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Submitted on October 16, 2006
Accepted on March 26, 2007
University of Wisconsin Osteoporosis Clinical Research Program, Madison, WI; Human Nutrition, Food and Animal Sciences, University of Hawaii at Manoa, Honolulu, HI; Laboratory Medicine, University of Wisconsin, Madison, WI; Medical University of South Carolina, Charleston, SC
* To whom correspondence should be addressed. 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 which might have been confounded by imprecision of the assays employed.
Objective: The objective of this study 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, HI, (latitude 21°).
Participants: The study population consisted of 93 adults 30 women and 63 men, mean (SEM) age and BMI of 24.0 (0.7) years and 23.6 (0.4) kg/m2 respectively. Their self-reported sun exposure was 28.9 (1.5) hours/week 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 < 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.
Conclusion: These data suggest that variable responsivity to UVB radiation is evident among individuals, causing some to have low vitamin D status despite abundant sun exposure. Additionally, as the maximal 25(OH)D concentration produced by natural UV exposure appears to be 
60 ng/ml, it seems prudent to utilize this value as an upper limit when prescribing vitamin D supplementation.
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