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Low Vitamin D Status despite Abundant Sun Exposure

University of Wisconsin Osteoporosis Clinical Research Program (N.B., D.K., T.K., M.K.D.), Madison, Wisconsin 53705; Human Nutrition, Food and Animal Sciences (R.N., Y.G.D.), University of Hawaii at Manoa, Honolulu, Hawaii 96822; Laboratory Medicine (G.L.), University of Wisconsin, Madison, Wisconsin 53792; and Medical University of South Carolina (B.W.H.), Charleston, South Carolina 29425

Address all correspondence and requests for reprints to: Neil Binkley, M.D., University of Wisconsin Osteoporosis Research Program, Suite 100, 2870 University Avenue, Madison, Wisconsin 53705. E-mail: nbinkley{at}wisc.edu.

Context: Lack of sun exposure is widely accepted as the primary cause of epidemic low vitamin D status worldwide. However, some individuals with seemingly adequate UV exposure have been reported to have low serum 25-hydroxyvitamin D [25(OH)D] concentration, results that might have been confounded by imprecision of the assays used.

Objective: The aim was to document the 25(OH)D status of healthy individuals with habitually high sun exposure.

Setting: This study was conducted in a convenience sample of adults in Honolulu, Hawaii (latitude 21°).

Participants: The study population consisted of 93 adults (30 women and 63 men) with a mean (SEM) age and body mass index of 24.0 yr (0.7) and 23.6 kg/m² (0.4), respectively. Their self-reported sun exposure was 28.9 (1.5) h/wk, yielding a calculated sun exposure index of 11.1 (0.7).

Main Outcome Measures: Serum 25(OH)D concentration was measured using a precise HPLC assay. Low vitamin D status was defined as a circulating 25(OH)D concentration less than 30 ng/ml.

Results: Mean serum 25(OH)D concentration was 31.6 ng/ml. Using a cutpoint of 30 ng/ml, 51% of this population had low vitamin D status. The highest 25(OH)D concentration was 62 ng/ml.

Conclusions: These data suggest that variable responsiveness to UVB radiation is evident among individuals, causing some to have low vitamin D status despite abundant sun exposure. In addition, because the maximal 25(OH)D concentration produced by natural UV exposure appears to be approximately 60 ng/ml, it seems prudent to use this value as an upper limit when prescribing vitamin D supplementation.

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