Vitamin D

Vitamin D is a fat-soluble secosteroid hormone produced by the skin in response to UV-B light and obtained in smaller amounts from diet and supplements. It regulates over 1,000 genes and plays foundational roles in calcium absorption, bone remodeling, immune modulation, and cellular differentiation. Despite being producible through sun exposure, deficiency is widespread , affecting an estimated 40–50% of adults globally, with higher rates among older adults, people with darker skin, and those in northern latitudes.

Mortality Reduction {#mortality-reduction}

The most compelling longevity evidence for vitamin D comes from large-scale meta-analyses of randomized controlled trials. A 2023 systematic review and meta-analysis of 80 RCTs encompassing over 163,000 participants found that vitamin D supplementation was associated with a statistically significant reduction in all-cause mortality (OR: 0.95, 95% CI 0.91–0.99). An earlier meta-analysis of 50 trials with 74,655 participants similarly found a significant 7% reduction in all-cause mortality with supplementation. The benefit appears most pronounced when correcting pre-existing deficiency , supplementation in already-replete individuals shows weaker effects. Animal research in C. elegans models shows vitamin D3 directly extends lifespan and improves healthspan through protein homeostasis pathways involving genes SKN-1, IRE-1, and XBP-1, suggesting mechanistic plausibility beyond observational data.

Bone Health {#bone-health}

Vitamin D’s role in bone metabolism is the best-established benefit. It increases intestinal calcium absorption from approximately 10–15% (when deficient) to 30–40% (when sufficient), directly enabling bone mineralization. Deficiency causes rickets in children and osteomalacia in adults , conditions of impaired bone mineralization. In older adults, adequate vitamin D status is associated with reduced fracture risk and slower bone density loss. Supplementation trials consistently show reduced fracture rates in populations with baseline deficiency, though supplementation without confirmed deficiency shows more modest effects.

Immune Function {#immune-function}

Vitamin D receptors are present on virtually all immune cells, and active vitamin D (1,25-dihydroxyvitamin D) directly modulates both innate and adaptive immune responses. It enhances the pathogen-killing capacity of macrophages, regulates T-cell differentiation, and promotes immune tolerance to reduce autoimmune activity. Examine.com’s meta-analysis of 107 trials involving over 1.27 million participants identifies upper respiratory tract infection reduction as a well-supported benefit (grade A–B evidence). Low vitamin D levels are consistently associated with higher incidence and severity of respiratory infections, with supplementation studies showing meaningful protective effects particularly in deficient individuals.

Mood Support {#mood-support}

A systematic review and meta-analysis of 14 studies with over 31,000 participants found significantly lower vitamin D levels in people with depression compared to controls, with those in the lowest vitamin D quartile showing substantially higher depression risk. While this association does not establish causation, and the mechanism is not fully characterized, vitamin D receptors are present throughout the brain , including areas regulating mood such as the prefrontal cortex, hippocampus, and hypothalamus. Current evidence is strongest for individuals with confirmed deficiency, where correcting low levels may improve depressive symptoms. Routine supplementation for mood benefit in replete individuals lacks robust RCT support.

Telomere Preservation {#telomere-preservation}

A 2025 RCT from the VITAL trial (1,054 participants, 4 years, double-blind placebo-controlled) found that 2,000 IU/day of vitamin D3 significantly reduced telomere attrition by 0.14 kilobase pairs over four years compared to placebo. Telomeres are protective caps at the ends of chromosomes that shorten with age; their rate of attrition is a validated biomarker of cellular aging and disease risk. The 140 base-pair preservation corresponds to an estimated 3 years of reduced cellular aging. Notably, omega-3 supplementation in the same trial showed no significant effect on telomere length, suggesting this is a vitamin D-specific mechanism. Animal model research in C. elegans has identified the molecular pathways involved: vitamin D3 engages longevity-associated genes SKN-1, IRE-1, and XBP-1 to promote protein homeostasis and extend lifespan, providing a mechanistic basis for the RCT findings.

Dosing

The commonly recommended supplementation dose for most adults is 1,000–2,000 IU per day, with higher doses (up to 4,000 IU/day) used to correct confirmed deficiency under physician guidance. The daily upper tolerable limit is typically set at 4,000 IU/day for long-term use. Vitamin D is fat-soluble, so it should be taken with a meal containing fat for optimal absorption. Testing 25(OH)D blood levels is the most reliable way to confirm status, with sufficiency generally defined as 30–50 ng/mL. People with dark skin, limited sun exposure, or aged 65 and older are at elevated risk for deficiency and most likely to benefit from supplementation.