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So you have heard about the wonderful health benefits of vitamin D supplements and are now shopping, scrolling your way through the endless list of options online. Your screen fills with a bewildering variety of available products, and you may wonder: Why is it vitamin D3, and not simply D? And what does IU mean? Is it the higher the IU, the better the supplement? In this blog, we give you a few pointers so that you can make an informed decision about your vitamin D supplementation…
Just like with vitamin B, there exist several different forms of vitamin D—five, to be exact. In terms of their chemical composition, all the different vitamin D forms are secosteroids, a sub-class of steroids with one broken bond of steroid rings.
For our purposes, vitamin D2 and D3 are the most important forms. Both promote the absorption of calcium in your gut, which in turn regulates and promotes bone growth, and they support your immune and neuromuscular systems. The chart below shows their characteristics in greater detail:
| Vitamin D2 | Vitamin D3 | |
| Also known as… | Ergocalciferol | Cholecalciferol |
| Can the human body produce it? | No | Yes, within the human skin, from 7-dehydrocholesterol, when activated by sunlight |
| From which (nutritional) sources can you get it? | Fungi grown in UV light Dietary supplements Foods fortified with vitamin D | Oily fish (salmon, sardines, etc), Fish oil, Egg yolk, Liver, Butter, Dietary supplements, Foods fortified with vitamin D |
| How is it manufactured for use in supplements and fortified foods? | Yeast is exposed to UV light to convert ergosterol into ergocalciferol | 7-dehydrocholesterol from lanolin (an oily substance found in sheep’s wool, for example) is exposed to UV light |
| How effectively can it raise your vitamin D level? | Less effective at higher doses | Very effective |
Your body can produce vitamin D3 when it is exposed to sunlight; therefore, it is also called the “sunlight vitamin.” Vitamin D2, however, needs to come from your diet. Most fortified foods contain greater amounts of vitamin D2, partially because it is cheaper to produce; however, this form is of lower quality and tends to degrade more easily.
If you want to raise your vitamin D level, look for D3, as this form has been proven to be better absorbed and more effective for this purpose.[1]
IU stands for International Unit. This unit of measurement is used for biologically active substances such as vitamins, hormones, vaccines, medications and the like. The basis for determining this unit of measurement is not mass or volume, but the biological effect. For example, in terms of the effect vitamin A has in your body, 0.3 micrograms of vitamin A in the form of retinol equals 0.6 micrograms vitamin A in the form of beta-carotene, and therefore 1 IU of beta-carotene weighs twice as much as 1 IU of retinol. This unit of measurement is employed by the World Health Organization (WHO) to ensure that we do not compare apples and oranges, but apples and apples when it comes to figuring out how effective supplements and medications are.
For vitamin D3, 1 IU equals 0.025 micrograms. For instance, one tablespoon of cod liver oil contains 1,360 IU, or 34 micrograms. Vitamin D supplements are available in a large range of IU, from 400 to 50,000.
You should be aware that a higher amount of IU does not necessarily mean more benefits, for three reasons. First, excessive doses of vitamin D have been linked to excess calcium in the body (hypercalcemia), which can then cause painful kidney stones.[2] As with most things in life, moderation is key. Secondly, whether you choose vitamin D2 or D3 will make a difference. In other words, vitamin D3 will give you more bang for your buck, even at lower amounts of IU. Thirdly, your vitamin D3 supplement’s other ingredients also play an important role in how effectively your body can process this important micro-nutrient.
All forms of vitamin D are fat-soluble. This means that not only are they more likely to occur in oily substances (such as salmon and butter), but they also more easily dissolve in fats and oils. In other words, vitamin D’s bioavailability—the rate at which it is absorbed into your blood stream to take full effect on the cellular level—is much greater when you take it together with some type of fat or oil.
Have we answered your vitamin D supplement questions? Make sure to consult with your healthcare provider to find the supplement that is best suited to your needs… and happy (informed) shopping!
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Sources
- Arnarson, Atli, PhD, “Vitamin D2 vs. D3: What’s the Difference?” Healthline, https://www.healthline.com/nutrition/vitamin-d2-vs-d3
- Brown, Jessica, “Should Everyone Be Taking Vitamin D?” https://www.bbc.com/future/article/20181010-do-vitamin-d-supplements-work
- “Calcium and Vitamin D,” National Osteoporosis Foundation, https://www.nof.org/patients/treatment/calciumvitamin-d/
- Somerville, Judson, MD, The Optimal Dose: Restore Your Health with the Power of Vitamin D3. Big Bend Press, 2018.
- “Taking Too Much Vitamin D Can Cloud Its Benefits and Create Health Risks,” Harvard Health Publishing, https://www.health.harvard.edu/staying-healthy/taking-too-much-vitamin-d-can-cloud-its-benefits-and-create-health-risks
- Tripkovic, Laura et al. “Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D status: a systematic review and meta-analysis.” The American Journal of Clinical Nutrition vol. 95,6 (2012): 1357-64. doi:10.3945/ajcn.111.031070
- “Vitamin D: Fact Sheet for Health Professionals,” National Institutes of Health: Office of Dietary Supplements, https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/#h8
[1]Laura Tripkovic et al. “Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D status: a systematic review and meta-analysis.” The American Journal of Clinical Nutrition vol. 95,6 (2012): 1357-64.
[2]“Vitamin D: Fact Sheet for Health Professionals,” National Institutes of Health: Office of Dietary Supplements, https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/#h8; “Taking Too Much Vitamin D Can Cloud Its Benefits and Create Health Risks,” Harvard Health Publishing, https://www.health.harvard.edu/staying-healthy/taking-too-much-vitamin-d-can-cloud-its-benefits-and-create-health-risks
