NASHVILLE, TENN. — Physicians may need to rethink their advice to patients to get some casual sun to ensure vitamin D sufficiency, according to new data presented at the annual meeting of the American Society for Bone and Mineral Research.
In a study of young adults in Hawaii, abundant sun exposure did not guarantee adequate serum levels of vitamin D, said Dr. Neil C. Binkley of the osteoporosis clinical center and research program at the University of Wisconsin in Madison.
“This suggests that some individuals—for an unclear reason or reasons—do not achieve a high circulating D3 concentration despite abundant sun exposure,” he said. “It seems logical that the clinical approach that many of us take to recommend casual sun exposure for patients is certainly not a guarantee of vitamin D adequacy.”
Initially, individuals with abundant sun exposure were studied in order to identify the upper limit of the normal range of 25-hydroxy vitamin D3 (25[OH]D3).
The investigators recruited 100 young adults, most of whom were surfers, from the Honolulu area in March 2005.
Participants were required to have had 3 or more hours of sun exposure per day, 5 or more days per week, for the previous 3 months. Information about sun exposure, sunscreen use, and dietary vitamin D intake was obtained by questionnaire. Skin color was measured by reflectance colorimetry.
Forty-one percent of the cohort used no sunscreen. On average, participants were exposed to the sun for about 4 hours per day, or 28 hours weekly.
This estimate included hours exposed to the sun with sunscreen.
As part of the self-questionnaire, respondents were asked to fill in areas of skin coverage on human figures. The researchers were then able to calculate that the group had an average of 10.6 hours per week of whole-body sun exposure.
A group of 174 healthy controls were recruited from the University of Wisconsin between January and April of 2005. No data were obtained from this group regarding sun exposure, sunscreen use, and dietary vitamin D.
The two groups were similar in terms of average age, 24, and average body mass index, 23. There were slightly more men in the Hawaii cohort (67% vs. 46%).
Serum 25(OH)D3 was measured by a precise high-performance liquid chromatography assay.
Results correlated well with those obtained by tandem mass spectroscopy.
Using a cutoff of 30 ng/mL to define inadequate vitamin D status—a point of some contention among researchers—86% of the Wisconsin cohort was vitamin D inadequate. And “somewhat surprisingly, 51% of the Hawaii cohort was below this arbitrary cutoff.” The mean concentration of serum 25(OH)D3 was 31.4 ng/mL in the Hawaii cohort, compared with 18.3 ng/mL in the Wisconsin cohort.
The highest levels of 25(OH)D3 were almost identical in the Hawaii and Wisconsin groups (62.0 ng/mL vs. 62.3 ng/mL). Levels of 25(OH)D3 in the Hawaii group were unrelated to hours of sun exposure or skin color.
Among those in the Hawaii cohort, the 10 participants with the lowest serum 25(OH)D3 levels were similar to the rest of the cohort in terms of parathyroid hormone levels, age, body mass index, serum chemistries, multivitamin use, and number of hours of sun exposure without sunscreen.
They did have a lower average number of hours per week of whole-body sun exposure (6 hours).
Serum vitamin D3 levels were also measured in a random subset of 20 individuals in the Hawaii cohort. “Despite abundant sun exposure, some of these individuals do not maintain a high serum concentration of D3,” Dr. Binkley said.
The individuals with the highest 25(OH)D3 levels in both cohorts had levels slightly higher than 60 ng/mL.
This suggests that the upper limit for normal 25(OH)D3 levels is 60–80 ng/mL, which is consistent with published reports, Dr. Binkley said.
The results are limited by the self-reporting of sun exposure.