https://www.ncbi.nlm.nih.gov/pubmed/17426097
Low vitamin D status despite abundant sun exposure.
Abstract
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.
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.
Finally, the data reported here are consistent with prior reports of highly sun-exposed individuals that also demonstrate substantial variability in 25(OH)D status. For example, in 18 Puerto Rican farmers with self-reported sun exposure from 32–70 h/wk, two individuals had a 25(OH)D level less than 30 ng/ml (31). Similarly, low 25(OH)D values were reported in some subjects who used a tanning bed at least once a week for 6 wk (32) and among outdoor workers with a sun index of 11.5 (22). Thus, even substantial sunlight or UV exposure does not ensure maintenance of vitamin D adequacy for all individuals, according to currently accepted standards. This implies that the common clinical recommendation to allow sun exposure to the hands and face for 15 min may not ensure vitamin D sufficiency.
This research has been replicated in other populations as wellhttps://www.ncbi.nlm.nih.gov/pubmed/18673464
Vitamin D deficiency in rural girls and pregnant women despite abundant sunshine in northern India.
People who live farther North and South often cannot make any vitamin D3 in their skin for up to 6 mo of the year.41 For example in Boston at 42° North essentially no vitamin D3 can be produced in the skin from November through February. Inhabitants living in Edmonton Canada at 52° North, Bergen Norway at 60° North, or Ushuaia Argentina at 55° South are unable to produce any significant vitamin D3 for about 6 mo of the year. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3897598/