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.

What is Vitamin D?

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.

The Science: Vitamin D and Immune Function

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?

History of Vitamin D Levels and Deficiency

There’s been controversy on what constitutes Vitamin D deficiency through the years.

Historically, any level less than 10 ng/mL was deficient.

In 2011, the Endocrine Society issued a report encouraging a higher minimum blood level of 30 ng/mL based on their research (7).

Then, the America Heart Association encouraged higher levels. It cited data that showed Vitamin D’s correlation with less degenerative disease including heart failure, depression, coronary artery disease, hypertension, and kidney failure. However, it discouraged levels greater than 100 ng/mL since these were linked to arterial fibrillation.

More studies piled up that connected Vitamin D to bone health, muscle strength and less fat accumulation, athletic performance, blood sugars, mental health, and reduced risk of some cancers.

At present, there is still no exact optimal number. However, many experts recommend 40-60 ng/mL, a possible immune- and health-enhancing range.

To get there, you may need to both avoid any risk factors you can for deficiency and proactively increase Vitamin D intake.

Risk Factors for Vitamin D Deficiency

Factors that increase the risk of Vitamin D deficiency include:

1. Lack of direct sunlight exposure in winter months. Even though you are in the sunshine in the winter, the axis of the Earth reduces the number of UV rays that directly hit us.
2. Life indoors. Compared to generations before us who mostly worked outside, modern adults now largely live indoors.
3. Limited natural food sources of Vitamin D and pollution. There simply aren’t many great natural sources of Vitamin D in our food supply. Additionally, ocean pollutions threaten the fish that do supply it.
4. Dark-pigmented skin cells. If you have darker skin, the amount of melanin in it reduces its ability to make Vitamin D from sunlight.
5. Age. As you age, your kidneys cannot convert as much Vitamin D to its active form.
6. Digestive health. If you suffer from Crohn’s disease, cystic fibrosis, and celiac disease, you may not absorb as much Vitamin D.
7. Obesity. Obesity in adults is correlated to significantly lower levels of Vitamin D.
8. Sunscreen Use. While sunscreen is very important, its use decreases Vitamin D production by the skin.

Are You Deficient? A Look at Your Numbers

If you are at risk of Vitamin D deficiency, it may be worthwhile to get your levels tested.

The most accurate measurement is the 25-hydroxy vitamin D blood test. While there are different target levels among professionals, consider a goal of 40-60 ng/mL with your doctor (8).

How to Increase Low Vitamin D Levels

If you have insufficient Vitamin D levels, you can increase them by:

• Getting outside for direct sunlight exposure to start. Try exposing skin for just 10-15 minutes per day.
• Eating wild fish such as herring, wild salmon (try this Keto Zone chowder recipe), and tuna. Cage-free eggs and cheese also supply small amounts.
• If overweight, using Keto Zone guidelines to lose weight.
• Supplementing Vitamin D3 as needed. Just 1000-2000 IU per day in the winter can raise insufficient levels, and up to 4000 IU per day can be used to correct a deficiency. Retest to monitor.

Bottom Line

Vitamin D levels decrease during the winter months and may be linked to more respiratory infections and illness. Do what you can to avoid deficiency risk factors, increase your intake as needed, and optimize your Vitamin D and immune function connection.