Vitamin D

What is Vitamin D?

Vitamin D (25-hydroxyvitamin D) is a fat-soluble secosteroid hormone synthesized in the skin from cholesterol upon UVB exposure, and obtained from fatty fish, fortified foods, and supplements. The liver converts it to 25(OH)D (the measured form), then the kidneys convert it to active 1,25(OH)2D (calcitriol). It regulates over 1000 genes.

What might a high or low Vitamin D mean?

Vitamin D deficiency is very common worldwide. Beyond its role in calcium absorption and bone health, vitamin D is involved in immune function, inflammation, muscle function, and mood. Low vitamin D is associated with greater risk of infections, autoimmune conditions, cardiovascular disease, and other outcomes.

What is the typical reference range for Vitamin D?

The general-population reference range shown here is 75 – 250 nmol/L. Reference ranges describe the general population and are not a personal target — discuss your results with your physician.

What can affect Vitamin D?

It may be higher with: Sun exposure (UVB — 15-30 min midday, skin type dependent), Vitamin D3 supplementation (cholecalciferol — preferred form), Fatty fish (salmon, mackerel, sardines), Cod liver oil, Fortified foods (milk, cereals), UV lamps (medical-grade). It may be lower with: Limited sun exposure (northern latitudes, indoor lifestyle), Dark skin pigmentation (more melanin blocks UVB), Obesity (vitamin D sequestered in fat tissue), Aging (reduced skin synthesis capacity), Malabsorption (celiac, Crohn's, gastric bypass), Kidney disease (impaired activation), Sunscreen use (SPF 30 blocks 97% UVB).