Vitamin D

Synthesis and metabolism: Cutaneous production: 7-dehydrocholesterol in skin exposed to UVB (290-315 nm) is converted to cholecalciferol (D3). 15-30 min/day without sunscreen at sufficient UVB hours (10 AM-2 PM in temperate latitudes) produces between 5,000-25,000 IU depending on phototype and exposed surface. Hepatic and renal conversion: D3 → 25-OH-D (measured in blood tests, reflects body status) → 1,25-OH-D (active form, renal regulation).

25-OH-D values: Deficiency: <20 ng/mL — high bone and immune risk Insufficiency: 20-30 ng/mL — most Spaniards Optimal general sufficiency: 40-60 ng/mL Optimal longevity range: 50-80 ng/mL (Holick, Hollis) Toxicity: >150 ng/mL chronic — hypercalcaemia

Factors that reduce status: latitude >35° with low winter exposure, dark skin pigmentation, constant sunscreen use, pollution (UVB does not cross pollution), ageing (skin production drops 50% at age 70), obesity (sequestration in adipose tissue), kidney/liver insufficiency, VDR polymorphisms, intestinal malabsorption.

Supplementation protocol: Generic dose: 2,000-4,000 IU/day D3 (cholecalciferol, NOT ergocalciferol D2 which is less potent). If deficiency: 10,000 IU/day for 8-12 weeks, then maintenance. Essential cofactors: vitamin K2 (MK-7, 100-200 μg/day — directs calcium to bone rather than arteries), magnesium (cofactor of hydroxylases), zinc. Measurements: 25-OH-D every 3-6 months until target reached, then annually.

Robust clinical evidence: fracture prevention in older adults, reduction of respiratory infections (especially in deficient — Martineau 2017 meta-analysis), immune modulation (multiple sclerosis, autoimmunity), prevention of some cancers (VITAL studies).