What is it?

We’ve all heard of vitamin D. Whether it’s from your doctor, on TV, on a cereal box, or in the health food store.  It’s an extremely common supplement, and for good reason! Vitamin D is involved with several important organ systems in the body and its deficiency can cause many different health problems.

Elmer McCollum discovered vitamin D in 1912 after noticing the positive effects of cod liver oil on rickets. It wasn’t initially clear what ingredient in the cod liver oil was providing the health effects, but this led to vitamin D being isolated and extracted. Until 1912, vitamin D was only ever available in cod liver oil! (1) The ability to take vitamin D on its own meant that vitamin D could now be shipped far and wide with far fewer concerns for whether the cod liver oil would go bad while in transit.

Vitamin D is a pro-hormone, which means that it’s an ingredient in the formation of hormones like testosterone, progesterone, estrogen, and cortisol. As a non-essential vitamin, your body does have the ability to produce it in the skin. That said, it can be hard to produce enough unless you live in a country that is close to the equator or you spend lots of time outside.

You’ll notice two letters after the dose of vitamin D on a supplement bottle or a nutrition facts panel: “IU”. This stands for “International Units”. Vitamin D can be measured in a few different ways, but the IU measurement is the most common term. Occasionally, vitamin D will be measured in micrograms (mcg).

For example:

1 IU Vitamin D = 0.025mcg Vitamin D

1000 IU Vitamin D = 25 mcg Vitamin D

Supplements Vitamin D

How is it made?

Vitamin D is one of the only vitamins that can be made by the human body. In order to have sufficient pro vitamin D3 in the skin, you need to eat enough healthy cholesterol in your diet.   When UV-B rays from the sun are absorbed by your bare skin, they cause the conversion of 7-dehydrocholesterol (or pro vitamin D3) to cholecalciferol (vitamin D3). Cholecalciferol is converted by the liver and then the kidney into the active form of vitamin D3 called Calciferol.

Chemical structure flow chart: 

Chemical Structure Flow Chart https://www.researchgate.net

  • Cholesterol
  • PRO Vitamin D
  • UVB Rays
  • PRE Vitamin D
  • Vitamin D3
    • Vitamin D2 can also convert to D3 at this step
  • Circulating Vitamin D
  • Active Vitamin D

You can also get vitamin D from plant sources like mushrooms. This form is vitamin D2 and also called ergocalciferol. It’s widely accepted that vitamin D3 is better absorbed and utilized by the body, so the recommendation when supplementing is to stick with vitamin D3.

The large majority of the vitamin D3 (cholecalciferol) on the market comes from the same source, which is a lanolin base D3. Lanolin is the waxy substances secreted by the sweat glands of sheep! Lanolin is rich in provitamin D and once it has been exposed to UV-B rays, vitamin D3 is formed. Lanolin is harvested by shearing sheep and doesn’t hurt the animal at all. In fact, it’s just like a haircut. Once cut, the fresh wool is stewed gently in hot water and over time, the lanolin separates just like oil on top of the water. Some breeds of sheep, like the merino sheep, can yield up to a whopping 300mL of lanolin from a single coat! (2)

The alternative to lanolin-based D3 is lichen-based D3. Lichen, a kind of plant that grows in moist environments, also known as moss, is one of the only non-lanolin sources that can provide vitamin D in its previtamin form. Vitamin D is also found in mushrooms, in the previtamin  form of vitamin D2 or ergocalciferol. Prior to lichen-based D3’s prominence, D2 products were the solution for patients who needed exogenous vitamin D, but chose not to use lanolin-based D3.

If you’re trying to follow a vegan diet, or trying to avoid products made from animal byproducts, choose a vegan vitamin D product, which is made from lichen (a type of seaweed) instead of sheep’s wool.

If your bottle doesn’t say the source material of the vitamin D and doesn’t say that it’s a vegan vitamin D on the bottle, then it’s most likely made from lanolin. This can happen with other products as well: source material isn’t always communicated on dietary supplements, and the lack of information about sourcing can make it difficult to choose products because you don’t know enough about it.

