Known for its link to sunshine, vitamin D is a micronutrient associated with a variety of vital functions in the body. Particularly important is vitamin D’s support of the immune system, both the non-specific innate response and the targeted adaptive response.
According to the NIH Office of Dietary Supplements, adults between 19 and 70 years old should aim for 15 micrograms, or 600 International Units (IU), of vitamin D every day. A 2017 Reuters Health report found that almost one in five American adults take supplemental vitamin D in effort to reach that goal. However, many still fall short.
Vitamin D as A Hormone
1,25(OH)2D is a bioactive vitamin D metabolite that is also a steroid hormone responsible for activating the vitamin D receptor (VDR).1 Vitamin D’s activity as a hormone distinguishes it from other micronutrients. Both vitamins and hormones are essential for life, but by definition, vitamins cannot be produced on their own in the human body; they must be obtained from the diet. However, hormones are produced within the human body and serve to transmit chemical messages all over the body.
Vitamin D can be thought of as a hormone because direct skin exposure to ultraviolet B (UVB) rays in sunlight stimulate vitamin D production in the body. Vitamin D is rarely found as a natural ingredient of food, but it can be found in foods fortified with various vitamins and minerals. More direct sources of vitamin D come from dietary supplements and sunlight.
Vitamin D Metabolites in the Immune System
Research shows that vitamin D metabolizing enzymes (CYP27B1) and receptors (VDR) are present in various immune cells, including antigen-presenting cells, T cells, B cells, and monocytes. Calcitriol, the active form of vitamin D normally produced in the kidneys, is associated with B cell homeostasis, especially in the context of autoimmunity.2 Respiratory monocytes and macrophages and epithelial cells express VDR, allowing vitamin D to support protection from respiratory infections.3
Vitamin D is involved in a variety of immune activities. First, this micronutrient modulates innate immune cells, improving the antimicrobial effects of macrophages and monocytes that serve as “first-responders” when the innate immune system indicates a pathogenic attack.4
Vitamin D is also important for supporting regulatory T cells that control immune tolerance.5 This regulation is important for resolving necessary immune attacks on external invaders in a timely manner as well as supporting the immune system’s ability to recognize benign external components, such as food particles, and to distinguish between the body’s own cells (“self”) and potentially harmful pathogenic cells. If the immune system can’t tell friend from foe, it might accidentally trigger friendly fire, resulting in various autoimmune conditions.
Lastly, vitamin D supports the immune system through regulating the inflammatory process. Vitamin D metabolites are associated with both inhibiting pro-inflammatory cytokines and promoting anti-inflammatory cytokines.6,7
Vitamin D Deficiency: Immune Consequences
Data from the 2005-2006 National Health and Nutrition Examination Survey (NHANES) showed that over 40 percent of Americans have insufficient vitamin D levels.8 Studies have found that indicators of suboptimal vitamin D status are associated with upper respiratory tract infections.9-11 Additionally, vitamin D deficiency is associated with the development of autoimmune disorders such as:12
- Type 1 diabetes
- Multiple sclerosis
- Inflammatory bowel disease
There are also many consequences of vitamin D insufficiency not related to immune health, such as rickets in children and periosteal bone pain in adults (more common in less developed countries) as well as links to cardiovascular disease, cancer, and diabetes (more common in developed countries).8
Vitamin D is an important micronutrient for multiple systems in the body, the immune system included. It plays a particularly vital role in supporting antimicrobial activity, immune tolerance, and inflammatory regulation. And while accessing vitamin D through sunlight is effective, supplementation may be necessary to reach optimal levels of this significant micronutrient.