Upper and Lower Limits for Vitamin D

by: Dr. Gabe Mirkin M.D.

image from blog.scientificamerican

The safe upper limit of vitamin D has been identified for the first time. A study of more than a million people older than 45 shows that having blood levels of calcidiol (hydroxyl vitamin D) below 20ng/mL (50 nmol/L) or above 36 ng/mL (90 nmol/L) increases risk for heart attacks and premature death (J Clin Endocrinol Metab, published online March 26, 2013). An overdose, as well as a deficiency, of vitamin D markedly increases heart attack risk.

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.