When we think of immune defense arsenal, vitamin C is the most popular nutritional weapon of choice. Vitamin C earns its rightful place at the top of the immune boosting nutrition list as a powerful antioxidant and its inflammation reducing powers, but vitamin D plays a surprisingly large part fighting foreign invaders and might be more important to supplement than vitamin C during the cold and flu season.

Why would vitamin D supplementation be more beneficial than vitamin C?

All vitamins are important for optimal healthy body functions, but during the cold and flu season where immunity needs to be battle ready, vitamin D levels might be a larger concern than vitamin C.

25-hydroxyvitamin D levels were inversely correlated with acute respiratory infection, as reported in the National Health and Nutrition Survey


Vitamin D is important for immune response.

Vitamin D is mostly known for calcium and bone homeostasis. An underrated job of the bones is the bone marrow which is responsible for creating blood cells including immune cells. An immune response is activated when a foreign invader enters our body such as a virus during cold and flu season. An immune response includes increasing immune cell creation to get as many body soldiers out in our system as we can to fight the invader as quickly and efficiently as possible. This immune response is inhibited with lower levels of vitamin D.

Deficiency in vitamin D is associated with increased autoimmunity as well as an increased susceptibility to infection.

Cynthia Aranow, MD, Investigator

Vitamin D deficiency is most prevalent during the peak of cold and flu season.

Cold and flu season starts in December and usually lasts until March. These are the winter months which are mostly known for winter depression. Winter depression (seasonal affective disorder) is commonly associated with vitamin D deficiency.

One way to get your daily dose of vitamin D is through a skin mechanism utilizing the sun rays. During the winter months, the Earth is tilted farther away from the sun and it tends to be more cloudy reducing those beneficial UV rays.

During the winter, our bodies are fighting to survive with less sunlight and more foreign invaders than usual.

Vitamin D deficiency is more common than vitamin C deficiency.

Vitamin C deficiency rates in the United States is about 7% (statpearls) where as vitamin D deficiency rates are as high as 42% according to the Cleveland Clinic.

That’s 7% vs 42%. That’s a difference of 35%.

New Study Found 80% of COVID-19 Patients Were Vitamin D Deficient


There are less food sources of vitamin D then vitamin C food sources available.

Vitamin C is found in many citrus fruits and vegetables such as oranges, strawberries, lemons, brussels sprouts, and peppers.

Vitamin D is found in oily fish, red meat, liver, mushrooms and egg yolks. This list is much shorter and less appetizing to most compared to vitamin C foods. This is why it is usually fortified in cereals and milk. Cow’s milk is not a good source of vitamin D unless it is added.

If you eat an adequate amount of vitamin D food sources, there are some risk factors you should know that can still lead to vitamin D deficiency.

Risk factors for vitamin D deficiency

  • Age
  • Smoking
  • Obesity
  • Diabetes
  • Hypertension
  • Digestive disorders that affect vitamin D absorption
  • Kidney disease preventing the conversion of vitamin D to it’s active form


Vitamin D is just as important as vitamin C and many other vitamins and minerals but the NEED to supplement or understand the vital role of this vitamin in immune defense is far greater.

Women tend to be more susceptible to low levels of vitamin D along with other important nutrients explained in the ingredients of this woman created multivitamin.

Vitamin D may also help get your through the next pandemic period!

To help prepare your natural medicine cabinet with more cold and flu season arsenal, see our best natural medicine list here!