The Importance of Vitamin D for the Proper Function of the Immune System

Key Mechanisms of Vitamin D Action in the Body

Understanding the physiological significance of vitamin D in the function of the immune system has been primarily contributed by increasing knowledge from epidemiological studies on the association between vitamin D deficiency and various health problems. Foremost among these are autoimmune diseases such as multiple sclerosis, type 1 diabetes, and Crohn's disease, as well as tuberculosis.

The second factor that has clarified the role of vitamin D in the functioning of the immune system is the understanding of the mechanisms of this relationship. We now know that immune system cells can convert 25-hydroxyvitamin D to the active 1,25-hydroxyvitamin D and respond to the presence of this active form of vitamin D. These mechanisms are involved in the macrophage response to the presence of pathogens and in the regulation of antigen-presenting dendritic cell maturation. Through this last mechanism, vitamin D could regulate the function of T lymphocytes, which show a direct response to the presence of 1,25-hydroxyvitamin D, particularly in the development of suppressor regulatory T lymphocytes.

Findings from Studies

Studies with human cell cultures and animal models have shown strong effects of vitamin D on both innate and adaptive immunity in various tissues. The results indicate that vitamin D plays a role in eliminating pathogens such as Mycobacterium tuberculosis, and also suppresses the potential adverse effects of chronic inflammation. Therefore, it has the ability to influence the course of many immune diseases, particularly infectious and autoimmune.

All these findings indicate that vitamin D is a crucial factor linking innate and adaptive immunity and that their functions can be impaired in its deficiency.

International Recommendations

Given that vitamin D deficiency is very common across all age groups, the international expert working group for practical recommendations in endocrinology – Endocrine Practice Guidelines Committee – recommends determining the serum level of 25-hydroxyvitamin D in all individuals at risk of deficiency. Supplementation is then recommended for those with deficiencies. The recommended daily intake of vitamin D for different age groups and the upper tolerable intake limits are shown in the table below.

Table.  Recommended daily intake of vitamin D and upper intake limits

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Sources:
1. Holick M. F., Binkley N. C., Bischoff-Ferrari H. A. et al.; Endocrine Society. Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab 2011; 96 (7): 1911–1930, doi: 10.1210/jc.2011-0385. 
2. Hewison M. Vitamin D and immune function: an overview. Proc Nutr Soc 2012; 71 (1): 50–61, doi: 10.1017/S0029665111001650.