Vitamin D3 (cholecalciferol) is the bioactive form your skin makes from UVB exposure. Most people are deficient by lab standards. D3 supports testosterone production, immune modulation, mood, and calcium absorption.
Why K2. D3 raises serum calcium; K2 (MK-7) directs that calcium to bone and teeth, away from arteries and soft tissue. Don't take D3 without K2 long-term.
Dose: 5,000 IU D3 + 90-180 mcg K2 daily, with fat. Test 25-OH-D every 3-6 months; target 50-80 ng/mL.
Caveats: Toxicity is rare but possible at sustained mega-doses without testing.