Vitamin D Supplements: What You Need to Know

Dermveda explores the different forms of vitamin D supplements

Credits: Brad Huchteman on
Anna Pleet
Edited By:
Dermveda Content Team ,

How Do I Get Vitamin D?

UVB irradiation induces the vast majority of active vitamin D formation in the body.[1] In fact, only about 10-20% of our vitamin D needs can be met by dietary sources alone,[1] and individuals often have difficulty meeting their full daily requirement. One study of the Dutch elderly population found that about 99% of individuals aged 65-80 years were not meeting their average vitamin D requirements.[2] In a survey of about 60,000 French adults, researchers found that the majority of individuals misidentified key roles of vitamin D and very inconsistently guessed their vitamin D status, indicating a lack of knowledge and understanding about the health effects of vitamin D and how to get it.[3]

Since diet provides insufficient amounts of vitamin D and many people live in areas with limited sunlight exposure, many physicians and health practitioners turn to vitamin D supplementation. This can be a confusing area, though. Many different types and forms of vitamin D supplements are available over-the-counter and in varying amounts. So which type is best?


Vitamin D Supplementation

Vitamin D Types and Sources

There is much controversy over whether different types of vitamin D supplementation provide equal effects in maintaining active vitamin D levels in the body. The main types of vitamin D used in supplementation are D2 (ergocalciferol) and D3 (cholecalciferol). One trial found that supplementation of either D2 or D3 was equally effective in raising serum vitamin D concentrations over 11 weeks.[4]

A second study found contradicting results, stating that over the winter months in a New Zealand population, supplementation of D3 was found to be superior to D2 in raising serum vitamin D concentrations.[5] A meta-analysis and systematic review of randomized controlled trials (RCTs) comparing vitamin D2 to D3 supplementation in changing serum vitamin D levels also determined that D3 supplements are more effective than D2.[6] However, the authors of this systematic review state that additional research is required to tease out metabolic pathways and confounding variables of these results.[6]

While the efficacy of different types of vitamin D may affect one’s choice in supplementation, the source of the supplemental vitamin is also a factor. Vitamin D2 comes from plant sterols, commonly found in different varieties of fungi and yeasts.[7,8] This differs from the D3 form of the vitamin, which is commonly found in animal sources, such as cod liver oil.[7] Fish contain the highest natural content of vitamin D (in D3 form) available to humans,[8] but it is important to note that they contain such high concentrations of vitamin D3 due to their consumption of microalgae in the water.[8] This discovery points out that plant sources of vitamin D3 do in fact exist, debunking historic thinking that plants only provide the D2 form.[8] For individuals who are vegan or vegetarian, it is important that they identify the exact source of the vitamin to assure it is plant-based. 


Historically, high doses of oral vitamin D supplementation (10,000 IU and higher daily doses) were administered to treat asthma and rheumatoid arthritis,[9] and current research still explores this intervention. A preliminary case report showed that doses up to 60,000 IU of daily oral vitamin D3 supplementation increased serum vitamin D levels and improved asthma symptoms in three different subjects.[9] Furthermore, this case report indicates that these high doses were safely tolerated. It should be noted that these subjects were only given vitamin D3. 

A separate study explored whether high doses of D3 compared to high doses of D2 in 109 adult subjects with known severe vitamin D deficiencies.[10] The results of the study indicated that participants given high doses of oral vitamin D3 (50,000 IU daily over 10 consecutive days) showed significant increases in serum vitamin D concentrations maintained over several weeks compared to those given high doses of oral D2 (60,000 IU daily for 10 consecutive days).[10] In fact, the single mega-dose of vitamin D2 (600,000 IU) was not effective in raising serum vitamin D concentrations and actually decreased levels in subjects given this dosage.[10] The investigators in the study attributed vitamin D3’s success to the subjects’ high compliance over a short 10-day administration period.

Supplement Forms and Products

Evidence suggests that not all forms of vitamin D supplements are equal. One preliminary study compared vitamin D3 supplements in three different forms: chewable tablets, oil-emulsified drops, and an encapsulated powder, each labeled at 10,000 IU.[11] Pairwise comparisons of administration of each type of vitamin D3 supplement in 10,000 IU daily dosages (results adjusted to actual determined amounts) showed that the oil-emulsified drops provided greater increases in serum vitamin D levels on a per weight basis than the tablets or drops.[11]

Table 1 – Dosage of Vitamin D Based on Formulation[11]


Actual Dose Based on

Third-Party Testing

(Mean dose measured in labeled 2,000 IU dose)

Mean Increase in

Serum Vitamin D3

(ng/mL 25(OH)D/mcg D3)


3,943 IU

(197% of labeled dose)

0.125 ± 0.015



2,208 IU

(110% of labeled dose)

0.106 ± 0.017



4,131 IU

(207% of labeled dose)

0.068 ± 0.016


Third-party testing of the different D3 products (oil drops, chew tablets, and powder capsules) from the above trial showed that two out of the three supplement forms indicated wide variability in the actual dose contained within the supplement, differing from what was indicated on the label (Table 1).[11] This phenomenon is seen in other studies as well. A pilot study analyzed vitamin D concentrations in 29 different fortified foods and 15 different supplements intended for infants.[12] Results showed that compared to labeled amounts, concentrations differed 50-153% in fortified foods and 8-177% in supplements tested.[12] Based on both studies and tests, it can be concluded that there may be over and under-estimation of vitamin D concentrations in different vitamin D products.


Risks of Supplementation

As with many other nutrients, a “U-shaped” curve suggestively describes the relationship of the risks associated with different physiologic levels of vitamin D.[13] What this means is that there are risks with both low and high levels of vitamin D in the body, while average or moderate levels indicate the lowest risk. More studies have assessed the risks of low vitamin D levels compared to studies that have evaluated the risks of high vitamin D levels. As a result, little is known about the risks of high vitamin D in the body, and more research is needed to understand exactly how high vitamin D levels may be harmful.[13]

With regards to the risks associated with high levels of vitamin D, although more research is needed regarding high serum levels, the most well known risk is hypercalcemia or increased calcium levels in the blood.[13] Risks for some types of cancers are also associated with high vitamin D levels.[13] The medical research community actively seeks to establish safe threshold levels for supplementation and to set optimal serum ranges for the general public.[13] Until then, medical researchers advise clinicians to first determine a patient’s active serum vitamin D level and make recommendations for supplementation based upon it.


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