Frequently Asked Questions

I read about vitamin D3, cholecalciferol, calcifediol, 25-hydroxyvitamin-D3, ampli-D®, are these all the same?

There are different forms of vitamin D found in certain foods like fatty fish, milk, eggs, or mushrooms. The most abundant forms of vitamin D found in these foods are known as vitamin D3, vitamin D2 and calcifediol. We have created an overview with the different forms of vitamin D for you:

Forms of vitamin D

Also known as


Vitamin D3

Colecalciferol, cholecalciferol

Sun, fatty fish, milk, eggs, fortified foods or supplements

Vitamin D2


Mushrooms, fortified foods or supplements


25-hydroxyvitamin-D3, ampli-D®

Fatty fish, milk, eggs or supplements with ampli-D® such as FORTARO®

Calcifediol is a faster and more effective form of vitamin D. Calcifediol is also scientifically named 25-hydroxyvitamin-D3, and the brand name is smpli-D®, so these are all the same.

How is calcifediol different from vitamin D3?

Our bodies get vitamin D predominantly from sunlight, and from some food sources. Additionally, vitamin D supplements contribute significantly to our vitamin D levels. Before our body puts these sources of vitamin D for further use, it must be processed by the liver and converted into calcifediol. This is a slow process and can easily take months to achieve an optimal vitamin D level. Ampli-D® is already in calcifediol form and it is also the more bioavailable vitamin D form, hence better absorbed by the body. This helps in reaching optimal vitamin D levels faster. FORTARO® powered by ampli-D® is clinically proven to optimize vitamin D status 3x more quickly than ordinary vitamin D.

Besides supplements, where else can I find calcifediol?

Calcifediol is the form of vitamin D which is most common in our bodies. Most of us get this very early in life through breast milk. Then as we grow older fish, eggs, meat, and milk are sources which contain calcifediol. It is the second most abundant form of vitamin D in food after vitamin D3.

How do I best use calcifediol?

Dietary supplements aren’t intended to replace a healthy diet.
Calcifediol can be consumed any time of the day with or without a meal. To reach the full effect, daily consumption is recommended.

Can I get too much vit D because it’s faster and more potent? If it’s faster, does that mean I also need to stop at some stage?

It takes several days to weeks rather than months of daily calcifediol intake to reach a new steady state level (this level depends on the daily dose), which will not significantly increase anymore. Even more, this level will decrease to a lower level again if daily supplementation is ceased similarly to cholecalciferol.

How many IU is one tablet of FORTARO®?

Many know their vitamin D product dosage in international units (IU) which is a unit of activity. Another way to express the dosage is to list the vitamin D dose in microgram. For vitamin D3, the official conversion factor is:  1 microgram of vitamin D3 = 40 IU of vitamin D activity. And a high dose vitamin D3 supplement in Australia has 25 micrograms of vitamin D3 which therefore equals to 1000IU.

FORTARO® contains 10 micrograms of calcifediol and it acts 3x faster and more effective in raising vitamin D levels than ordinary vitamin D3. This seemingly easy question of “How many IU is one tablet of FORTARO®” is quite complicated because there is no official conversion factor for calcifediol from microgram to IU.

When we look at efficiency to reach vitamin D levels in serum (i.e. get the vitamin D levels up in your blood), calcifediol per microgram has demonstrated to be 3 times more effective than ordinary vitamin D3 on a microgram per microgram comparison. Therefore, one could apply the calculation rule: 40*3*10 micrograms (which is the dose of calcifediol in the tablet) = 1200 IU. Applying this calculation gives a guidance for FORTARO® which contains 10 micrograms of calcifediol and corresponds to 1200 IU.

If calcifediol is 3x faster and more effective than regular vitamin D3, can I just take 3x more D3 and have the same effect?

In practice, all of us vary in how efficient we are in absorbing vitamin D3 and converting this to calcifediol. With vitamin D3 it is difficult and can take months to reach sufficient/optimal vitamin D levels in the body if at all, even with 3x the recommended dose. Since calcifediol is a faster and more effective form of vitamin D with superior bioavailability, you are more likely to reach sufficient vitamin D levels. Moreover, optimal levels are achieved in days to weeks rather than months with ordinary vitamin D3 supplements.

The subsequent question we then get is: does 30mcg of D3 have the same benefit as 10 micrograms of calcifediol, given the factor 3? This was confirmed in a study by Shieh et al [2] where it was shown that with 20 micrograms of calcifediol it took 28 days to reach optimal serum levels, whereas with 60 micrograms of vitamin D3 it took over 122 days. Thus, for most people, FORTARO® containing 10 micrograms of calcifediol would still be more effective, as calcifediol is the already metabolized form of vitamin D3, and therefore easier and better used by the body.

[1] JM Quesada-Gomez and R Bouillon, Osteoporos Int 29 (8), 1697 (2018).
[2] A Shieh et al., J Clin Endocrinol Metab 102 (4), 1133 (2017).
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