Today we will finish up our discussion of vitamin D.
Vitamin D2 and D3 are changed in the liver to 25-Hydroxyvitamin D. That is the last time you will see that tough word since I will refer to it as 25(OH)D. This is what is measured in the blood, and it determines if we need to have extra vitamin supplementation. Vitamin D insufficiency is usually defined as a serum 25(OH)D level of 30 to 32 ng/ml.
In regard to calcium absorption, most studies suggest that with a 25(OH)D level of at least 20 ng/ml, there will be maximum absorption.
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The daily recommendation for vitamin D is 600 IU for healthy non-pregnant women under 70 years of age and 800 IU for women older than 70 years.
Extra vitamin D should be added if the level is below 30 ng/ml. As a general rule of thumb, an extra 1000 IU per day of vitamin D3 will increase the 25(OH)D levels by 6-10 ng/ml. The levels should be checked after a three month trial.
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Lastly, some studies point to problems if the 25(OH)D levels are greater than 50 ng/ml, so always check levels after adding extra vitamin D.
-Dr. P