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A major function of vitamin D is to increase absorption of calcium from the intestines. Some people who take vitamin D supplements raise their blood levels of calcium as well as vitamin D above normal. These people are at increased risk for heart attacks and premature death.
The only dependable blood test for vitamin D deficiency is called calcidiol or hydroxy-vitamin D. If you have a calcidiol level below 20 ng/mL, you can raise it to 32 ng/mL by taking 30 µg (1200 IU) per day. If your blood level of calcidiol is 30 ng/mL, you need to take 5 µg (200 IU) per day, to raise blood levels to 32 ng/mL. If your blood calcidiol level is too high (above 90 nmol/L), check with your doctor about reducing your dose of vitamin D pills.