Vitamin D3
What it is
About 40% of adults worldwide are deficient, especially those who spend little time outdoors. Essential for bone density, calcium absorption, immune function, and muscle strength. The VITAL trial found 2,000 IU/day reduced autoimmune disease risk by 22%. Deficiency is linked to higher infection rates and fractures. Test your blood level (25-OH vitamin D) before supplementing.
Dose
1,000–2,000 IU/day for maintenance; up to 4,000 IU/day to correct deficiency. Pair with Vitamin K2.
Time of day & tips
Take with a meal containing fat for best absorption. Best taken in the morning to avoid interfering with sleep. Pair with K2 to direct calcium into bones.
Cycling
Safe for continuous daily use. Retest blood levels (25-OH-D) every 6-12 months to ensure you stay in the 40-60 ng/mL range. Reduce dose in summer if sun exposure increases.
Compare or learn more
Compare with another supplement →
Browse by symptom →
Open the full interactive view →