Vitamin D in the D2 or D3 form is “equally bioavailable in orange juice and capsules”, according to a study led by Michael Holick from Boston University and funded by the US National Institutes of Health (NIH) and the Beverage Institute for Health & Wellness, a division of Coca-Cola North America.
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 American Journal of Clinical Nutrition, could challenge this view however. Holick and his co-workers arrived at their conclusion after performing a randomized, placebo-controlled, double-blind study with over 100 health adults.
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.
While our bodies do manufacture vitamin D on exposure to sunshine, the levels in some northern countries are so weak during the winter months that our body makes no vitamin D at all, meaning that dietary supplements and fortified foods are seen by many as the best way to boost intakes of vitamin D.
According to Holick and his colleagues, vitamin D is already used to fortify orange juice, and is often added in combination with calcium. “[However,] it is unknown whether vitamin D is as bioavailable from orange juice as it is from supplements,” they said.
In order to answer this question, the researchers randomly divided the participants into six groups, each with between 15 and 20 people per group. The adults, aged between 18 and 84, received 1,000 international units (IU) of vitamin D2 or D3 in orange juice or capsule form, or placebo in juice or capsule form, for 11 weeks. The study occurred at the end of winter.
Results showed that 64 per cent of the participants were vitamin D deficient at the start of the study, with 25(OH)D blood levels lower than 20 nanograms per millilitre (ng/ml). At the end of the study period, the data showed no difference between 25(OH)D levels in any of the groups receiving the vitamin.
The results appear to support earlier findings by Holick’s team. In 2008, the Boston-based researchers reported finding from a three-month study with 68 subjects found that supplementation with both forms of the vitamin produced similar results. The findings were published in the Journal of Clinical Endocrinology & Metabolism (Vol. 93, pp. 677-681).
However, 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: American Journal of Clinical Nutrition
Published online ahead of print, doi: 10.3945/ajcn.2009.27972
“Fortification of orange juice with vitamin D2 or vitamin D3 is as effective as an oral supplement in maintaining vitamin D status in adults”
Authors: R.M. Biancuzzo, A. Young, D. Bibuld, M.H. Cai, M.R. Winter, E.K. Klein, A. Ameri, R. Reitz, W. Salameh, T.C. Chen, M.F. Holick