Vitamin D Testing: Optimal Levels and What Your Result Means

The Test: 25-Hydroxyvitamin D

The correct test for vitamin D status is serum 25-hydroxyvitamin D (written as 25-OH D or 25(OH)D). This is the storage form of vitamin D — the form that circulates in blood and reflects your body's total vitamin D supply from sun exposure and supplementation combined.

Note: This is different from 1,25-dihydroxyvitamin D (calcitriol) — the activated hormonal form. Calcitriol is tightly regulated and stays normal even in severe deficiency, making it useless for status assessment.

Interpreting Your Result

The Standard Thresholds (Designed for Bone Health Only)

Most laboratory reference ranges classify vitamin D status as:

• Deficient: Below 20 ng/mL (50 nmol/L)
• Insufficient: 20–29 ng/mL
• Sufficient: 30 ng/mL and above

These thresholds were established specifically to prevent rickets and osteomalacia — the bone-softening diseases caused by severe vitamin D deficiency. They say nothing about optimal immune function, cardiovascular health, or neuromuscular performance.

The Optimal Range for Full Systemic Function

Researchers studying vitamin D's non-skeletal effects — immune regulation, cardiovascular risk, depression, cognitive performance, cancer prevention — consistently find that benefits plateau and plateau in the range of 40–60 ng/mL (100–150 nmol/L).

Above 100 ng/mL, toxicity risk begins to emerge (hypercalcemia). Below 40 ng/mL, immune and systemic functions remain suboptimal even in the absence of bone disease.

What Affects Your Level

Latitude and season: Above 35° north latitude (roughly, north of Los Angeles), UVB angles are insufficient for vitamin D synthesis during winter months — sometimes 4–6 months per year.

Skin pigmentation: Melanin is natural sunscreen. Darker skin requires 3–6 times more sun exposure to produce equivalent vitamin D. This is a primary driver of the substantially higher deficiency rates documented in Black and South Asian populations in Northern latitudes.

Body fat: Vitamin D is fat-soluble. Adipose tissue sequesters vitamin D, reducing its circulating bioavailability. Higher body fat means higher supplementation requirements for equivalent blood levels.

Age: Skin synthesis efficiency decreases approximately 75% between age 20 and 70.

Gut absorption disorders: Celiac disease, Crohn's disease, and gastric bypass reduce fat-soluble vitamin absorption.

Supplementation Protocol

Dosing by Current Level

• Below 20 ng/mL: 5,000–8,000 IU D3 daily for 8–12 weeks to load, then maintenance
• 20–30 ng/mL: 3,000–5,000 IU D3 daily
• 30–40 ng/mL: 2,000–3,000 IU D3 daily
• 40–60 ng/mL: Maintenance at 1,500–2,000 IU daily; continue monitoring

Always co-supplement with vitamin K2 (MK-7, 100–200 mcg) to direct calcium to bones rather than arteries. Take with the largest fat-containing meal for optimal absorption.

Monitoring

Retest every 3 months when adjusting doses. Once stable in the optimal range, annual testing is sufficient.