What it measures.
Vitamin D is actually a hormone that influences over 200 genes affecting immune function, bone health, mood, cardiovascular health, and cancer prevention. Deficiency is pandemic, affecting an estimated 1 billion people worldwide.
25-hydroxyvitamin D [25(OH)D] is the primary circulating form of vitamin D and the best marker of vitamin D status. It reflects both sun exposure and dietary/supplemental intake, with a half-life of about 3 weeks.
Why it matters.
Vitamin D deficiency is associated with increased risk of osteoporosis, autoimmune diseases, infections (including respiratory viruses), cardiovascular disease, depression, and cancer. Optimal levels are associated with reduced all-cause mortality. Many people need levels higher than the outdated 'sufficient' threshold of 30 ng/mL.
Physiology.
Vitamin D is produced in skin when UVB rays convert 7-dehydrocholesterol to vitamin D3. This is then converted in the liver to 25(OH)D (the measured form), then in the kidneys to active 1,25(OH)2D (calcitriol). Calcitriol acts as a hormone, binding to vitamin D receptors (VDRs) found in nearly every cell, regulating gene expression for immunity, cell growth, and calcium metabolism.
Testing & preparation.
How to prepare
- No fasting required
- Note recent supplementation (affects interpretation)
- Consider seasonal timing—levels are lowest in late winter
- Request 25-hydroxyvitamin D, not 1,25(OH)2D
When to test
Baseline test for everyone; recheck 2-3 months after starting or changing supplementation; annually once stable.
How often
Every 3-6 months while optimizing; annually once at target level.
Interpretation.
High vitamin d (25-oh)
Common causes:
- Excessive supplementation (>10,000 IU daily long-term)
- Granulomatous diseases (sarcoidosis)—unregulated conversion to active form
- Lymphoma
- Rare: Williams syndrome
Implications:
- Hypercalcemia risk (usually only >150 ng/mL)
- Kidney stones
- Soft tissue calcification
- At moderate elevations (80-100 ng/mL): likely safe but no additional benefit
Low vitamin d (25-oh)
Common causes:
- Inadequate sun exposure (latitude, season, sunscreen, indoor lifestyle)
- Darker skin pigmentation (requires more sun exposure)
- Older age (reduced skin synthesis)
- Obesity (vitamin D sequestered in fat tissue)
- Malabsorption (celiac, IBD, gastric bypass)
- Liver or kidney disease
- Medications (anticonvulsants, glucocorticoids)
Implications:
- Increased fracture and osteoporosis risk
- Weakened immune function
- Higher risk of respiratory infections
- Increased autoimmune disease risk
- Association with cardiovascular disease
- Depression and cognitive decline
- Increased cancer risk
Optimization.
Diet
- Fatty fish (salmon, mackerel, sardines): 400-1000 IU per serving
- Cod liver oil: very high in D3
- Egg yolks: ~40 IU each (pasture-raised higher)
- Fortified foods: milk, orange juice, cereals
- Mushrooms exposed to UV light
- Note: Diet alone rarely provides adequate vitamin D
Lifestyle
- Safe sun exposure: 10-30 minutes midday with arms and legs exposed
- Sun exposure between 10 AM-3 PM when UVB is sufficient
- More exposure needed at higher latitudes, in winter, and with darker skin
- Glass blocks UVB—outdoor exposure required
- Maintain healthy weight (obesity reduces bioavailability)
Supplements
- Vitamin D3 (cholecalciferol) preferred over D2
- Typical maintenance: 2,000-5,000 IU daily for most adults
- Loading dose for deficiency: 50,000 IU weekly for 8 weeks
- Take with fat-containing meal for absorption
- Consider vitamin K2 (MK-7) to direct calcium to bones
- Obese individuals may need 2-3x higher doses
FAQs.
Why is my vitamin D still low despite supplementation?
Common reasons: (1) Insufficient dose—most people need 2,000-5,000 IU daily, not 400-800 IU; (2) Not taking with fat—vitamin D is fat-soluble; (3) Obesity—D gets trapped in fat tissue; (4) Malabsorption issues; (5) Genetic variations in vitamin D metabolism. Consider higher doses and retest in 3 months.
Can I get enough vitamin D from the sun?
In theory, yes—10-30 minutes of midday sun with significant skin exposed produces ~10,000-20,000 IU. In practice, most people can't: they live at high latitudes, work indoors, wear sunscreen, or have darker skin. Between October-April at latitudes above 35°N, UVB is insufficient for vitamin D synthesis regardless of sun exposure.
Is vitamin D toxicity a real concern?
Toxicity is rare and typically requires >10,000 IU daily for months, reaching blood levels >150 ng/mL. Symptoms include hypercalcemia (nausea, weakness, kidney stones). At levels of 60-100 ng/mL, there's no evidence of harm. Regular testing allows safe optimization.
Should I take vitamin K2 with vitamin D?
K2 (especially MK-7 form) helps direct calcium to bones rather than arteries. While not strictly required, it's a reasonable addition when taking higher doses of vitamin D. Typical dose: 100-200 mcg MK-7 daily. K2 is found in fermented foods, hard cheeses, and egg yolks.
What's the difference between vitamin D2 and D3?
D3 (cholecalciferol) is the form your body makes from sun and is found in animal sources. D2 (ergocalciferol) comes from fungi/plants and is less effective at raising blood levels. D3 raises 25(OH)D more effectively and maintains levels longer. Choose D3 when possible.