Vitamin D3 (cholecalciferol) is the form of vitamin D the body synthesizes in skin from UVB exposure. As a supplement, D3 raises and maintains 25(OH)D levels more efficiently than D2. Most adults need 1000-4000 IU/day to reach functional optimal levels.
Adults over 60 — reduced skin synthesis efficiency
Higher body weight — vitamin D distributes into adipose tissue
Dosing notes
Start with 2000 IU/day if deficient, re-test in 8-12 weeks
Higher body weight often needs 4000-5000 IU/day to reach optimal range
Pair with vitamin K2 (MK-7, 90-200 mcg) and 200-400 mg magnesium for proper calcium handling
Avoid sustained dosing above 4000 IU/day without monitoring
Genetic context
VDR variants (FokI, BsmI, TaqI) influence how strongly tissues respond to a given 25(OH)D level. Some VDR carriers feel best at the higher end of the optimal range (50-60 ng/mL). CYP2R1 variants reduce conversion to 25(OH)D and can require slightly higher D3 doses.
VDR — VDR variants change how strongly tissues respond to a given vitamin D level; some carriers target the higher end of optimal. Moderate evidence
FAQ
How much vitamin D3 should I take?
Most adults need 1000-4000 IU/day to maintain 30-50 ng/mL. Body weight, baseline, latitude and VDR genotype all matter — re-testing after 8-12 weeks is the only way to know if your dose is right.
Do I really need K2 with vitamin D3?
If you're taking more than 2000 IU/day chronically, pairing with vitamin K2 (MK-7) is reasonable. K2 directs calcium toward bone and away from soft tissue.
References
Tripkovic et al., Am J Clin Nutr 2012 — PMID 22552031. Vitamin D3 raises serum 25(OH)D more effectively than D2.
Holick et al., J Clin Endocrinol Metab 2011 — PMID 21646368. Endocrine Society guideline on diagnosis and treatment of vitamin D deficiency.