There are studies that show that Vitamin D3 is the more effective of the two. Below is a reference article regarding some of the research. I take a very high quality multi-vitamin by Pure Encapsulations called "multi t/d" and they chose to use D3.
Here is the research info:
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Vitamin D3 more potent than D2, further evidence
6/21/2004- Vitamin D3 has significantly greater potency than another form of the vitamin, D2, shows a small study, presented last week.
The researchers say D3 supplements will be much more effective at preventing deficiency of the vitamin, a condition gaining increasing attention from the medical community.
Researchers writing in The Lancet last year warned that pregnant women and children should take vitamin D supplements to prevent a resurgence of rickets, the failure to mineralise new bone tissue leading to poorly formed bones. Its incidence is thought to be increasing due to several current lifestyle factors, including greater use of sun protection creams.
Sunlight is the best source of vitamin D but for those exposed to little sunlight, supplements are recommended. Even vitamin D-fortified foods, such as cereals or yellow spreads, do not offer the recommended daily allowance, according to recent surveys.
Since the 1930s, researchers have believed that vitamin D2 and D3 haveessentially the same benefits in people. However, recent studies have suggested that vitamins D2 and D3 are not the same and D3 tends to be more widely used. New findings, presented by Dr Laura Armas from Creighton University in Omaha and colleagues at ENDO 2004 in New Orleans, US, last week, appear to confirm this difference.
The researchers examined the potency of both forms by randomly giving a single 50,000 IU dose of either vitamin D2 or D3 to 20 healthy male volunteers. Over a period of 28 days, blood levels of vitamin D and its product 25-hydroxyvitamin D were measured.
The two calciferols produced similar rises in serum concentration of the native vitamin, indicating equivalent absorption. But while both produced similar initial rises in serum 25 hydroxyvitamin D over the first three days, serum 25OHD fell rapidly in the D2 treated subjects and was not different from baseline at 14 days.
Meanwhile 25OHD continued to rise in the D3 treated subjects, peaking by 14 days and continuing to remain high.
Calculating the difference in potency by measuring the area under the curve revealed an even greater difference with D3 more than nine times more effective than D2.
The fall in 25OHD2 may reflect differences in the affinity of the vitamin D binding protein for the two calciferols, said the researchers.
"Clearly vitamin D3 is the preferable form of vitamin D," they concluded, adding that "physicians using vitamin D2 should be aware of its markedly reduced potency and shorter duration of action".
The D3 form does however pose some limitations. Some manufacturers are reluctant to use it as lanolin is used in its manufacture and this presents issues for vegans.
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