Variety of ingredients with vitamin C and D to help boost immune system

Using Vitamin C and D to Boost Your Immune System

Vitamins C, D and the mineral zinc have long been touted for anyone looking for ways to strengthen immune defenses against respiratory infections. While media can often unfairly represent important nutrients, this trio has consistently remained popular, and for good reason. All three have been shown to support the activity and function of the cells within the immune system.


Why are vitamins C and D important?

Vitamin C has long been recognized as an important antioxidant to support the immune system and regular use has been shown to shorten the duration of colds and reduce their severity.1,2 Vitamin D, once most commonly associated with its vital role in bone health, has gained in popularity in more recent years with an association based on a significant amount of research showing it too is a valuable asset to the immune system.3-5 Both these vitamins’ role in human health continue to impress researchers.

Unfortunately, vitamin D deficiency and insufficiency is all too common.6 So, how do you know if you’re getting enough of this valuable vitamin? A simple 25-Hydroxy Vitamin D Blood Test can tell you your levels, and the test is available in most states without requiring a doctor visit or insurance. Life Extension has long advocated an optimal vitamin D blood level between 50-80 ng/mL.


How much vitamin C and D per day?

Functional medicine experts often recommend more than the 15 mcg (600 IU) RDA of vitamin D daily in order to support optimal health. The RDA of vitamin C for adults is 75–90 mg for women and men, respectively, and slightly higher for smokers and pregnant or breastfeeding women. However, the Linus Pauling Institute recommends 200–400 mg of vitamin C per day. Even many conventional medical experts support supplementing with 1000 mg of vitamin C daily due to its antioxidant support.

Heart shape tray with lean protein and nutritious veggies and a stethoscope


According to the 2015-2020 Dietary Guidelines, vitamins C & D, among other nutrients, are underconsumed by Americans. The guidelines go on to state that “of the underconsumed nutrients, calcium, potassium, dietary fiber and vitamin D are considered nutrients of public health concern because low intakes are associated with health concerns.” Life Extension generally suggests 5000 IU of vitamin D daily for most adults in order to support optimal levels.

Heart shape tray with lean protein and nutritious veggies and a stethoscope


Vitamin C and D food sources

Humans are unable to synthesize their own vitamin C and must obtain it from food and supplementing. Skin has the ability to manufacture vitamin D from sunlight but with limitations such as one’s age, skin color, geographic latitude and sunscreen use. Furthermore, food is often an unreliable way to obtain enough vitamin D required to reach and maintain optimal levels.

Heart shape tray with lean protein and nutritious veggies and a stethoscope

Vitamin D

  • Fatty fish such as salmon and trout
  • UV exposed mushrooms
  • Egg yolks
  • Fortified cereal, dairy and non-dairy milks
Heart shape tray with lean protein and nutritious veggies and a stethoscope

Vitamin C

  • Kiwi fruit
  • Citrus fruits
  • Strawberries
  • Broccoli
  • Peppers


Vitamin C’s role in the immune system

Vitamin C has been shown to support the immune system by several mechanisms:7,8

  1. Helps the activity of our immune cells
  2. Reduces inflammation caused by infections
  3. Supports collagen synthesis, which is important for maintainings protective tissue barriers such as those found in the lining of our lungs.

How vitamin D benefits the immune system

Like vitamin C, vitamin D is useful when it comes to supporting the immune system; it too can help the activity of our immune cells and reduce inflammation caused by infection—this includes moderating the amount of inflammatory cytokines produced when the imune system responds.9,10 Vitamin D may also play a role in the prevention of autoimmune disease and acute respiratory infections.11

Vitamins C & D: The dynamic duo

From helping with the common cold to more advanced roles in health, vitamin C remains an immune system superstar. And because vitamin D receptors are present in most tissues and cells within the body, including immune cells of course, it too has proven to be a tremendous asset to our health. Ensure you are getting enough of these immune nutrients today!

About the Author: Holli Ryan is a food & nutrition expert, registered & licensed dietitian-nutritionist, health & wellness writer, blogger, and social media specialist. She graduated from Florida International University and is a member of the Academy of Nutrition and Dietetics. In her free time she enjoys photography, travel, cooking, art, music, and nature.


  1. Schloss J, Lauche R, Harnett J, et al. Efficacy and safety of vitamin C in the management of acute respiratory infection and disease: A rapid review. Adv Integr Med. 2020.
  2. Ran L, Zhao W, Wang J, et al. Extra Dose of Vitamin C Based on a Daily Supplementation Shortens the Common Cold: A Meta-Analysis of 9 Randomized Controlled Trials. Biomed Res Int. 2018;2018:1837634.
  3. Martineau AR, Jolliffe DA, Hooper RL, et al. Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. BMJ (Clinical research ed). 2017;356:i6583.
  4. Poorna R, Biswal N. Respiratory infections: Role of Vitamin D and surfactant proteins A and D. Lung India. 2020;37(5):421-424.
  5. Kowalówka M, Główka AK, Karaźniewicz-Łada M, Kosewski G. Clinical Significance of Analysis of Vitamin D Status in Various Diseases. Nutrients. 2020;12(9).
  6. Amrein K, Scherkl M, Hoffmann M, et al. Vitamin D deficiency 2.0: an update on the current status worldwide. European journal of clinical nutrition. 2020;74(11):1498-1513.
  7. Jariwalla RJ, Harakeh S. Antiviral and immunomodulatory activities of ascorbic acid. Subcell Biochem. 1996;25:213-231.
  8. Murad S, Grove D, Lindberg KA, Reynolds G, Sivarajah A, Pinnell SR. Regulation of collagen synthesis by ascorbic acid. Proc Natl Acad Sci U S A. 1981;78(5):2879-2882.
  9. Sassi F, Tamone C, D'Amelio P. Vitamin D: Nutrient, Hormone, and Immunomodulator. Nutrients. 2018;10(11):1656.
  10. Cannell JJ, Grant WB, Holick MF. Vitamin D and inflammation. Dermatoendocrinol. 2015;6(1):e983401.
  11. Charoenngam N, Holick MF. Immunologic Effects of Vitamin D on Human Health and Disease. Nutrients. 2020;12(7).