Vitamin D: The Complete Guide to the Sunshine Vitamin

Vitamin D deficiency is one of the most common nutritional deficiencies worldwide, affecting an estimated 1 billion people globally. In the United States, approximately 42% of adults have insufficient vitamin D levels (below 20 ng/mL), with rates significantly higher among certain populations: 82% of Black Americans, 69% of Hispanic Americans, and 60% of adults over 70 are deficient. The primary reason is simple — modern humans spend most of their time indoors, and when they do go outside, they often wear sunscreen that blocks UVB radiation needed for vitamin D synthesis. Geographic location matters enormously: people living above the 37th parallel (roughly above Atlanta, Georgia) cannot produce adequate vitamin D from sunlight during the winter months, regardless of how much time they spend outdoors. The angle of the sun is simply too low for UVB rays to penetrate the atmosphere effectively.

The gold standard test for vitamin D status is the serum 25-hydroxyvitamin D [25(OH)D] test, which measures the circulating form of vitamin D in your blood. This test reflects vitamin D from both sun exposure and supplementation. The test is widely available and can be ordered by any physician, or through direct-to-consumer lab services. Results are reported in ng/mL (nanograms per milliliter) or nmol/L (nanomoles per liter). To convert: ng/mL × 2.5 = nmol/L. Most experts recommend testing at least once per year, ideally at the end of winter when levels are lowest. If you're supplementing, retest 3 months after starting or changing your dose to allow levels to stabilize.

There are two main supplemental forms of vitamin D: D3 (cholecalciferol) and D2 (ergocalciferol). D3 is the form naturally produced by human skin in response to sunlight and is found in animal-based foods. D2 is produced by fungi and yeast when exposed to UV light and is the form found in fortified foods and some prescription supplements. The evidence overwhelmingly favors D3. A landmark study published in the Journal of Clinical Endocrinology & Metabolism found that D3 was approximately 87% more potent than D2 in raising and maintaining serum 25(OH)D concentrations. D3 also has a longer half-life in the body, meaning it maintains blood levels more effectively between doses. The only advantage of D2 is that it's traditionally been considered vegan-friendly (though vegan D3 from lichen is now widely available).

The recommended daily allowance (RDA) of 600 IU for adults was set by the Institute of Medicine in 2011 and is considered by many experts to be inadequate for achieving optimal blood levels. The Endocrine Society recommends 1,500–2,000 IU daily for adults at risk of deficiency, while many functional medicine practitioners recommend 4,000–5,000 IU daily based on individual blood levels. The key principle is that dosing should be personalized based on your current 25(OH)D level, body weight, and goals. A general rule of thumb: every 1,000 IU of D3 supplementation raises blood levels by approximately 10 ng/mL, though this varies significantly between individuals. Obese individuals typically need 2–3 times more vitamin D due to sequestration in fat tissue. Daily dosing is preferred over weekly or monthly mega-doses, as it more closely mimics natural production and maintains more stable blood levels.

Vitamin D doesn't work in isolation — it requires several co-factors for optimal function. The most important is vitamin K2 (menaquinone-7 or MK-7). Vitamin D increases calcium absorption from the gut, but it's vitamin K2 that directs that calcium to your bones and teeth rather than your arteries and soft tissues. Without adequate K2, high-dose vitamin D supplementation could theoretically contribute to arterial calcification. A typical recommendation is 100–200 mcg of K2 (MK-7) for every 5,000 IU of D3. Magnesium is another critical co-factor — it's required for the enzymes that convert vitamin D to its active form (1,25-dihydroxyvitamin D). An estimated 50% of Americans are magnesium deficient, which can impair vitamin D metabolism even when supplementing. Other important co-factors include zinc (required for vitamin D receptor function) and boron (may reduce vitamin D excretion).