized D3 (cholecalciferol) and plant-synthesized D2 (ergocalciferol).

While humans can make the active form of Vitamin D, calcitriol, from D2 or D3, the latter is considered more effective at increasing levels.


Vitamin D directly supports the immune system (2). It’s been shown to:

  • Increases the production of disease-fighting cytokines
  • Amplifies the recognition of viruses and bacteria
  • Improves the body’s anti-microbial activity
  • Supports the disruption of bacterial membranes and virus replication

There’s been mixed results on Vitamin D levels and specific disease outcomes. For example:

  • A 2007 study showed the rate of winter-time colds and flu symptoms was reduced by more than 300% when participants supplemented with 2000 IU Vitamin D3 per day. However, the quality of the questionnaire used to gather data has come into question (3).
  • In 2010, researchers found an inverse association between total Vitamin D levels and recent upper respiratory tract infections (URTI). Vitamin D levels <30 ng/mL were associated with more recent URTI than those higher (4).
  • Also in 2010, scientists reported an approximate 8% reduction of seasonal influenza A in schoolchildren with Vitamin D supplementation of 1200 IU per day (5).
  • Most recently, a 2017 analysis of 11,321 participants in the British Medical Journal found that those with Vitamin D levels of <10 ng/mL had a higher risk of respiratory infections compared to those at 10 ng/mL or greater (6).

There have also been studies that don’t show significant correlations. So, how much is enough?

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