Vitamin D is technically a hormone that regulates over 1,000 genes, plays fundamental roles in immune function, bone health, cardiovascular health, and mood regulation. According to the CDC, 42% of Americans are deficient. Here’s everything you need to know.
The Consequences of Deficiency
- Bone health: Poor calcium absorption, low bone density, increased fracture risk
- Immune dysfunction: Increased susceptibility to respiratory infections and autoimmune conditions
- Mood and depression: Vitamin D receptors in the brain regulate serotonin. Deficiency is strongly linked to depression and seasonal affective disorder
- Muscle weakness: Increases fall risk in older adults
- Cardiovascular risk: Independently associated with hypertension and cardiovascular mortality
Why Deficiency Is So Widespread
Vitamin D requires UVB radiation hitting bare skin. Multiple factors reduce synthesis: living north of the 37th parallel (above Atlanta, GA), working indoors, sunscreen use, darker skin pigmentation, winter months, and aging (skin becomes less efficient over time).
Getting Enough
Sunlight (best source): 10–30 minutes of midday summer sun on arms and legs produces 10,000–20,000 IU. But most people outside the tropics can’t maintain optimal levels year-round from sun alone.
Food (limited): Wild salmon (450–600 IU/3 oz), egg yolks (40 IU each), fortified foods (100 IU/serving). Inadequate as a sole source for most people.
Supplementation: Most adults need 2,000–4,000 IU of vitamin D3 daily to reach optimal levels (40–60 ng/mL). Take with fat and vitamin K2 (MK-7 form). Get your level tested with a 25-hydroxyvitamin D blood test first.
The Bottom Line
Get your levels tested. Spend time in midday sun when possible. Supplement intelligently. The downstream benefits to immune function, mood, bone health, and cardiovascular health make this one of the highest-return interventions available.