Maintaining adequate levels of vitamin D during winter months requires a daily dose of 20 micrograms, four times the current recommended dose, says a new study.
The study, led by Susan Sullivan from the University of Maine, has important implications for ongoing consultations on vitamin D recommendations, with the current level of five micrograms (200 International Units) seen by many as insufficient.
Current recommended daily intakes (RDIs) of vitamin D are 200 IU for people up to 50 years of age, 400 IU for people between 51 and 70, and 600 IU for over the 70s years.
However, according to new findings published in the Journal of Nutrition, daily intakes of 20 micrograms are required for twenty-something women to achieve optimised vitamin D status living in Maine.
Vitamin D refers to two biologically inactive precursors - D3, also known as cholecalciferol, and D2, also known as ergocalciferol. The former, produced in the skin on exposure to UVB radiation (290 to 320 nm), is said to be more bioactive.
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.
In adults, it is said vitamin D deficiency may precipitate or exacerbate osteopenia, osteoporosis, muscle weakness, fractures, common cancers, autoimmune diseases, infectious diseases and cardiovascular diseases. There is also some evidence that the vitamin may reduce the incidence of several types of cancer and type-1 diabetes.
Sullivan and her co-workers recruited 112 women with an average age of 22.2 were assigned to receive a placebo from March 2005 until September 2005, and then randomly assigned to receive either placebo or a daily vitamin D3 supplement (20 micrograms) until February 2006.
Serum levels of 25- hydroxyvitamin D (25(OH)D), the non-active 'storage' form, were assessed, and the researchers found that, among the 86 women who completed the study, those receiving the daily vitamin D supplements experienced increases on 35.3 nanomoles per litre, compared to only 10.9 nanomoles per litre in the placebo group.
“Daily supplementation with 20 micrograms of D3 during winter achieved optimal 25(OH)D concentrations (at least 75 nmol/L) in 80 per cent of participants, indicating that this dose is adequate to optimize vitamin D status in most young women in Maine,” concluded the researchers.
Institutes of Medicine gets involved
There have been repeated calls in scientific and public circles for a hike in recommended consumption levels of the vitamin in light of research indicating the protective effects that vitamin D may have against conditions such as diabetes, heart disease and cancer.
At the end of 2008, the US and Canadian governments announced they would be sponsoring a review of vitamin D and calcium that may lead to the establishment of higher recommended daily intakes.
The review, which is to be conducted by the Institute of Medicine (IOM), will involve an examination of all available science to date.
Source: Journal of Nutrition
2009, Volume 139, Pages 540-546, doi:10.3945/jn.108.096180
"Supplements of 20 ug/d Cholecalciferol Optimized Serum 25-Hydroxyvitamin D Concentrations in 80% of Premenopausal Women in Winter”
Authors: M.L. Nelson, J.M. Blum, B.W. Hollis, C. Rosen, S.S. Sullivan