In which forms is it available?

There are two different kinds of vitamin D commonly available on the market: vitamin D2 and vitamin D3. Vitamin D2 is one of the precursors to active vitamin D, and needs to go through more changes than supplemental vitamin D3 to get to the active form. Fifteen years ago, there was a lot more parity between the amounts of vitamin D3 and vitamin D2 on the market, but vitamin D3 has dominated in recent years because of the additional research and confidence that it’s more bioavailable and more effective at high doses.(3)

When possible, if you’re deficient in vitamin D or could benefit from supplemental vitamin D, consider using D3 to optimize absorption and bioavailability of oral vitamin D instead of D2. (4) Spend time outdoors so the body continues to convert it’s dietary D2 into D3 as well through UVB light exposure to the skin.

When taking oral supplements, absorption can be optimized by using lipid suspensions of vitamin D3, either as a gelcap or a liquid, and by co-ingesting vitamin D with a fatty meal. While the first generation of vitamin D supplements came in the form of dry tablets, the large majority of products have now shifted to liquids and fatty liquid suspensions. An example of a fatty liquid suspension is good old cod liver oil!

Vitamin D is also added to a lot of food products that wouldn’t normally have it, and is one of the best examples of “healthwashing” products. Most people think of vitamin D as a health-promoting vitamin, so adding it to food helps to increase the public’s opinion of the health of that food. The vitamin D that is added to some orange juice products is no different than the vitamin D in a supplement.

How does it work?

Since vitamin D is a fat-soluble vitamin, it will accumulate in the body if it’s taken for a long time or in high doses. This also means that taking it daily isn’t absolutely necessary. There are even some trials that demonstrate vitamin D can be taken as infrequently as once a week in a higher dose. (5) Regardless of how you take it, vitamin D is best taken alongside a fatty meal, bound to fat, suspended in an oil-based liquid, or in an oil-suspension gelcap.

As a prohormone, vitamin D is a precursor to many other important hormones in the body. Vitamin D supplies the backbone structure of other steroid hormones through its own precursor: cholesterol. Once vilified, we now know that cholesterol is pivotal for the body’s overall function. Without that steroid backbone that cholesterol provides, we wouldn’t be able to make vitamin D in the skin, and without vitamin D, we wouldn’t be able to make testosterone, estrogen, progesterone, and cortisol properly.

How can vitamin D help?

In the developed world, vitamin sufficiency is taken for granted due to food security and fortification, or the addition of supplemental vitamins and minerals to foods. However, in developing countries where vitamin D-rich foods and food fortification aren’t as common, vitamin D supplements play a particular role in preventing osteomalacia (rickets), a common condition caused by vitamin D deficiency. It results in children with soft and brittle bones that don’t develop properly, and they end up with a bow-legged appearance. Vitamin D supplementation in the food system and through dietary supplements has significantly reduced the risk and subsequent global impact of this disease. Vitamin supplementation in the food system has also helped to prevent adult bone loss, and increase calcium absorption; another factor in keeping the bones of adults healthy.

Vitamin D’s role in immune health cannot be understated. By regulating hormone production and function, as well as being a pivotal precursor in the adaptive immune response, vitamin D is helpful in both acute and chronic immune disease states. Vitamin D insufficiency is linked with increased rates of pneumonia and the common cold in both children and adults (so you want to make sure you have adequate levels), but supplemental vitamin D above normal levels hasn’t shown to be effective for the treatment of common colds. (6) As long as you have adequate vitamin D levels, you’re getting the immune benefit of vitamin D; you don’t need to ingest excess levels in times of sickness if you’re already at a normal level.

