If you want to optimize your vitamin D levels, you'll need to know how much vitamin D you need in your diet to get there. You can get vitamin D from your diet and also after the sun stimulates your skin to make vitamin D. The best way to know for certain whether you have enough vitamin D in your body is to get a specific blood test that measures 25-hydroxyvitamin D.
Unfortunately, tests that measure this form of vitamin D can vary in accuracy and are expensive. You want to make sure that you use the best test available so that you get the most accurate results.
The question of how much vitamin D your body needs can be asked in several ways:
What level of the serum 25-hydroxyvitamin D3 in the body avoids any of the problems associated with vitamin D3 deficiency?
How much time in the sun will help you reach the necessary blood level of vitamin D3?
How many international units (IU) of vitamin D do you need to get in food or from supplements to reach the necessary blood level of vitamin D3?
Of course, the answer to the last two questions depends on the answer to the first question.
Looking at recommended vitamin D levels
Scientists believe that the serum level of 25-hydroxyvitamin D is critical for protecting health. Because of this, when you go to your doctor to figure out whether you have enough vitamin D in your system, they do a blood test for this form of vitamin D.
The idea is that when you have enough 25-hydroxyvitamin D in your blood, you will be able to make all the calcitriol that you need, and as a result you will have the strongest bones, and you may also have a healthy immune system, protection against several cancers, reduced rates of diabetes, and lower risk of heart disease.
In late November 2010, an expert panel of scientists published a detailed report that explains how much 25-hydroxyvitamin D you need in your blood and provides recommendations for how much vitamin D you need in your diet to get to those serum levels. The following table shows critical levels of serum 25-hydroxyvitamin D based on this report.
| ng/ml (nmol/L) | Health Status |
|---|---|
| Less than 10 (25) | Vitamin D deficient. Leads to rickets in infants and children, and osteomalacia in adults. |
| 10 to 20 (25 to 50) | Inadequate for normal bone mineralization and overall health. |
| Greater than 20 (50) | Generally considered adequate for normal bone growth and mineralization. |
| Greater than 50 (125) | Considered potentially toxic; leads to hypercalcemia and hyperphoshatemia (also called “vitamin D toxicity”) |
Whereas the clinical labs in the United States report your blood levels of 25-hydroxyvitamin D as nanograms per milliliter (ng/ml), Canada and many other countries express serum 25-hydroxyvitamin D levels in nanomoles per liter (nmol/L). Don’t get confused by this — just be certain what units your test result were measured in. To turn nmol/L into ng/ml, all you need to do is divide the nmol/L value by 2.5.
You may notice in the table that the levels are associated with bone health. That’s because the expert panel felt that only the evidence to link dietary vitamin D or serum 25-hydroxyvitamin D to protection from rickets, osteomalacia, low bone mass, and osteoporosis was strong enough.
Computing the correct IU level of vitamin D
So, if you should have a blood level of 25-hydroxyvitamin D of at least 20 ng/ml (50 nmol/L), how much vitamin D do you need in your diet to get there?
Most of the information that is available about the relationship between dietary vitamin D and serum 25-hydroxyvitamin D comes from studies on adults. These studies show that there’s not a simple linear relationship between the two factors. They find that if you have low serum 25-hydroxyvitamin D levels, you need very little dietary vitamin D to get to 20 ng/ml (50 nmol/L) — just 600 international units (also called IU) are needed to get and keep you there.
If you don’t get any vitamin D from exposing your skin to the sun, you’ll need to get all of it from diet or supplements.
You can calculate that every 30 IU of vitamin D you consume will raise your serum 25-hydroxyvitamin D by 1 ng/ml (600 IU divided by 20 ng/ml) or 2.5 nmol/L. Unfortunately it isn’t as simple as that. After you have 20 ng/ml (50 nmol/L) of 25-hydroxyvitamin D in your blood, your body changes the way you use the vitamin D from the diet.
At that point, it takes a lot more vitamin D to raise your serum 25-hydroxyvitamin D levels — to raise your serum 25-hydroxyvitamin D from 20 to 30 ng/ml (50 to 75 nmol/L) you would need an additional 900 IU per day or more! (That’s 600 IU + 900 IU or 1,500 IU per day total to get your serum levels to 30 ng/ml [75 nmol/L].)