The Nation's Health + [vitamin D]

Topping up your vitamin D tank

Now that my vitamin D replacement experience dates back nearly 5 years, I've been witnessing an unusual phenomenon:

The longer you take vitamin D, the less you need.

Let me explain. You take 10,000 units D3 in gelcap form. 25-hydroxy vitamin D levels, checked every 6 months, have remained consistently between 60 and 70 ng/ml. Three years into your vitamin D experience and 25-hydroxy vitamin D level rises to 98 ng/ml--an apparent need for less vitamin D.

So we cut your intake from 10,000 units per day to 8000 units per day. Another 25-hydroxy vitamin D level 6 months later: 94 ng/ml. We cut dose again to 6000 units, followed by another 25-hydroxy vitamin D level of 66 ng/ml.

This has now happened in approximately 20% of the people who have been taking vitamin D for 3 or more years. I know of no formal analysis of this effect, what I call the "topping up" phenomenon. Reasoned simply, it seems to me that, once your vitamin D "tank" is topped up (i.e., tissue stores have been replenished), it requires less to keep it full.

No one has experienced any adverse consequence of this topping up effect though it has potential for some people to develop toxic levels if 25-hydroxy vitamin D levels are not monitored long-term. In my office, I measure 25-hydroxy vitamin D levels every 6 months.

It means that long-term monitoring of 25-hydroxy vitamin D is crucial to maintain favorable and safe levels.