Vitamin D (25-hydroxyvitamin D)
A fat-soluble vitamin essential for bone health, immune function, and mood. Commonly deficient in populations with limited sun exposure.
What is Vitamin D?
Vitamin D is a fat-soluble vitamin that your body produces when your skin is exposed to sunlight. It's also found in certain foods and supplements. The 25-hydroxyvitamin D test measures the total amount of vitamin D in your blood.
Despite being called a vitamin, D functions more like a hormone, with receptors found in nearly every cell in your body.
Normal Ranges
| Status | Level (ng/mL) | Level (nmol/L) |
|---|---|---|
| Deficient | Below 20 | Below 50 |
| Insufficient | 20-29 | 50-72 |
| Sufficient | 30-100 | 75-250 |
| Potentially Harmful | Above 100 | Above 250 |
Note: Optimal levels are debated. Many functional medicine practitioners aim for 40-60 ng/mL.
Why It Matters
Bone Health
Vitamin D helps your body absorb calcium. Without enough vitamin D, bones can become thin, brittle, or misshapen — leading to conditions like osteoporosis.
Immune Function
Vitamin D plays a crucial role in immune regulation. Deficiency has been linked to increased susceptibility to infections and autoimmune conditions.
Mental Health
Low vitamin D levels are associated with depression, seasonal affective disorder (SAD), and cognitive decline in older adults.
Muscle Function
Adequate vitamin D is necessary for muscle strength. Deficiency can lead to muscle weakness and increased fall risk.
Risk Factors for Deficiency
- Limited sun exposure — Working indoors, living at high latitudes
- Darker skin — More melanin reduces vitamin D synthesis
- Age — Older adults produce less vitamin D
- Obesity — Vitamin D gets sequestered in fat tissue
- Malabsorption conditions — Crohn's disease, celiac disease
How to Optimize Your Vitamin D
- Sunlight exposure — 10-30 minutes of midday sun several times per week
- Dietary sources — Fatty fish, egg yolks, fortified foods
- Supplementation — Vitamin D3 is preferred over D2
- Monitor levels — Test every 3-6 months when supplementing
Supplementation Guidelines
Most adults need 1,000-4,000 IU daily to maintain optimal levels. If deficient, higher doses may be needed initially under medical supervision.
How Often to Test
- If never tested: Get a baseline
- If deficient: Retest 3 months after starting supplementation
- If optimal: Annually, or seasonally if you live at high latitude
Related Biomarkers
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