Vitamin D: The Deficiency You Probably Have

Vitamin D is technically a hormone, not a vitamin. Its receptor is found in virtually every cell in the body, and it regulates the expression of over 1,000 genes. Despite its fundamental biological importance, roughly 75% of adults in the United States have insufficient levels — a figure that rises sharply in northern latitudes, darker-skinned populations, and anyone who spends most of their time indoors.

Why Deficiency Is So Widespread

The primary source of vitamin D is cutaneous synthesis from UVB radiation. When UVB light hits the skin, 7-dehydrocholesterol is converted to previtamin D3, which isomerizes to vitamin D3 (cholecalciferol). This process is highly efficient in direct summer sun — 20–30 minutes of full-body sun exposure can generate 10,000–20,000 IU.

The problems are structural:

• Latitude: Above approximately 35° North (roughly the latitude of Los Angeles), UVB intensity is insufficient for adequate synthesis from October through March.
• Indoor lifestyle: The average American spends 93% of their time indoors. Windows block UVB.
• Sunscreen: SPF 15 reduces vitamin D synthesis by 99%.
• Skin pigmentation: Melanin is a natural UVB filter. Darker skin requires 3–5× longer sun exposure to produce equivalent vitamin D.
• Age: Vitamin D synthesis efficiency declines by approximately 75% between age 20 and 70.

Dietary sources are limited. Fatty fish (salmon, mackerel), egg yolks, and fortified foods provide modest amounts — typically 100–400 IU per serving — far below what the skin can produce or what research suggests is optimal.

The Consequences of Deficiency

Vitamin D is essential for calcium absorption, bone mineralization, and immune function — this much is widely taught. But the research has expanded significantly:

Immune system: Vitamin D activates T-cells and macrophages and is required for the production of antimicrobial peptides like cathelicidin. Low vitamin D is associated with increased susceptibility to respiratory infections, including influenza and COVID-19 in observational studies.

Cardiovascular health: Multiple large observational studies link low vitamin D to higher rates of hypertension, coronary artery disease, and stroke. The proposed mechanisms include effects on the renin-angiotensin system and vascular smooth muscle tone. Interventional trial results have been more mixed, suggesting deficiency correction is important but supraphysiologic dosing provides no additional benefit.

Mental health: Vitamin D receptors are densely expressed in the brain regions involved in mood regulation. Low vitamin D correlates with depression and seasonal affective disorder. Meta-analyses show supplementation modestly but consistently improves depressive symptoms, particularly in deficient individuals.

Mortality: The VITAL trial (n=25,871) found that vitamin D3 supplementation at 2,000 IU/day significantly reduced cancer mortality (by 17%) compared to placebo over a 5-year follow-up — one of the more striking findings from a large RCT in preventive medicine.

D3 vs. D2: The Form Matters

Vitamin D comes in two supplemental forms: D3 (cholecalciferol) and D2 (ergocalciferol). D3 is the form produced by the skin and found in animal products. D2 is derived from plant sources (irradiated yeast and fungi).

Multiple head-to-head trials confirm that D3 is significantly more effective at raising and sustaining serum 25(OH)D levels. D3 has a longer half-life, binds more effectively to vitamin D-binding protein, and generates more potent active metabolites. D2 is what most prescription vitamin D in the US is made from — a historical accident driven by patent considerations, not pharmacology. Always use D3.

The Critical Pairing: Vitamin K2

This is where most supplementation approaches go wrong. Vitamin D increases intestinal calcium absorption. Vitamin K2 (MK-7 form) is required to direct that calcium into bones and teeth rather than into arterial walls and soft tissues.

Low K2 combined with high-dose vitamin D creates the conditions for vascular calcification — the exact opposite of the cardiovascular benefit you are seeking. K2 activates matrix GLA protein (MGP) and osteocalcin, two proteins that regulate where calcium deposits. The Rotterdam study found that high dietary K2 intake reduced aortic calcification by 52% and cardiovascular mortality by 57%.

If you supplement vitamin D, supplement K2 alongside it. 100–200 mcg of MK-7 (menaquinone-7) is the standard recommendation. MK-7 is the preferred form over MK-4 due to its substantially longer half-life.

Optimal Dosing

The official RDA of 600–800 IU was set to prevent rickets, not to optimize health. Endocrinologists and vitamin D researchers increasingly use 25(OH)D blood levels as the relevant target.

Optimal serum level: 40–60 ng/mL (100–150 nmol/L).

Dosing to achieve this:

• Those with levels below 20 ng/mL often require 4,000–6,000 IU/day to reach optimal range.
• Maintenance for most adults who are insufficient: 2,000–4,000 IU/day of D3.
• Obese individuals require higher doses (vitamin D is fat-soluble and sequesters in adipose tissue).
• Toxicity (hypercalcemia) is a real but rare concern — it generally requires sustained doses above 10,000 IU/day for months without monitoring.

Testing

Ask your doctor for a 25-hydroxyvitamin D [25(OH)D] blood test. This is the standard measure of vitamin D status. Do not confuse it with 1,25-dihydroxyvitamin D (the active hormone), which is often normal even in deficiency and is not the appropriate screening test.

Test, supplement, retest at 3 months. Adjust dose as needed. This is the only way to know if your supplementation approach is working.