Vitamin D is technically a hormone, not a vitamin — your body synthesizes it from sunlight exposure rather than obtaining it primarily from food. It regulates over 1,000 genes, plays fundamental roles in immune function, bone health, cardiovascular health, mood regulation, and cancer prevention. And according to the CDC, 42% of Americans are deficient. Here’s everything you need to know.
What Vitamin D Does in Your Body
Vitamin D receptors exist on virtually every cell in the human body. When activated by vitamin D (specifically the active form, calcitriol), these receptors regulate gene expression involved in calcium absorption and bone mineralization, immune cell activation and modulation, insulin secretion and blood sugar regulation, cardiovascular function and blood pressure, muscle function and strength, and mood regulation via serotonin synthesis.
The Consequences of Deficiency
- Bone health: Vitamin D deficiency causes poor calcium absorption, leading to low bone density, osteomalacia (soft bones), and increased fracture risk
- Immune dysfunction: Low vitamin D is associated with increased susceptibility to respiratory infections, autoimmune conditions, and slower wound healing
- Mood and depression: Vitamin D receptors in the brain regulate serotonin synthesis. Deficiency is strongly associated with depression and seasonal affective disorder
- Muscle weakness: Vitamin D regulates muscle protein synthesis and neuromuscular function. Deficiency causes proximal muscle weakness and increased fall risk in older adults
- Cardiovascular risk: Low vitamin D is independently associated with higher risk of hypertension, heart failure, and cardiovascular mortality
Why Deficiency Is So Widespread
Vitamin D synthesis from sunlight requires UVB radiation hitting bare skin. Multiple factors reduce this: living above the 37th parallel (roughly north of Atlanta, GA), spending most daylight hours indoors, applying sunscreen (which blocks UVB), darker skin pigmentation (melanin reduces UVB absorption), winter months when UVB is too weak in most latitudes, and aging (skin becomes less efficient at vitamin D synthesis over time).
Food Sources of Vitamin D
Natural food sources are limited. The best sources: fatty fish (salmon 450–600 IU per 3 oz), egg yolks (40 IU each), fortified foods (milk, orange juice, and cereals typically contain 100 IU per serving). Getting adequate vitamin D from food alone is extremely difficult for most people.
Sunlight: The Most Effective Source
In summer months (April–October in most of the US), 10–30 minutes of midday sun on the arms and legs without sunscreen produces 10,000–20,000 IU of vitamin D. This is far more than any supplement. However, time of day, season, cloud cover, and latitude all dramatically affect synthesis. For most people outside the tropics, sunlight alone cannot maintain optimal levels year-round.
Should You Supplement?
If you live in a northern climate, work indoors, have darker skin, or are over 50, supplementation is strongly worth considering. The optimal blood level is 40–60 ng/mL (100–150 nmol/L). Most adults need 2,000–4,000 IU of vitamin D3 (not D2) daily to reach and maintain this range, though individual variation is significant. Get your level tested with a 25-hydroxyvitamin D blood test — it’s inexpensive and provides the only accurate picture of your actual status. Take vitamin D3 with a fat-containing meal and ideally with vitamin K2 (MK-7 form) to support calcium routing to bones rather than arteries.
The Bottom Line
Vitamin D deficiency is one of the most common, most consequential, and most easily corrected nutritional deficiencies in the modern world. 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.