Evidence check: Vitamin D 25-OH in the context of longevity

The evidence for vitamin D (25-OH) as a direct longevity factor is mixed. Observational studies consistently show that low 25(OH)D levels (< 30 nmol/l) are associated with increased all-cause mortality, even after adjusting for age, comorbidities, and lifestyle. Mechanistically, vitamin D influences calcium homeostasis, immune function, inflammation regulation, and cell differentiation, which could theoretically modulate aging-related processes. However, randomized controlled trials (RCTs) of vitamin D supplementation in generally healthy adults without proven deficiency have not demonstrated a significant survival benefit. Post-hoc analyses suggest potential benefit in those with very low baseline levels, but overall evidence does not support a general recommendation for longevity enhancement. Importantly, very high doses (> 4000 IU/day long-term) can be toxic (hypercalcemia, kidney stones). A 25(OH) blood test is useful to rule out deficiency before starting supplementation. The evidence grade is therefore 'mixed': strong association in observational data but weak to absent confirmation in interventions for the endpoint longevity.