One Simple Tool to Support Your Immune System This Winter

Dr. Tieraona Low Dog

Vitamin D is crucial to our body’s natural defenses. Read on to learn how the sunshine vitamin can support your immune system this winter!

— Tieraona Low Dog, M.D.

Most people reach for vitamin C to boost their immune system, but there’s another powerful player that often gets overlooked—vitamin D. As we transition to winter, I wanted to share a few thoughts about the role vitamin D plays in our overall health, as well as our immune health specifically.

What is vitamin D?

Vitamins are essential micronutrients our bodies require to function properly, but can’t synthesize and therefore require intake through the diet. However, vitamin D isn’t technically a vitamin, as our bodies can produce it when we expose our skin to sunlight. Our bodies convert cholesterol in our skin into vitamin D3 and store it in our fat. Then, when called upon, it is transported to the liver and kidneys and converted to the active form, calcidiol. 

There are two primary forms of vitamin D:  cholecalciferol (vitamin D3), which is found only in animal sources (e.g., fatty fish, liver, egg yolks), and ergocalciferol (vitamin D2), which comes from plants, yeast, and mushroom sources. Most of the vitamin D3 used in dietary supplements is from the lanolin in sheep’s wool. There are also vegan sources of vitamin D3 produced from lichen or algae.

What does vitamin D do?

Vitamin D receptors are in 30 different body tissues, and activated vitamin D interacts with more than 1,000 genes! Without vitamin D, our bodies can’t absorb calcium, leading to rickets in children or osteomalacia in adults. Vitamin D is also essential for bone health throughout our lives, preventing muscle weakness, reducing the risk of falls—critically important as we age. 

Additionally, considerable evidence suggests vitamin D offers some protection against certain cancers. However, the evidence is stronger for improving survival in those who develop cancer. A review of studies found a significantly lower risk of cancer death in those assigned to receive vitamin D versus placebo. 

Good hygiene, adequate rest, and a healthy diet are foundational to a healthy immune response, but vitamin D also plays an important role. A 2021 review found vitamin D is essential in regulating our innate immune system’s response to microbial threats, and deficiency is associated with a greater risk of viral infections, particularly those in the respiratory tract. These findings are consistent with a review of 25 clinical trials published in the British Medical Journal, which found vitamin D supplementation to safely protect against acute respiratory tract infection, particularly in vitamin D deficient patients. Did you know that over 2.5 million people die every year from respiratory infections? And that’s BEFORE the recent pandemic. In fact, a recent large meta-analysis found low vitamin D levels were associated with increased severity of COVID-19, resulting in more hospitalizations and death.  

With a growing body of evidence supporting vitamin D’s significant role in immune health, knowing what healthy levels are and how to meet them is very important.

Vitamin D + winter

Exposure to sunlight is the most effective and efficient way to get vitamin D. But what happens when sun exposure becomes limited in the winter months? 

Besides the natural inclination to stay indoors during winter months, some places simply don’t get the sunlight required for the adequate production of vitamin D. This is the case for regions above 35 degrees latitude for several months each winter, referred to as “vitamin D winter.” So it’s no surprise respiratory infections are worse in winter when vitamin D levels are lowest for most of us.

Alternative sources and adequate levels of vitamin D

Insufficient exposure to sunlight is as concerning as excessive exposure, linked to increased risk of skin cancer and sun-damaged skin. However, as we age, there is a decline in 7-dehydrocholesterol levels in our skin, making it hard to make enough vitamin D to meet our needs, even with solar exposure. 

Vitamin D is not readily available in food. In the United States, most dietary vitamin D comes from fortifying milk or nondairy milk products, breakfast cereals, and orange juice. It is also naturally present in fatty fish, herring, sardines, egg yolks, and mushrooms. However, while I advocate getting nutrients from food when possible, getting adequate amounts of vitamin D from diet alone can be challenging.

According to The Endocrine Society, the recommended daily intake amounts of vitamin D are: 

  • 0–12 months old:  400–1,000 IU
  • 1–8 years old:  600–1,000 IU
  • 9–18 years old:  600–1,000 IU
  • 19 years old + :   1,500–2,000 IU
  • Pregnant or breastfeeding, 14–18 years old:  600–1,000 IU
  • Pregnant or breastfeeding, 19–50 years old:  1,500–2,000 IU

Given these guidelines, to get a mere 600 IUs of vitamin D, you would need to consume: 

While salmon is an excellent source of vitamin D, it is difficult to eat daily for most folks. Do you love sardines and mushrooms, or live somewhere sunny year-round? If not, ensuring sufficient vitamin D levels will likely require supplementation.

Supplement considerations

In choosing any supplement, it’s essential to know what form to take, how much of it to take, and when to take it.

Which form?

Our bodies produce cholecalciferol (vitamin D3). Most clinical guidelines recommend vitamin D3 for increasing and maintaining serum vitamin D levels. Vitamin D3 is the most common form in dietary supplements, derived from the lanolin in sheep’s wool, fish, or liver oil. It is also extracted from vegan sources such as lichen and algae. Ergocalciferol (vitamin D2) is also available in supplements, often prepared from irradiated yeast, another option for vegans. 

When to take it?

As a fat-soluble vitamin, vitamin D needs to be taken with some fat. One study reported a significant increase in vitamin D absorption when consumed with a high-fat meal. Therefore, I usually recommend taking your vitamin D with dinner, as it generally contains more fat.

How Much?

I typically recommend 2,000-3000 IU per day of vitamin D3 for most adults, especially during winter. That is a “safe” dose, as the tolerable upper limit (UL) is 4,000 IU per day for nine-year-olds and above. However, if your vitamin D levels are low, you will need to take larger doses until your levels are corrected (see below).

Note: When considering supplements, consult your professional healthcare provider to discuss what is best for you.

Checking your levels

Whether clinicians should check vitamin D levels is debated amongst health professionals. Personally, I believe it makes sense to test vitamin D levels in November/December (in the northern hemisphere) to ensure adequate levels during the winter months—especially for those who:

  • are over age 65
  • don’t spend much time outdoors
  • have melanin-rich skin
  • live in very northern regions
  • have underlying health risks

It is almost impossible to know if your levels are within the normal range without testing. Your health professional can order a test, or you can order your own through Everlywell or RequestATest. The Endocrine Society states that sufficiency for serum 25(OH)D is 30 ng/mL or higher; insufficiency is 21-29 ng/mL; vitamin D deficiency is 20 ng/mL or lower. I generally aim for 40-50 ng/mL.


Vitamin D is a crucially important piece of the human health puzzle. As we look for ways to boost our body’s natural defenses this winter, remember:  getting enough of this essential vitamin is a must! For more of my winter health tips, click here. You can also click here to check out some of my immune-boosting winter recipes.