Form in Dietary Supplements

What forms of vitamin D are found in dietary supplements?
The two forms of vitamin D used in dietary supplements are ergocalciferol (vitamin D2) and cholecalciferol (vitamin D3). Ergocalciferol is sometimes considered a vegetarian source of vitamin D since it can be plant-derived. However, yeast is also commonly used as a source of D2 as are other fungi (like ergot). Some individuals would regard these microbially produced forms of D2 as animal-based, while others would not.
There's also one "synthetic versus natural" issue involved with supplemental vitamin D. When the plant building block for vitamin D2 (ergosterol) is used to produce vitamin D2 (ergocalciferol), it's usually irradiated in a chemistry lab in order to produce this D2 form.
Cholecalciferol, the D3 form of the vitamin, can be obtained from animal or microbial sources. One practice for generating the D3 found in supplements involves sheep's wool. Sheep (and many other animals) have sebaceous glands in their skin that secrete a complex variety of substances, including cholesterol (in the form of 7-dehydrocholesterol). The secretions from the sebaceous glands naturally find their way into the animal's fur. A supplement manufacturer wanting to produce vitamin D3 supplements can remove the secretions from the fur (in this case sheep's wool), process and purify the 7-dehydrocholesterol, expose it to UVB (ultra-violet B) light, and thereby convert it into cholecalciferol.
From our perspective, very few individuals will benefit from supplementation with vitamin D2 versus vitamin D3. Although we might not go as far as the American Journal of Clinical Nutrition�which argued in 2006 that D2 should no longer be considered as a nutrient �suitable for fortification or supplementation� given the strong hormonal advantages of D3�we believe that the vast majority of unwanted consequences from vitamin D deficiency stem from deficiency of the hormonally active, dihydroxyvitamin D form of this nutrient. For this reason, we believe that D3 is the delivery form of choice when supplementing with vitamin D.
Within the category of D3 supplements, the most common form for vitamin D3 delivery is gel caps containing D3 in liquid form (and often dispersed in flax oil or olive oil). D3 is also available in powdered form in D3 capsules, in powdered form involving pressed tablets, and in unencapsulated liquid form (D3 drops). We have not seen non-proprietary, peer-reviewed studies comparing these various delivery forms of vitamin D3. In general, we have seen more problems with absorption of powdered vitamins from tablets versus capsules due to problems in digestive track breakdown of pressed tablets. For this reason, we favor use of vitamin D3 capsules over vitamin D3 tablets when selecting from dry powder versions of vitamin D3. However, with respect to encapsulated powders versus encapsulated liquids (or liquid drops in unencapsulated form), we believe that your most important consideration is reliability of the manufacturer and quality control/ assurance provided by that manufacturer.
Vitamin D is often measured in International Units (IU) or micrograms. One microgram of cholecalciferol is equal to 40 IU of vitamin D.

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