How Vitamin D Status Is Measured
Vitamin D status is gauged by a blood test for 25-hydroxyvitamin D [25(OH)D], the main circulating form. The NIH Office of Dietary Supplements groups results as [1]:
| 25(OH)D level | Interpretation |
|---|---|
| Below 30 nmol/L (12 ng/mL) | Deficiency |
| 30 to under 50 nmol/L (12–20 ng/mL) | Inadequate |
| 50 nmol/L (20 ng/mL) or above | Adequate for most people |
Note that very high levels aren't better — adequacy tops out around these thresholds for bone and general health.
Recommended Intake
The RDA is 600 IU (15 mcg)/day for adults 19–70 and 800 IU (20 mcg)/day after age 70 [1]. Vitamin D comes from sunlight on skin, a few foods (fatty fish, fortified dairy), and supplements.
Who Is Most at Risk of Deficiency
Factors that lower vitamin D status include:
- Limited sun exposure — indoor lifestyles, higher latitudes, winter, heavy sunscreen or covering.
- Darker skin — more melanin reduces vitamin D synthesis from sunlight.
- Older age — skin makes less vitamin D over time.
- Higher body weight — vitamin D is sequestered in fat tissue.
- Malabsorption — Crohn's, celiac, and similar conditions.
- Breastfed infants — human milk is low in vitamin D, so supplementation is commonly advised.
What to Do
If you have risk factors, ask a clinician whether a 25(OH)D test makes sense. Treat the number as one input — interpretation depends on your health and the lab's reference range. For broader background see the Vitamin D Complete Guide; for the flip side of intake, see Vitamin D Toxicity.