What vitamin D actually is
Vitamin D is a fat-soluble hormone (technically; called a "vitamin" historically) produced in skin from cholesterol when exposed to UVB light. The active form (calcitriol) is produced from 25-OH-vitamin D in kidney and other tissues.
Reference vs optimal
- Reference "normal": 30-100 ng/mL
- Optimal: 40-60 ng/mL
- Below 30: deficient
- Below 20: severely deficient
- Above 100: potentially excessive (rare from sun; from supplementation)
Deficiency prevalence
In temperate climates with limited winter sun, deficiency is extremely common, often 40-80% of adults during winter months. Risk factors:
- Limited sun exposure
- Darker skin (more melanin requires more sun for synthesis)
- Higher latitude
- Older age (reduced skin synthesis)
- Obesity (sequestered in fat)
- Malabsorption
What it does
- Bone health (calcium absorption, bone mineralization)
- Immune function (T-cell regulation)
- Mood (low D associated with depression)
- Cardiovascular function
- Insulin sensitivity
- Testosterone production (low D is associated with low T)
- Cancer risk (signals from observational data)
Supplementation
- Typical dose: 2,000-5,000 IU daily
- Severe deficiency: 5,000-10,000 IU daily for repletion
- Take with fat for absorption
- D3 form preferred over D2
- K2 (MK-7) cofactor sometimes paired
Monitoring
- Test baseline
- Re-test at 3 months when starting supplementation
- Re-test annually once stable
- Adjust dose to maintain 40-60 ng/mL
The clinical pearl: Vitamin D deficiency is one of the easiest deficiencies to fix. Test baseline; supplement if needed; re-test. Most adults benefit from 2,000-5,000 IU daily during low-sun months.
Bottom line
Vitamin D affects far more than bone. Optimal range is 40-60 ng/mL, higher than basic reference. Deficiency is common and easy to fix. Test routinely and supplement appropriately.
40-60
ng/mL optimal
40-80%
winter deficiency rates in temperate climates
2,000-5,000
IU typical daily dose