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Research conducted at Winthrop University Hospital in Mineola, New York, has determined that individuals with vitamin D insufficiency need an average dose of 3800 to 5000 international units (IU) per day in order to attain optimal levels. One hundred thirty-eight men and women with baseline serum values of 25-hydroxyvitamin D of less than 80 nanomoles per liter (nmol/L) were included in the current study. Optimal levels have been proposed to be at least 75 nanomoles per liter by U.S. experts, with an upper limit of 220 nmol/L. In order to determine the amount of supplemental vitamin D3 necessary to achieve this concentration, participants whose serum concentrations of the vitamin were greater than 50 nmol/L were started on 2,000 IU per day or a placebo, and those whose levels were below 50 nmol/L were given 4000 IU or a placebo. Follow up visits occurred at eight week intervals over the six month study period, during which serum vitamin D levels were assessed and vitamin D dosage adjusted accordingly.
All of the subjects who received vitamin D were able to attain the target level by the end of the study. No evidence of toxicity of the doses administered was observed. The authors note that the selection of 2000 IU vitamin D per day as a safe tolerable upper intake level by the Food and Nutrition Board is now considered to be based on insufficient evidence. According to the current study's findings, the authors suggest that individuals whose 25-hydroxyvitamin D levels are above 55 nmol/L consume 3800 IU per day vitamin D to achieve optimal serum levels, and that those whose levels are less than 55 nmol/L consume 5000 IU. The findings were published in the June, 2008 issue of the American Journal of Clinical Nutrition.