Vitamin A and its Role in the Immune System

Vitamin A is an essential micronutrient, meaning it cannot be made by the body and thus must be obtained from the diet¹. Vitamin A is used in various physiological processes such as regulating immune function, vision, reproduction and cellular differentiation².

Vitamin A is important in regulating innate and adaptive immunity. It promotes the normal function of cells of the immune system (natural killer cells, neutrophils and macrophages), and the cells that create antibody responses (T and B lymphocytes) ³. Vitamin A also helps to control inflammation by promoting T cells that control pro-inflammatory cytokines ⁴.

A large part of the immune system lies in the gut, where multiple food particles and external pathogens are encountered every day. Vitamin A plays an essential role in maintaining immune tolerance of the gut lining, as well as the function and integrity of all mucosal surfaces that act as first-line barriers in the immune system (skin, respiratory tract, genitourinary tract, eye, etc.) ^{1,3, 4}.

Research conducted on patients infected with Ebola virus and HIV virus showed that vitamin A supplementation reduced mortality rates^5,6. Studies conducted in children with measles found that only 2 days of supplementation of vitamin A (200,000 IU on each day) reduced mortality rates, reduced the incidence of croup and shortened the duration of fever, pneumonia and diarrhea².

A deficiency in vitamin A can lead to higher levels of pro-inflammatory cytokines, which can lead to autoimmune disease⁴. Vitamin A deficiency can also increase the risk of severe infection, during which vitamin A demands are higher and therefore depleted, leading to a vicious cycle¹. Those at risk of vitamin A deficiency include premature infants, pregnant and lactating women, individuals with cystic fibrosis, and populations in developing countries².

Two forms of vitamin A can be found in the diet. One form comprises the animal-based retinoids (retinol, retinyl esters) ⁷. The other form is the plant-based carotenoids (including beta-carotene), some of which also act as antioxidants. Both forms require fat with consumption in order to optimize absorption¹. Once ingested, the body converts these pre-formed vitamin A molecules into retinal and retinoic acid. These active vitamin A products are then used to perform physiological functions. Foods such as leafy green vegetables and orange or yellow vegetables are rich in beta-carotene. Dietary sources of retinol include mainly animal sources such as beef liver, cod liver oil, salmon, mackerel, tuna, egg yolks, and dairy products ⁷. Supplemental forms of vitamin A include retinyl palmitate/acetate, or beta-carotene². Vitamin A is stored in the liver, so there can be toxic side effects with higher doses of more active forms of vitamin A. It is important to speak with your naturopathic doctor to determine the right form and dosage for your specific needs.

References:

1. Gilbert, C. (2013). What is vitamin A and why do we need it?. Community eye health, 26(84), 65.
2. https://ods.od.nih.gov/factsheets/VitaminA-HealthProfessional/
3. https://lpi.oregonstate.edu/mic/health-disease/immunity
4. Ash, M., & ION, D. D. (2010). Vitamin A: The Key to a Tolerant Immune System?. Clinical Education, August, 18.
5. Aluisio, A. R., Perera, S. M., Yam, D., Garbern, S., Peters, J. L., Abel, L., … & Levine, A. C. (2019). Vitamin A supplementation was associated with reduced mortality in patients with Ebola virus disease during the West African outbreak. The Journal of nutrition, 149(10), 1757-1765.
6. Fawzi, W. W., Mbise, R. L., Hertzmark, E., Fataki, M. R., Herrera, M. G., Ndossi, G., & Spiegelman, D. (1999). A randomized trial of vitamin A supplements in relation to mortality among human immunodeficiency virus-infected and uninfected children in Tanzania. The Pediatric infectious disease journal, 18(2), 127-133.
7. https://www.mountsinai.org/health-library/supplement/vitamin-a-retinol