Having demonstrated the importance of optimal vitamin D at all stages of life, from fetal development to old age, dosage recommendations for vitamin D can be addressed. The most important consideration is serum 25(OH)D levels. The consensus of scientiﬁc understanding13,14,140-143 is presented in Table 1. Several studies have found calcium absorption and parathyroid hormone (PTH) levels plateau for 25(OH)D levels near 30 ng/mL.140,144-147 Although the optimal range of 25(OH)D is still the subject of debate, it is assumed to be approximately 30-50 ng/ mL (75-125 nmol/L) or higher.142 Exposure to solar UVB irradiation as it contributes to serum 25(OH)D levels depends on latitude, time of day, season, fraction of body exposed, whether one visits indoor tanning facilities,76 skin pigmentation, body mass index, and amount of body fat.148 Non-UVB factors include diet, vitamin D supplementation, and use of certain pharmaceutical drugs, such as glucocorticoids.149,150
The guidelines currently in place in the United States recommend 5 µg/day (200 IU/day) of vitamin D for children and younger adults, 400 IU/day for those ages 51-70, and 600 IU/day for those over age 70.151 These guidelines are based on maintaining bone health. Since 1997, much has been learned about the non-calcemic benefits of vitamin D, essentially making these guidelines obsolete. From evaluation of vitamin D consumption among nurses and male health professionals in cohort and other studies, the mean intake of vitamin D at age 50 and older is approximately 320 IU/day in the United States, with about 200 IU/day coming from dietary sources.125,136 By one assessment, no child or adult received the recommended vitamin D dose from dietary sources alone.125 The average summertime serum 25(OH)D levels for white adults in Canada and the northern portions of the United States are in the range of 30-35 ng/mL, dropping to 25 ng/mL in winter (Table 2), putting most people in the insufficient range.
In France, where food fortiﬁcation with vitamin D is perhaps lowest,161 the wintertime serum 25(OH)D level for adolescents drops to as low as 8-10 ng/mL, clearly in the deﬁcient range. From the average adult vitamin D intake of 320 IU/day and the wintertime 25(OH)D level of 25 ng/mL, minus the value of 8-10 ng/mL from France, the ratio of vitamin D intake to serum 25(OH)D levels is 0.05 ng/mL/IU/ day. Clinical studies found 500-1,000 IU of vitamin D/day maintains serum levels of 30 ng/mL (0.06 ng/ 123,162,163 Thus, using the clinical value, to reach the upper end of the optimal range (50 ng/mL) in the absence of solar or artiﬁcial UVB irradiation, vitamin D intake should be 1,000 IU/day. Levels as high as 4,000 IU/day have been demonstrated to be safe for up to six months.141,164,165 However, there are concerns that at higher doses (>1,000 IU/day) over extended periods of time, some adverse effects may occur, such as increased risk of prostate cancer.166,167 At higher values of 25(OH)D, vitamin D resistance may occur.168 However, modest levels of 25(OH)D (15-25 ng/mL) seem to provide the optimal reduction of risk for prostate cancer.166,167,169