A recent small study of 129 young women in Maine (average age 22.2 years) showed that an intake of 1000 IUs, which is 4 times the current recommended daily intake (RDI) of 200 International Units (IUs) or 5 micrograms (which is thought by many experts to be insufficient), was needed to achieve the accepted optimal serum levels cited in the above studies during the winter months. A daily level of 1000 IU’s was able to raise the levels of the study participants to optimal levels in 80% of those women participating in the study, which sounds like a large number, but even an intake of 1000 IU’s per day clearly was not high enough to achieve optimal levels in 100% of the participants.
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.
Vitamin D collectively refers to two biologically inactive precursors - D3, also known as cholecalciferol, and D2, also known as ergocalciferol. Vitamin D3, produced in the skin on exposure to sunshine, has a higher rate of absorption when consumed orally and is also thought to be more bioactive. However the sunshine levels above a latitude of 37 degrees (roughly a line from Richmond, VA to San Francisco, CA in the US) 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.
So many studies are now indicating a relationship between higher levels of vitamin D intake and reduced risk of various cancers (in addition to better prognosis and/or quality of life) and other costly and debilitating diseases such as diabetes, osteoporosis, heart disease, and several autoimmune diseases, that there have been repeated calls in scientific and public circles for an increase in the recommended consumption levels of the vitamin.
At the end of 2008, both US and Canadian governments announced they would be sponsoring a review of vitamin D that may lead to the establishment of higher recommended daily intakes plus higher levels of recommended upper limits for vitamin D intake.
Bottom line: it is reasonable to consume 1000 IU of vitamin D through foods and dietary supplements as a daily amount. I still suggest having your serum level tested (25-hydroxy vitamin D, which is the storage form) and developing a plan for achieving and monitoring optimal levels from foods, supplements, and sensible sun exposure with your physician.
Source: Journal of Nutrition 2009, Volume 139, Pages 540-546
"Supplements of 20 microgram/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
Diana Dyer, MS, RD
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