Vitamin D3

A fat-soluble hormone essential for immune function, bone health, and metabolic regulation.

VitaminCholecalciferolVitamin DThe Sunshine Vitamin

Vitamin D3 (cholecalciferol) is a fat-soluble secosteroid hormone produced in the skin upon UVB exposure. Despite being called a "vitamin," it functions as a hormone with receptors in virtually every tissue in the body. An estimated 42% of American adults are deficient, with rates significantly higher in northern latitudes, darker-skinned individuals, and those who spend limited time outdoors. Deficiency is linked to immune dysfunction, bone loss, muscle weakness, depression, and increased mortality.

Evidence-Based Benefits

Immune Function

Vitamin D activates antimicrobial peptides and modulates both innate and adaptive immunity. Deficiency is associated with increased susceptibility to infections.

Bone Health

Essential for calcium absorption. Deficiency causes osteomalacia in adults and rickets in children. Critical for preventing osteoporosis.

Muscle Function

Vitamin D receptors in muscle tissue affect strength and function. Supplementation reduces fall risk in deficient older adults.

Mood & Depression

Meta-analyses show significant antidepressant effects of vitamin D supplementation, particularly in deficient individuals.

Testosterone Support

Vitamin D receptors exist in testicular Leydig cells. Studies show testosterone increases with supplementation in deficient men.

Insulin Sensitivity

Research links adequate vitamin D status with improved insulin sensitivity and reduced type 2 diabetes risk.

Dosing Guide

Common Dose

2,000-5,000 IU per day (adjust based on blood levels)

Best Timing

Morning with a fat-containing meal

With Food?

Yes — take with a meal

Dose should be guided by blood levels. Target 40-60 ng/mL (100-150 nmol/L). Most adults need 2,000-5,000 IU daily to achieve optimal levels. Co-supplement with K2 (MK-7) to direct calcium to bones instead of arteries. Retest levels after 3 months.

Forms & Comparison

Vitamin D3 (Cholecalciferol)

The preferred form — same as what your skin produces. 87% more effective than D2 at raising blood levels.

Vitamin D2 (Ergocalciferol)

Plant-derived form. Less effective at raising and maintaining blood levels. Not recommended unless D3 is unavailable.

D3 + K2 Combination

Ideal pairing. K2 (MK-7 form) activates proteins that direct calcium to bones and teeth instead of soft tissues.

Cautions & Interactions

Toxicity is possible but rare — typically from doses above 10,000 IU/day for extended periods without monitoring.
Always test blood levels before and during high-dose supplementation.
Take with K2 (MK-7) to prevent calcium deposition in arteries.
Caution with granulomatous diseases (sarcoidosis) — increased sensitivity to vitamin D.
May interact with certain medications (thiazide diuretics, corticosteroids, statins).

Frequently Asked Questions

What blood level should I target?

Optimal is 40-60 ng/mL (100-150 nmol/L). Most labs list "normal" as >30 ng/mL, but research suggests benefits plateau around 40-60 ng/mL. Below 20 ng/mL is frank deficiency.

Can I get enough from sunlight?

Depends on latitude, season, skin color, and sun exposure. At latitudes above 37°N (e.g., north of San Francisco), UVB is insufficient for vitamin D production from October through March. Most people benefit from supplementation.

Why take K2 with D3?

Vitamin D increases calcium absorption. Vitamin K2 (MK-7 form) activates the proteins that direct that calcium to bones and teeth rather than depositing it in blood vessels. They work synergistically and should be taken together.