Several studies have reported that vitamin D2 is between 30 and 50 per cent less effective as the D3 form in maintaining blood levels in humans.
The new study, published in the Journal of Clinical Endocrinology & Metabolism , could challenge this view however.
The three-month study with 68 subjects found that supplementation with both forms produced similar results.
Vitamin D refers to two biologically inactive precursors - D3, also known as cholecalciferol, and D2, also known as ergocalciferol.
The former is produced in the skin on exposure to UVB radiation (290 to 320 nm).
The latter is derived from plants and only enters the body via the diet.
Both D3 and D2 precursors are hydroxylated in the liver and kidneys to form 25- hydroxyvitamin D (25(OH)D), the non-active 'storage' form, and 1,25-dihydroxyvitamin D (1,25(OH)2D), the biologically active form that is tightly controlled by the body.
"Our results suggest that vitamin D2 is as effective as vitamin D3 in sustaining both 25(OH)D and 1,25(OH)2D levels and improving bone health," wrote lead author Michael Holick.
"Our observations also suggest that 1,000 International Units (IU) of vitamin D2 or vitamin D3 is required to sustain blood levels of 25(OH)D above a mean of 20 ng/ml, but was insufficient in raising the levels above a mean of 30 ng/ml."
Currently, the vitamin's RDA is 400 IU, but calls have been growing louder to increase this level considerably.
The new study appears to support this view.
Holick and co-workers randomly assigned the subjects (average age 38.6) to one of four intervention groups for 11 weeks.
The first received placebo, the second received 1,000 IU (25 micrograms) vitamin D2, the third received 1,000 IU (25 micrograms) vitamin D3, and the fourth received 500 IU vitamin D2 plus 500 IU vitamin D3.
All supplements were manufactured by Tishcon Corp. Sixty percent of the adults were vitamin D deficient at the start of the study.
The Boston-based researchers report that adults in the placebo group experienced no significant change in their total 25(OH)D levels during the winter and early spring.
Adults receiving the vitamin D supplements all experienced similar 25(OH)D level increases, however.
In addition, the 1,000 IU dose of vitamin D2 or D3 did not raise 25(OH)D levels in vitamin D deficient subjects above 30 ng/ml.
"One thousand IU of vitamin D2 daily was as effective as 1,000 IU of vitamin D3 in maintaining serum 25-hydroxyvitamin D levels and did not negatively influence serum 25-hydroxyvitamin D levels," said Holick.
"Therefore, vitamin D2 is equally as effective as vitamin D3 in maintaining 25-hydroxyvitamin D status."
"More studies are needed to determine whether the media (i.e., ethanol vs oil vs lactose) that vitamin D2 and vitamin D3 are dissolved in influence either their bioavailability or catabolism," concluded the researchers.
Commenting independently on the research, Daniel Fabricant, PhD, vice president, scientific and regulatory affairs for US trade association, the Natural Products Association (NPA) told NutraIngredients.com: "It's an interesting area of study, as with any such study it generally leads to more questions than it answers.
But it may bode well for vitamin D2, of course more research will be needed."
"While this study may continue to confound the understanding of optimal D status, until the body of D2 clinical studies produce the results the D3 studies have, D3 will most likely continue to be favored by the research community."
Dr. Fabricant added that the analytical methodology of measuring vitamin D also require attention since multitude challenges currently exist.
"One issue with Liquid Chromatography/Mass Spectrometry (LC-MS) is that it has a relative inability to discriminate between 25(OH)D3 and its inactive isomer 3-epi-25(OH)D3, additionally the production of 25(OH)D2 as a result of vitamin D2 supplementation may additionally hinder the assessment of total circulating 25(OH)D, apparently some common assay systems used for clinical purposes have either a diminished capacity or do not detect 25(OH)D2 with the same efficiency as 25(OH)D3," he said.
"It would be important to run some sort of comparative analysis to determine whether this was also a factor in LC-MS analysis of Vitamin D." Previously, researchers from Creighton University in Omaha reported in 2004 that while both forms of the vitamin do produce similar rises in serum concentration of the native vitamin, indicating equivalent absorption, only vitamin D3 sustained 25(OH)D levels over a 14 day period.
However, serum 25OHD fell rapidly in the D2-supplemented subjects and was not different from baseline at 14 days.
Source: Journal of Clinical Endocrinology & Metabolism Published online ahead of print, 18 December 2007.
doi:10.1210/jc.2007-2308 "Vitamin D2 is as effective as vitamin D3 in maintaining circulating concentrations of 25-hydroxyvitamin D" Authors: M.F. Holick, R.M. Biancuzzo, T.C. Chen, E.K. Klein, A. Young, D. Bibuld, R. Reitz, W. Salameh, A. Ameri, and A.D. Tannenbaum Read NutraIngredients.com's report of the 2004 Creighton University report here .