Medically reviewed by, Russell Braun RPH
Your standing in the pharmacy aisle looking at the various products. After a discussion with your doctor taking vitamin D supplement seems like a good idea. The problem is all the products have different strengths and marketing jargon on the box. You wonder what is the difference between vitamin D from a microgram to an IU?
How do you know which one is the dose your doctor wants you to use? What the heck is an IU anyway?
Medications and their strengths can be confusing. After all some medications come in micrograms (mcg), milligrams (mg) or even grams (g). Also, nobody has told you how any of those related to an IU.
Good news, we will explain how to convert from vitamin D microgram to IU and why it is important for you to know the difference.
1. What is an IU?
The abbreviation IU stands for International Unit. An international unit is an internationally standardized and accepted amount of a substance. It relates to the potency or how biologically active a substance is in the body.
This type of measure is often used for fat soluble vitamins and a few other chemicals like hormones and vaccines. Vitamins A, D, E and K are considered fat soluble vitamins (meaning they mix better with fats than with water).
Given this definition an IU of vitamin D is not the same amount as an IU of vitamin A, E or K. In 1949 the World Health Organization standardized the amount of vitamin D3 required for an IU. There are two forms of vitamin D, D2 (ergocalciferol) and D3 (cholecalciferol)…more on that below. For now just remember the IU is based off of D3, which is the active form in the body.
2. Label confusion?
In 2016, the Food and Drug Administration (FDA) revised the requirement for nutritional supplement labels. They required that manufacturers start showing the strengths in metric amounts (Ex: micrograms – mcg) rather than international units.
Metric amounts represent the mass or volume of a substance that is actually in a product. This is important for labels because you want to know how much of the active ingredient is actually in the product.
This meant that all the fat soluble vitamins mentioned above would now be required to have their dosage displayed as micrograms (mcg). Therefore, many manufactures only included the metric and left the international unit conversion off the packaging.
This causes confusion for patients because many doctors don’t realize this and continue to prescribe these supplements based on IU’s. The FDA requirement did not make it mandatory for IU’s to be part of the new labels.
Percent daily value
In addition to requiring the strength to be shown in metric quantities such as micrograms, the FDA has required a percent daily value as well. This helps consumers understand how much vitamin D they are getting in relation to how much they need per day from their diet. The amount that is suggested from the diet is based on Recommended Dietary Allowances (RDAs). Those values are set by the National Academy of Medicine.
What the new labels look like
The change on vitamin D product labels to microgram to IU was made for two reasons:
- The need for showing the volume of a substance in the product.
- IU was being confused for IV (Intravenous) by some healthcare providers, which could lead to medication errors.
New labels are supposed to look like this.
Changes to include both vitamin D in microgram and IU have been requested since the FDA’s original rule requiring these changes. By January 1, of 2021 all manufacturers should be displaying both micrograms and IU’s on their labels.
3. What is an IU of vitamin D?
As mentioned an IU is a measure of the biologically active amount of vitamin D, rather than the weight of it. The weight required of vitamin D to obtain 1 International Unit of activity depends on the potency. The international community has agreed on a standard way to convert between potency and weight.
To determine how much an IU equals in weight you must have a conversion. According to the Office of Dietary Supplements the vitamin D conversion is:
| International Unit | Microgram |
| 1 IU | 0.025 mcg |
As you can see 1 IU is a very small amount in weight. Especially when you consider how small a microgram is. One pound is about 453 grams and in the table below you can see that only one gram is one million micrograms
| Gram | Milligram | Microgram |
| 1 g | 1,000 mg | 1,000,000 mcg |
4. How do you convert IU to micrograms?
Suppose your doctor told you to take a Vitamin D supplement. She said, get something in the range of 400 IU to 800 IU. How do you know how many micrograms that becomes?
Calculating the dose of vitamin D from IU to micrograms is fairly simple once we know the conversion from above. For the dose range given above the conversion looks like this:
400 IU x 0.025 mcg = 10 mcg
800 IU x 0.025 mcg = 20 mcg
Vitamin D2 vs D3
Vitamin D is found in two forms in nature, D2 (ergocalciferol) and D3 (cholecalciferol). Cholecalciferol is found only in animals and is the active form in humans. Ergocalciferol is produced by plants and must be converted by the liver to D3 in humans to be used by the body.
Some supplements contain D2 which is cheaper to produce. The good news is they both have the same activity in the body if your liver is able to convert the D2 to D3.
Therefore the conversion from vitamin D from microgram to IU is the same for either D2 or D3.
5. What is 1000 IU in mg of vitamin D?
More and more evidence has shown that vitamin D is crucial to improving health and preventing many diseases. Some examples include:
- Osteoporosis
- Rickets
- Diabetes
- High blood pressure
- Asthma
- Alzheimers disease
- Depression
- Autoimmune diseases
Given those findings it is common to see higher doses of vitamin D being used. Quiet often doses of 1,000 IU or higher will be prescribed by doctors or taken by patients without their doctors knowledge.
To convert 1,000 IU of vitamin D to mcg simply do a similar formula to above. In fact you can easily convert from microgram to IU by using this formula for vitamin D:
1 microgram equals 40 IU
I included a helpful table for quick reference based on your dose.
| Microgram (mcg) | International Unit (IU) |
| 10 mcg | 400 IU |
| 15 mcg | 600 IU |
| 20 mcg | 800 IU |
| 25 mcg | 1000 IU |
| 50 mcg | 2000 IU |
| 125 mcg | 5000 IU |
| 1,250 mcg | 50,000 IU |
6. How to figure out if you need more vitamin D
Given that most people do not get enough sunlight or vitamin D in their diet, insufficiency is estimated to affect almost 50% of the population. The only way to know for sure if you are deficient is to get a blood test.
There are three easy ways to get a vitamin D blood test.
1. Schedule an office visit and see if your doctor will order a blood test.
2. Set up an in person lab blood draw yourself at a site such as www.HealthLabs.com
3. Order an at home test from a company such as Everlywell.
7. How many IU of vitamin D should you take per day?
Deciding how much vitamin D is needed per day starts by looking at the recommended dietary allowances (RDA). These values set by the National Academy of Medicine offer the amounts required so that you are not deficient.
These values look mainly on how much vitamin D is needed to maintain bone health.
However, with so much research showing that vitamin D is critical to preventing many other diseases many experts now recommend higher blood levels. In fact, blood levels in the 30 to 60 ng/ml range are said to be ideal for a variety of healthful benefits.
While food and sun exposure may keep people from coming deficient, most would need to supplement vitamin D to reach the 30 to 60 ng/ml range.
The vitamin D supplements shown below are the only two that are certified by the United States Pharmacopeia. This agency certifies supplements based on ingredients, quality and manufacturing process.
As with anything check with your doctor before starting vitamin D and get your level checked so you know where you stand and if the supplement is actually helping.
Savings, tip…. buy in bulk to get lower prices if you determine you will be on vitamin D supplements for an extended time.
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Have you tried using vitamin D supplements? Also, please share how they worked for you. Chime in below with your comments and thoughts.