When provided as a supplement at proper levels, vitamins A, D and E can all improve immune function. However, It's important to note that vitamin A may be toxic at excessive levels, especially to horses already consuming beta carotene from fresh pasture. Caution may need to be exercised when supplementing horses with vitamin A.
> Vitamin A stimulates systemic immunity. 1
The vitamin A metabolite (retinoic acid) plays a key role in mucosal immune responses, as it may favor protection in the gut mucosa by increasing humoral immune responses.2 Retinoic acid is central to immunological tolerance and acts as a trigger of adaptive immune responses.3 Vitamin A plays a significant role in immune function, including innate immunity, cell-mediated immunity and humoral antibody immunity.4
> Vitamin D boosts innate immunity.
Vitamin D modulates production of antimicrobial peptides and the cytokine response.5 The vitamin D metabolite (1,25-(OH)2D3) acts as an immune system modulator through its effects on the vitamin D receptor. Circulating vitamin D levels have a direct influence on macrophages. Vitamin D also modulates the immune system by direct effects on T-cell activation and on the phenotype and function of antigen-presenting cells, especially dendritic cells.
> Vitamin E enhances immune function.
Vitamin E is one of the most effective fat-soluble vitamin nutrients. It is a chain-breaking, lipid-soluble antioxidant present in cell membranes, especially in those of immune cells, which protects them from oxidative damage. Supplemental vitamin E enhances both humoral and cell-mediated immune functions associated with increased resistance against several pathogens.6,7,8
1. Tan L, Wray AE, Ross AC. Oral vitamin A and retinoic acid supplementation stimulates antibody production and splenic Stra6 expression in tetanus toxoid-immunized mice. J Nutr 2012;142(8):1590-1595.
2. Cassanti B, Villablanca EJ, De Calisto J, et al. Vitamin A and immune regulation: role of retinoic acid in gut-associated dendritic cell education, immune protection and tolerance. Mol Aspects Med 2012;33(1):63-76.
3. Hall JA, Grainger JR, Spencer SP, et al. The role of retinoic acid in tolerance and immunity. Immunity 2011;35(1):13-22.
4. Garcia OP. Effect of vitamin A deficiency on the immune response in obesity. Proc Nutr Soc 2012;71(2):290-297.
5. Youssef DA, Miller CW, El-Abbassi AM, et al. Antimicrobial implications of vitamin D. Dermatoendocrinol 2011;3(4):220-229.
6. Bendich A. Vitamin E and immune functions. Basic Life Sci 1988;49:615-20.
7. Kelleher J. Vitamin E and the immune response. Proc Nutr Soc 1991;50(2):245-249.
8. Beharka A, Redican S, Leka L, et al. Vitamin E status and immune function. Methods Enzymol 1997;282:247-263.