Vitamin D: Why It's Important in the Fall

Vitamin D: Why It's Important in the Fall

Along with pumpkin spice, changing leaves, and cooler weather, fall also ushers in the beginning of cold and flu season. While there’s no way to ensure you won’t get sick this fall or winter, having enough vitamin D can give your immune system a helping hand. How exactly does vitamin D help us stay healthier this fall and beyond?

What is Vitamin D?

Vitamin D is a fat-soluble vitamin that is slightly different from other vitamins. The body can make the vitamin D it needs from exposure to the sun. Most foods are poor sources of vitamin D with a few exceptions, such as UV-radiated mushrooms, fish, egg yolks, and fortified dairy products.1 

This essential vitamin plays many important roles in the body. These include:2

  • Calcium absorption in the gut
  • Maintaining normal calcium and phosphate levels in the blood
  • Supporting bone growth and remodeling
  • Preventing rickets in children and osteomalacia in adults
  • Supporting a normal inflammatory response
  • Metabolizing carbohydrates (glucose) in food
  • Supporting immune health 

Vitamin D has so many functions, but in the fall and winter months, it becomes even more critical because its levels tend to go down.3 

Vitamin D and Immunity

Nearly every immune cell has a receptor for vitamin D, which highlights its importance in keeping the immune system healthy and strong.4 It has been found to play a role in both innate and adaptive immune responses, supporting the immune system in several different ways. It also supports a normal inflammatory response, acting directly on certain inflammatory cells of the immune system.5

But in the fall season, we have a harder time getting enough of this important vitamin. When it gets cold, the days also get shorter. We’re exposed to less sun because there are fewer hours of sunlight, the sun isn’t as strong, and we can’t be outdoors for long periods. 

This means our bodies have fewer opportunities to make the vitamin D we need to help our immune system. A lower vitamin D level may impact our immune health, increasing our susceptibility to respiratory health issues.

Maintaining Healthy Vitamin D Levels

An optimal vitamin D level is between 25-80 ng/mL.  According to a 2022 study, approximately 40% of Americans have vitamin D insufficiency, meaning their levels are not optimal.6 For those who live in colder areas, levels of insufficiency may be as high as 73% during the winter months.7 

The best way to know whether your vitamin D levels are low is to get them checked. This requires a simple blood test that can be ordered by your doctor. Depending on the severity of the vitamin D deficiency, your doctor may recommend a high dose of vitamin D for a short period of time to get your levels up. 

If you are not deficient but want to maintain your vitamin D levels to support your immune and overall health, especially during those periods when the days get colder and shorter, consider a vitamin D supplement of at least 600 IU per day. Also if you are able, consider spending 10-15 minutes outdoors daily to help boost vitamin D levels.

Staying healthy during the fall and winter seasons isn’t just about vitamin D, but also involves a healthy diet and lifestyle. Consider adding foods and supplements to your routine that help support your immune health, so you can enjoy everything the fall season has to offer.


  1. Vitamin D. (n.d.). Retrieved October 11, 2023, from
  2. Vitamin D. (n.d.). Retrieved October 11, 2023, from
  3. Vitamin D. (2012, September 18). The Nutrition Source.
  4. Martens, P.-J., Gysemans, C., Verstuyf, A., & Mathieu, A. C. (2020). Vitamin D’s Effect on Immune Function. Nutrients, 12(5).
  5. L Bishop, E., Ismailova, A., Dimeloe, S., Hewison, M., & White, J. H. (2021). Vitamin D and Immune Regulation: Antibacterial, Antiviral, Anti-Inflammatory. JBMR plus, 5(1), e10405.
  6. Cui, A., Xiao, P., Ma, Y., Fan, Z., Zhou, F., Zheng, J., & Zhang, L. (2022). Prevalence, trend, and predictor analyses of vitamin D deficiency in the US population, 2001-2018. Frontiers in Nutrition, 9, 965376.
  7. Kennel, K. A., Drake, M. T., & Hurley, D. L. (2010). Vitamin D deficiency in adults: when to test and how to treat. Mayo Clinic Proceedings. Mayo Clinic, 85(8), 752–757; quiz 757–758.