Abundant sun exposure doesn't ensure adequate vitamin D status

Key Points

Madison, Wis. - A new study suggests that individuals have variable responsiveness to UV radiation, which causes some to have low vitamin D status despite abundant sun exposure, according to Neil Binkley, M.D., University of Wisconsin Osteoporosis Clinical Center and the Osteoporosis Clinical Research Program, Madison, Wis., and author of the study, published in the Journal of Clinical Endocrinology and Metabolism.

"Even substantial sunlight or UV exposure does not ensure maintenance of vitamin D adequacy for all individuals, according to currently accepted standard. This implies that the common clinical recommendations to allow sun exposure to the hands and face for 15 minutes may not ensure vitamin D sufficiency," says Dr. Binkley, who is board-certified in internal medicine and geriatrics.

In the event of vitamin D deficiency, he notes that the goal of vitamin D replacement therapy should be no greater than the maximum that appears attainable, a serum 25(OH)D concentration of approximately 60 to 65 ng/ml.

Studies have shown that low vitamin D status is extremely common, and may contribute to the development of osteoporosis and osteomalacia/ rickets, as well as increase the risk for falls.

In addition, low vitamin D status may play a role in nonmusculoskeletal diseases, including a variety of cancers, multiple sclerosis, infection, hypertension and diabetes mellitus, he notes.

"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(OH)D concentration, results that might have been confounded by imprecision of the assays used," he tells Dermatology Times.

Dr. Binkley and colleagues set out to document the 25(OH)D status of healthy individuals with habitually high sun exposure. This study was conducted in a convenience sample of adults in Honolulu, Hawaii (21 degrees north latitude). The 93 participants (30 women and 63 men) in the study spent an average of 22.4 hours per week outside without sunscreen and 28.9 hours per week outside with and without sunscreen. Mean age was 24 years.

This yielded a calculated total body sun exposure of 11.1 hours per week with no sunscreen used. 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 of less than 30 ng/ml.

"The mean serum 25(OH)D concentration was 31.6 ng/ml. Using a cutpoint of 30 ng/ml, 51 percent of this population had low vitamin D status. The highest 25(OH)D concentration was 62 ng/ml. Therefore, high amounts of sun exposure do not ensure what is currently accepted as vitamin D adequacy. Thus clinicians should not assume that individuals with abundant sun exposure have adequate vitamin D status," Dr. Binkley concludes.

He notes that because the maximal 25-hydroxyvitamin D [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. Given the widespread nature of low vitamin D status, routine supplementation with 800 IU to 1000 IU of vitamin D3 daily is indicated, he says.

More research

Some limitations of the study include the cross-sectional design and the self-reporting of sun exposure.

Researchers note that it is possible that some individuals incorrectly reported their sun exposure and/or body surface exposed.

"Despite this limitation, this population was clearly highly sun-exposed, as documented by darkening of exposed skin," Dr. Binkley says.

In addition, because this study included only highly sun-exposed individuals, these observations may not be generalizable to those with less sun exposure, he says. While some may argue that the use of 30 ng/ml as a cutpoint is inappropriately high, even if a more conservative cutpoint of 20 ng/ml, as suggested by some, is used, a substantial minority (about 10 percent) of these individuals would still be "low," according to Dr. Binkley.