Overview

Vitamin D is a fat-soluble hormone (technically not a vitamin) critical for calcium absorption, bone health, immune function, and numerous physiological processes. Unlike most nutrients, vitamin D is primarily obtained through sun exposure (UV-B radiation triggering skin synthesis) rather than diet, making deficiency extremely common, particularly in higher latitudes, during winter months, and in those with limited sun exposure or darker skin.

Primary applications focus on bone health and osteoporosis prevention, immune system support and infection resistance, mood support and seasonal affect ive disorder (SAD), deficiency correction (widespread issue affecting billions globally), and potential benefits for autoimmune conditions and cardiovascular health.

Evidence quality is extremely strong for bone health and deficiency consequences, good for immune function, moderate for mood and other applications.

Safety is excellent at recommended doses (1000-4000 IU daily) though toxicity can occur at very high chronic doses (typically above 10,000 IU daily for extended periods).

Functions, Deficiency, and Supplementation

Core Functions

Primary functions: calcium and phosphorus absorption (essential for bone mineralization), bone metabolism regulation, immune cell function modulation (T cells, macrophages), gene expression regulation (vitamin D receptors in most tissues), and neurotransmitter synthesis support.

Pandemic Deficiency

Deficiency is pandemic with estimates suggesting 1 billion+ people globally have insufficient vitamin D. Risk factors: limited sun exposure, living in northern latitudes (above ~35° north or below ~35° south), winter months, darker skin pigmentation (melanin blocks UV-B), sunscreen use, aging (reduced skin synthesis), obesity (fat-soluble D sequestered in adipose), malabsorption disorders, and strict veganism (dietary D limited to fortified foods).

Consequences of deficiency include rickets in children (bone deformities), osteomalacia in adults (bone softening, pain), osteoporosis risk, increased infection susceptibility, muscle weakness, depression and seasonal affective disorder, and potential increased risk for autoimmune diseases and certain cancers (associations, not definitively proven).

Testing and Optimal Levels

Testing: 25-hydroxyvitamin D [25(OH)D] blood test measures vitamin D status. Optimal levels are debated: <20 ng/mL is deficient, 20-30 ng/mL is insufficient, 30-50 mg/mL is generally considered optimal, and above 50 ng/mL is potentially excessive (though toxicity rare below 100+ ng/mL).

Forms: Vitamin D2 (ergocalciferol, plant-derived, less effective at raising blood levels), and Vitamin D3 (cholecalciferol, animal-derived or lichen for vegans, more effective, preferred form).

Dosing and Sun Exposure

Dosing: 1000-2000 IU daily for maintenance in most adults. 2000-4000 IU daily for deficiency correction or limited sun exposure. Higher doses (5000-10,000 IU) for severe deficiency under medical supervision. Take with fat-containing meals for optimal absorption (fat-soluble).

Sun exposure: 10-30 minutes of midday sun on arms and legs several times weekly can provide adequate D for many (varies by skin tone, latitude, season). Balance sun exposure benefits with skin cancer risks.

Safety and Nutrient Synergies

Toxicity is rare but possible with chronic megadoses. Symptoms: hypercalcemia (elevated blood calcium causing nausea, vomiting, weakness, kidney problems). Occurs typically at sustained intake above 10,000 IU daily or blood levels above 100-150 ng/mL.

Nutrient interactions: Vitamin D increases calcium absorption. Ensure adequate calcium intake but not excessive (to avoid hypercalcemia). Magnesium is required for vitamin D activation and function. K2 works synergistically to direct calcium to bones vs soft tissues.

Vitamin D supplementation is valuable for most people given widespread deficiency, with particular importance for those with limited sun exposure, darker skin in northern latitudes, and winter months, with excellent safety at appropriate doses.

References

Holick MF. Vitamin D deficiency. N Engl J Med. 2007;357(3):266-281.

Autier P, Boniol M, Pizot C, Mullie P. Vitamin D status and ill health: a systematic review. Lancet Diabetes Endocrinol. 2014;2(1):76-89.