In many people, vitamin D levels can remain low despite abundant exposure to sunlight, new research shows.
Inadequate sun exposure, especially in Northern climates, is often blamed for the high prevalence of low vitamin D status. However, this explanation does not hold for some people with low vitamin D levels, but high sunlight exposure. Dr. Neil Binkley with the University of Wisconsin Osteoporosis Clinical Research Program, Madison, and colleagues investigated the vitamin D status of people living in sun-drenched Hawaii.
The 93 participants in the study spent an average 22.4 hours per week outside without sunscreen and 28.9 hours per week outside with and without sunscreen. This translates to a mean of 11.1 hours per week of total body skin exposure with no sunscreen used, the authors calculated.
Despite this abundant sun exposure, 51 percent of these individuals were found to have low vitamin D levels, the researchers found. "This implies that the common clinical recommendation to allow sun exposure to the hands and face for 15 minutes may not ensure vitamin D sufficiency," Binkley reported. It should not be assumed that individuals with abundant sun exposure have adequate vitamin D status," the researchers concluded.
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 degrees ).
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/m(2) (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.
Binkley N, Novotny R, Krueger D, Kawahara T, Daida YG, Lensmeyer G, Hollis BW, Drezner MK.
Low vitamin D status despite abundant sun exposure. J Clin Endocrinol Metab. 2007 Jun;92(6):2130-5. Epub 2007 Apr 10. PMID: 17426097 [PubMed - in process]