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Vitamin D: An Overview, and Its Importance in Immune Health

By Dr. Alyssa Dweck, MS MD FACOG, practicing gynecologist in Westchester County, New York, and Tru Niagen Partner

Introduction to Vitamin D

Vitamin D is a fat-soluble vitamin also called calciferol. Its two main forms are cholecalciferol (also known as vitamin D3), and ergocalciferol (aka vitamin D2). When vitamin D is ingested through diet or synthesized in the skin from sunlight, it is then activated into the active forms in the liver and kidneys respectively. Vitamins D3 and D2 have subtle chemical differences, and both are well absorbed in the small intestine.  

Sources of Vitamin D

The primary source of vitamin D is through synthesis in the skin from sunlight and ultraviolet (UV) light exposure. Synthesis is dependent on multiple factors. Season, time of day, length of day and light exposure, cloud cover, smog, skin melanin content, and sunscreen can all affect UV light exposure and vitamin D synthesis. Older people and people with dark skin are less able to produce vitamin D from sunlight, and are thus more at risk for deficiency. It’s also worth noting that UV light is not absorbed through glass. Vitamin D is difficult to find in food. Natural food sources include fatty fish and fish liver oils.  Smaller amounts are found in beef liver, cheese, and egg yolks. Vitamin-D-fortified foods include dairy products like milk and yogurt, infant formula, orange juice, and breakfast cereals.  

Why Is Vitamin D Important?

Bone Health

We often think of Vitamin D as beneficial to bone health. Vitamin D is crucial to calcium and phosphorus homeostasis and subsequent bone metabolism. Vitamin D promotes calcium absorption in the gut and maintains adequate serum calcium and phosphate levels to enable normal bone mineralization. Normal bone mineralization is a result of highly specialized cells. Bone growth occurs via bone-building cells called osteoblasts. Bone remodeling occurs through osteoclasts—cells that break down bone. These cells work synergistically with one another to constantly remodel bone. 

Vitamin D along with calcium helps to protect against osteoporosis, or brittle bones.  Women are at substantial risk for osteoporosis and subsequent bone fracture during menopause, because estrogen levels decline significantly during this time, leaving bone strength vulnerable. Estrogen exposure protects bone integrity. As an aside, some younger women can also be at risk for bone loss and fracture, including those who fail to menstruate due to an eating disorder and excessive weight loss. These women also have limited estrogen exposure as a risk factor. Vitamin D may help to reduce risk of fractures and loss of bone density .

Vitamin D and Other Health Concerns

Vitamin D is associated with many other vital physiological functions. It helps to control inflammation, modulates cell growth, and is involved with neuromuscular and immune function and glucose metabolism.** Research suggests that low vitamin D is also associated with with a range of health conditions of varying severity. It’s important to note that association does not mean low vitamin D causes these conditions, or that taking a vitamin D supplement will adequately prevent or treat them.*****

Vitamin D and Immune Health

Suggestions linking vitamin D and immune function in part originate from observations of disease occurrence based on geographic location. Vitamin D is primarily obtained through synthesis in the skin following sun exposure, and levels of UV light vary according to latitude (distance from the equator). Higher latitude is often an indicator of lower levels of UV exposure and lower vitamin D status. The incidence or prevalence of a disease increases with increasing distance from the equator, and lower UV light exposure has been implicated in multiple sclerosis, type 1 diabetes, autoimmune vasculitis disorders, inflammatory bowel diseases, and asthma. Some observational studies have supported these findings. In case-control studies, participants with one of these autoimmune diseases tend to report lower past sun exposure, and/or have lower vitamin D status by serum vitamin D (25 hydroxy vitamin D, 25 (OH) D) levels, compared with healthy controls. In as much as autoimmune diseases affect women more than men, adequate vitamin D levels are of heightened significance for women, keeping in mind that further confirmatory research is needed.******

The recommended daily allowance (RDA) of vitamin D for people of ages 1 through 70 is 600IU (15mcg) daily, and 800IU (20mcg) is recommended for those over 70. Vitamin D is monitored with serum 1,25(OH)2 D or 25 (OH) D levels. Optimal levels vary based on stage of life, age, gender, race, and ethnicity. The Food and Nutrition Board of the National Academies (formerly the IOM) established an upper limit for vitamin D intake of 4000IU (100mcg)/day. In some cases, higher doses may be suggested based on individual needs.

Vitamin D Deficiency

In the past, severe vitamin D deficiency presented as rickets, which is  now-rare bone disease in children. Those who lack adequate sun exposure, have conditions of malabsorption, or diets poor in fortified foods are at risk for low vitamin D. Vitamin D deficiency can be diagnosed with a simple blood test, and supplementation can be suggested based on results.  

Older adults are at increased risk of developing vitamin D insufficiency, partly because the skin’s ability to synthesize vitamin D declines with age. In addition, older adults are likely to spend more time than younger people indoors, and they might have inadequate dietary intakes.

Vitamin D Excess

Excess amounts of vitamin D can be toxic. Because vitamin D increases calcium absorption in the gastrointestinal tract, vitamin D toxicity can result in excess calcium in the blood, aka hypercalcemia, which can lead to nausea, vomiting, muscle weakness, neuropsychiatric disturbances, pain, loss of appetite, dehydration, polyuria, excessive thirst, and kidney stones. In extreme cases, vitamin D toxicity causes renal failure, calcification of soft tissues throughout the body (including in coronary vessels and heart valves), cardiac arrhythmias, and even death. In general, signs and symptoms of toxicity are unlikely at daily intakes below 10,000 IU (250mcg).

Vitamin D Level Assessment

Serum blood testing for vitamin D (25(OH) D) can be measured. Interpretation and ranges may be lab–specific, but in general, levels of 20–50 ng/mL are considered adequate for most people for bone and overall health. Levels below 12 ng/mL are typically considered too low and may warrant supplementation. Levels above 50 ng/mL may warrant attention from your healthcare provider (HCP).   

Vitamin D Supplementation

Older adults may have limited ability to synthesize vitamin D from sunlight. Those who purposefully limit sun exposure with sunscreen, or keep their bodies and heads covered, or choose to remain indoors may warrant supplementation. Darker-skinned individuals tend to make less vitamin D from sunlight exposure. Those with conditions that limit fat absorption, like inflammatory bowel diseases or celiac disease, may also have lower vitamin D levels. Finally, those who are obese or have undergone gastric bypass procedures may need more vitamin D than others.

Avoidance of Vitamin D Supplements

Those with excessive serum vitamin D levels might have to diminish or discontinue supplementation. Vitamin D may interact with certain medicines, including Orlistat, a weight-loss drug, cholesterol-lowering statins, steroids, and specific diuretics. It’s important to check in with your HCP to make individual supplement decisions.


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