In a review of 7 different studies, The Cochrane Collaboration found that vitamin D supplementation was able to decrease the number of emergency room visits, decrease the need for systemic corticosteroids, and improve lung function in people with asthma. The exact dosing strategy wasn’t clarified, differing between the 7 trials and ranging from 400 IU per day to over 100,000 IU in a single dose. (7)

Multiple Sclerosis is an example of an autoimmune condition that is significantly impacted by vitamin D status. Adequate levels of vitamin D can ensure that the myelin sheaths (the fatty insulating layer around nerve cells) are allowing brain signals to transmit as well as possible, despite the body attacking those very cells and making nerve transmission difficult. Low vitamin status is a risk factor for developing multiple sclerosis, and there is a notable increase in multiple sclerosis rates in countries with long winters, and less sunny weather. (8)

Low vitamin D levels are a risk factor for various aspects of cardiovascular disease, including poor lipid values and higher risk for heart attacks. (9) Considering all of the benefits of adequate vitamin D status, it’s no surprise that the College of Family Physicians of Canada (10) has taken the stance that testing vitamin D is rarely necessary in healthy patients. Prior to low dose vitamin D administration (1000-2000 IU for adults), vitamin D testing isn’t performed, since there is a high proportion of adults that are already deficient in vitamin D (<50nmol/L or < 20ng/ml) and will have a hard time getting enough production from their own body due to limited sunlight in the winter

What are the side effects?

As mentioned above, vitamin D is a fat-soluble nutrient, so there is a chance that you can store excess amounts from high-dosage intake. While the negative side effects don’tt appear until extremely high levels are consumed, there is generally very low risk for side effects unless mega-dosing for a long time.  As we mentioned above, there’s such a low risk for dosing at levels around 1000 IU per day that the Canadian government suggests recommending it to patients prior to establishing a baseline if insufficiency is suspected and then re-testing later.

The most common concern to be aware of when it comes to vitamin D is whether there is an aversion to the source material (lanolin) or other non-medicinal ingredients in the product itself. It’s otherwise an extremely safe, well studied, and a relatively benign ingredient when dosed conservatively.

When dosed aggressively for chronic periods, or in large amounts all at once, it’s important to monitor negative outcomes to bone health. Extremely high levels of vitamin D can paradoxically cause the very same bone conditions it causes at lower levels, as excess vitamin D can alter calcium metabolism in a negative fashion.


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  1. Deluca, Hector F. “History of the Discovery of Vitamin D and Its Active Metabolites.” BoneKEy Reports, vol. 3, Jan. 2014, p. 479.
  2. WHAT IS LANOLIN? https://www.merinousa.com/about-lanolin.
  3. Office of Dietary Supplements – Vitamin D. https://ods.od.nih.gov/factsheets/Vitamin%20D-HealthProfessional/.
  4. Lehmann, Ulrike, et al. “Bioavailability of Vitamin D2and D3 in Healthy Volunteers, a Randomized Placebo-Controlled Trial.” The Journal of Clinical Endocrinology and Metabolism, vol. 98, no. 11, 2013, pp. 4339–45.
  5. Ish-Shalom, Sophia, et al. “Comparison of Daily, Weekly, and Monthly Vitamin D3 in Ethanol Dosing Protocols for Two Months in Elderly Hip Fracture Patients.” The Journal of Clinical Endocrinology and Metabolism, vol. 93, no. 9, Oxford University Press, Sept. 2008, pp. 3430–35.
  6. Linder, Jeffrey A. “Vitamin D and the Cure for the Common Cold.” JAMA: The Journal of the American Medical Association, vol. 308, no. 13, Oct. 2012, pp. 1375–76.
  7. Martineau, Adrian R., et al. “Vitamin D for the Management of Asthma.” Cochrane Database of Systematic Reviews , vol. 9, Sept. 2016, p. CD011511.
  8. Sintzel, Martina B., et al. “Vitamin D and Multiple Sclerosis: A Comprehensive Review.” Neurology and Therapy, Dec. 2017, doi:10.1007/s40120-017-0086-4.
  9. Gouni-Berthold, Ioanna, et al. “Vitamin D and Cardiovascular Disease.” Current Vascular Pharmacology, vol. 7, no. 3, July 2009, pp. 414–22.
  10. Lindblad, Adrienne J., et al. “Testing Vitamin D Levels.” Canadian Family Physician Medecin de Famille Canadien, vol. 60, no. 4, The College of Family Physicians of Canada, Apr. 2014, pp. 351–351.