Not a Vitamin — A Hormone
Vitamin D is technically a misnomer. Unlike true vitamins that must come from food, vitamin D is synthesized by your skin when exposed to UVB radiation from sunlight, then converted by your liver and kidneys into its active hormonal form: calcitriol. It acts on receptors in virtually every tissue in your body — from bones and muscles to the brain, immune cells, and heart.
This hormone-like behavior is why vitamin D deficiency has such wide-ranging consequences. And deficiency is the norm: globally, an estimated 1 billion people are deficient, with inadequate levels found in 40% of Americans and over 60% of adults in northern European countries.
What Vitamin D Does
Bone density and calcium metabolism — Vitamin D regulates calcium and phosphate absorption in the gut. Without adequate vitamin D, even a high-calcium diet cannot prevent bone loss. Deficiency is a primary driver of osteoporosis and rickets.
Immune function — Vitamin D activates T-cells and macrophages — front-line immune defenders. Low vitamin D is strongly associated with increased susceptibility to respiratory infections and autoimmune conditions.
Muscle function and strength — Vitamin D receptors are expressed in muscle tissue. Studies show that deficiency impairs muscle protein synthesis and reduces strength, while supplementation in deficient individuals improves both. Falls and fractures in older adults are significantly higher with low vitamin D.
Mood and mental health — Seasonal Affective Disorder (SAD) tracks with reduced sun exposure and lower vitamin D levels in winter months. Multiple meta-analyses link low vitamin D to increased risk of depression, anxiety, and cognitive decline.
Testosterone production — Leydig cells in the testes have vitamin D receptors. A 12-month randomized trial found that men supplementing with 3,332 IU of vitamin D daily had 25% higher testosterone levels than the placebo group at trial end.
Cardiovascular health — Deficiency is associated with higher rates of hypertension, heart failure, and cardiovascular events. Vitamin D influences the renin-angiotensin system, which regulates blood pressure.
How to Know Your Level
The only reliable way to know your vitamin D status is a blood test measuring 25-hydroxyvitamin D (25-OHD).
| Level | Classification |
|---|---|
| < 20 ng/mL | Deficient |
| 20–29 ng/mL | Insufficient |
| 30–39 ng/mL | Adequate (minimum) |
| 40–60 ng/mL | Optimal (most researchers' target) |
| > 100 ng/mL | Potentially toxic |
Most people without supplementation fall in the 15–30 ng/mL range. The target for optimal health and performance is generally considered 40–60 ng/mL.
Dosage: What the Research Supports
General population (without testing): 1,000–2,000 IU/day is a safe, broadly beneficial starting point.
To correct deficiency (with testing): 4,000–5,000 IU/day for 12 weeks is commonly used to bring levels from deficient into the optimal range, followed by a maintenance dose.
Upper tolerable limit: 4,000 IU/day is the official tolerable upper limit, though research suggests toxicity requires sustained doses above 10,000 IU/day over months. Many physicians supplement at 5,000 IU/day without concern.
Vitamin D3 vs. D2: Always choose D3 (cholecalciferol) — the form your skin produces. D2 (ergocalciferol) is less potent and shorter-acting.
The K2 Pairing
Vitamin D increases calcium absorption — but calcium must be directed to bones and teeth, not to arteries and soft tissue. Vitamin K2 (specifically MK-7 form) activates proteins (osteocalcin and matrix Gla-protein) that direct calcium to the right places. If you're taking high doses of vitamin D long-term, pairing with K2 (90–180mcg/day) is prudent.
Maximize from Sun
For those in sunny climates, direct midday sun exposure (face, arms, and legs) for 15–30 minutes produces 10,000–20,000 IU of vitamin D. Factors that reduce synthesis: sunscreen, glass windows, dark skin pigmentation, high latitude, winter months, and age (older skin produces less D per UV exposure).
The Bottom Line
Test your level, target 40–60 ng/mL, and supplement with D3 + K2 to get there. For most people in temperate climates who work indoors, 2,000–4,000 IU/day of D3 will close the gap. This single intervention has outsized effects on immunity, bone health, mood, muscle function, and hormonal balance.