In the aftermath of the COVID-19 pandemic, more research has come to light illustrating the role of vitamin D in regulating immune system function. As well, more people have become interested in supplementing with vitamin D to boost potentially deficient levels – especially those who live in regions with less direct sunlight.
Vitamin D is one of the most well-documented and studied supplements. A large body of evidence demonstrates vitamin D’s role in systemic health and immune system function.
Vitamin, or hormone? What is the role of vitamin D in endocrine function, cardiovascular health, and other metabolic pathways? More specifically, what is the role of vitamin D in immune system health?
While the primary role of vitamin D is to regulate calcium metabolism and bone mineralization, vitamin D is multifaceted. It regulates T and B cells, macrophages, and dendrites. This correlates to the role of vitamin D deficiency in autoimmune disease. Thus, vitamin D can be explored as a potential option for treating immune diseases, autoimmune conditions, and illness.
Safe Vitamin D Doses – How Much Is Too Much?
Research demonstrates a high-safety profile and tolerance, even when consumed in supraphysiological doses of 4,000 IU per day.
That isn’t to say vitamin D can’t be overdosed.
Some people are overdoing it on vitamin D supplements in the hopes of reducing risk of mood disorders, dementia, diabetes, heart disease, and cancer. These people are likely doing more harm than good.
Vitamin D isn’t just another nutrient, like A or C. It’s functionally a hormone with receptors in cells and tissues throughout the body (1).
Excess vitamin D supplementation can also lead to hypercalcemia, where too much calcium builds up in the blood, rather than being deposited into bones. This can predispose people to arterial deposits of calcium, increasing risk of stroke (2).
4,000 IU cholecalciferol (D3) per day is well defined as the upper limit for intake of supplemental vitamin D, and has proven efficacy in improving immune system function. However, some researchers contest this, as doses as low as 3,800 IU have been implicated in hypercalcemia when consumed long-term.
In one study, elevated serum calcium was found in 6 subjects consuming 3,800 IU cholecalciferol daily for 3 months. Comparatively, a larger 3-year study looked at doses of 400 IU, 4000 IU, and 10,000 IU cholecalciferol administered daily. Hypercalcemia occurred in 0% (400 IU), 3% (4,000 IU) and 9% (10,000 IU) of patient populations, respectively (3).
Long-term, we recommend a maximum of 1,000 IU per day, as D3, for prevention of vitamin D insufficiency and improved health biomarkers. This dose also completely reduces the risk of hypercalcemia in healthy adults.
How Does Vitamin D Impact the Immune System?
Vitamin D is a nutrient that is absorbed from exposure to UV-B rays (sunlight), and is also a hormone produced by our kidneys.
It is most commonly known as ergocalciferol (vitamin D2) or cholecalciferol (vitamin D3).
The liver and kidneys both convert vitamin D (produced in the skin from sunlight exposure, or ingested through diet) into the active hormone, which is called calcitriol or calcifediol. This active form of vitamin D helps to ensure calcium is properly absorbed from food into the bloodstream, and prevents calcium loss in the kidneys.
This relationship to calcium absorption, metabolism, and utilization means it has an important role in bone density. It’s important for proper bone formation in adults and children.
What about the role of vitamin D in immune system function?
- The active form of vitamin D, calcitriol, lowers the damaging immune response of white blood cells and pro-inflammatory cytokines.
- The active form of vitamin D, calcitriol, boosts the immune cell mediated production of “microbe-fighting” proteins – protecting against cold and flu.
- Adults with the highest serum levels of vitamin D had an up to 44% lower risk of developing Type 1 diabetes, compared to those with the lowest (4, 5).
- Adults with low vitamin D levels are more likely to report a cold, cough, or respiratory infection (6).
- Adults with the highest serum levels of vitamin D had a 62% lower risk of developing multiple sclerosis, compared to those with the lowest levels (7).
It is only within the last 30 years that the importance of vitamin D in function of the immune system has come to the forefront of related research.
It has been suggested that vitamin D is responsible for suppression of autoimmune disease by decreasing cytokines IL-2 and IFN-γ production, and increasing IL-4 expression. Administering a vitamin D analogue to diabetic mice, for example, inhibited the progression of diabetes (8).
Numerous other studies clearly show lower serum levels of vitamin D in patients with autoimmune disorders, including asthma, MS, diabetes, and reactive arthritis.
Vitamin D, thus, is suspected to help enhance and protect our immune system. It is produced by numerous cells within the immune system, and is involved in various immune regulated functions.
Supplementing Vitamin D Safely
When it comes time to supplement with vitamin D, most studies suggest vitamin D3 is more effective than vitamin D2 at raising blood levels. Vitamin D2 is routinely associated with lower increases in serum vitamin D levels, when compared to D3.
Vitamin D is a fat-soluble vitamin. As dietary fat increases vitamin D3 absorption, we always recommend opting for a supplement that contains dietary fatty acids (sunflower oil, olive oil, etc.) in a softgel or liquid form.
You don’t necessarily need a daily intake of vitamin D, as it is stored in your body fat. However, around 800 IU-1,000 IU daily has been administered in patients for years on a daily basis, without adverse reaction or hypercalcemia reported. We’d err on the side of caution when it comes to people going overboard with 10,000 IU daily doses.