When referring to vitamin D without a numeric qualifier, most literature intends the D2 form. The "Clinical Journal of the American Society of Nephrology" explains that D2, or ergocalciferol, is a synthetic form derived by irradiating plant sterols. Vitamin D3, or cholecalciferol, is the natural form synthesized in human skin by exposure to sunlight. The practical difference between the two became known by measuring the metabolized form of vitamin D known as 25-hydroxyvitamin D in the blood stream. In animal studies, D2 is only one tenth as potent as D3 in its ability to raise blood levels of this metabolite.
Potency
The "Journal of Clinical Endocrinology & Metabolism" reports vitamin D3 is impressively more potent than D2 in human metabolism. The study followed 33 healthy adults who supplemented 50,000 IU of either D2 or D3 for 12 weeks. At the end of the study, D3 was from 56 to 87 percent more effective in raising blood levels of 25-hydroxyvitamin D. The study also revealed that the body stores two to three times more D3 than it does of the synthetic form. The researchers urge that D3 be the preferred form of treatment when correcting for deficiency.
D2 Problems
The "American Journal of Clinical Nutrition" recommends that vitamin D2 no longer be regarded as a nutrient suitable for supplementation or fortification. The researchers make their recommendation out of concern for public health. Consumers are unaware that D3 is more potent, and the public expects to derive the same dose of vitamin D regardless of its form. Further, the most widely recognized benefit of vitamin D supplementation is increased bone strength, and all of the successful clinical trials examining the benefit used the D3 form. No such benefits are associated with D2.
D2 Toxicity
Researchers note that just 15 minutes of sun exposure wearing a bathing suit produces 10,000 to 20,000 IU of vitamin D3. Repeated day after day, this adds up to substantial vitamin D inputs, yet there has never been a reported case of vitamin D intoxication from sun exposure. On the other hand, there are documented cases of hypervitaminosis D, or vitamin D overdose, from drinking milk that was overly fortified with the D2 form. The "American Journal of Public Health" reports on 41 patients who were hospitalized and 2 who died from drinking overly fortified milk that was home delivered.
D3 Benefits
The "Journal of the American Geriatrics Society" looked at the blood serum levels of 25-hydroxyvitamin D in older adults in relation to mortality risk. Researchers followed 3,408 participants age 65 and older for 7 years. The researchers found that the higher the blood serum levels derived from D3, the lower the risk from any cause of death by disease. Subsequent studies are called for to assess the role of supplementing vitamin D3 to help determine if the effect is causal.
References
- "Clinical Journal of the American Society of Nephrology"; Vitamin D in Health and Disease; R. Heaney; September, 2008
- "Journal of Clinical Endocrinology & Metabolism"; Vitamin D3 is more potent than D2; R. Heaney et.al.; December, 2010
- "American Journal of Clinical Nutrition"; The case against vitamin D2; L. Houghton et.al.; October, 2006
- "American Journal of Public Health"; An outbreak of hypervitaminosis D; S. Blank et.al.; May, 1995
- "Journal of the American Geriatrics Society"; Prospective study of serum 25-hydroxy-vitamin D; A. Ginde et. al.; September, 2009
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