Vitamin D

The most common deficiency

Vitamin D deficiency affects an estimated one billion people worldwide, making it the most prevalent nutritional shortfall in industrialized societies. Despite its name, vitamin D is a secosteroid hormone with receptors (VDR) in nearly every cell type. Your skin produces it when exposed to UVB radiation at 290-315 nanometers, your liver converts it to 25-hydroxyvitamin D (25-OH-D), and your kidneys activate it into calcitriol, the potent hormonal form that regulates over 1,000 genes ^[1].

Optimal blood levels for longevity

Standard reference ranges classify 25-OH-D above 30 ng/mL (75 nmol/L) as sufficient. Longevity-focused research sets the bar higher: 40-60 ng/mL (100-150 nmol/L). A large meta-analysis found that serum levels in this range were associated with the lowest all-cause mortality risk ^[2]. A 2025 study of U.S. adults calculated that reaching sufficient vitamin D levels could prevent substantial premature deaths annually, adding measurable years of life expectancy at a population level ^[3]. Levels below 20 ng/mL are classified as deficient and are linked to significantly increased risks of osteoporosis, autoimmune disease, cardiovascular events, depression, and certain cancers.

Vitamin D and biological aging

Recent research has moved beyond deficiency correction into active anti-aging territory. The VITAL trial, a randomized controlled study of over 25,000 participants, found that 2,000 IU of daily vitamin D3 reduced confirmed autoimmune disease by 22% over 5.3 years ^[4]. A 2025 analysis from the same trial showed that vitamin D supplementation preserved telomere length in white blood cells, equivalent to roughly 3 years of reduced biological aging ^[5]. When combined with omega-3 fatty acids and exercise in the DO-HEALTH trial, vitamin D showed additive protective effects on epigenetic aging clocks ^[6].

Dosing and supplementation

Vitamin D3 (cholecalciferol) is the preferred form, being approximately 87% more effective than D2 (ergocalciferol) at raising serum levels ^[7]. Maintenance doses of 1,000-2,000 IU daily work for people with adequate baseline levels. Those with deficiency often need 4,000-10,000 IU daily for 8-12 weeks. D3 is fat-soluble, so take it with a meal containing fat. A cohort study documented a 50% increase in absorption when taken with a fatty meal versus on an empty stomach.

Two cofactors matter. Vitamin K2 (MK-7 form, 100-200 micrograms daily) directs calcium toward bones and teeth instead of arteries and soft tissue. Magnesium is required for vitamin D metabolism, and deficiency impairs conversion to active calcitriol. Without adequate magnesium, supplementing vitamin D alone may not raise your levels effectively.

Immune function

Vitamin D stimulates production of cathelicidin and defensins, antimicrobial peptides that form the first line of defense against pathogens. It simultaneously dampens excessive inflammatory cytokine production, helping prevent autoimmune overreaction. Randomized controlled trials show that daily supplementation reduces acute respiratory infections by about 12%, with stronger benefits in people who were deficient at baseline ^[8].

Who needs to pay extra attention

• At latitudes above 35 degrees North, UVB intensity is insufficient for vitamin D production during winter months
• People with darker skin pigmentation produce less vitamin D per unit of sun exposure
• Obesity sequesters vitamin D in fat tissue, requiring 2-3x higher doses for equivalent serum levels
• Genetic polymorphisms in VDR, CYP2R1, and GC genes can alter individual requirements and response to supplementation
• Test your 25-OH-D levels at least annually, ideally at the end of winter when levels are lowest