Written by Marcia J. Egles, MD.
In the United States, vitamin D levels have been diminishing significantly for more than a decade, with a majority of the population presently insufficient in vitamin D (1, 2). For people with darkly pigmented skin and for those with limited sunlight exposure, the currently recommended vitamin D allowances may be especially inadequate, according to a comprehensive new study (3) from the University of California, Davis.
The human skin is able to make the precursors of vitamin D if exposed to ultra-violet light (UVB) radiation from sunlight. Ultra-violet light also causes sunburn and, in the long term, may cause skin cancers. Melanin, the brown pigment in skin, acts as a sun filter, protecting the skin from burning. Highly pigmented skin can make vitamin D precursors, but the more pigment there is, the more sun exposure is required. The purpose of this newly reported study was to find out how much vitamin D is made through various levels of sun exposure in a group of individuals representing a range of skin pigmentation (3).
Seventy-two healthy male and female students from the Davis campus (38 degrees north latitude) of the University of California were studied for 7 to 8 weeks each season (fall, winter, spring, summer) over the course of an entire year. They were trained to maintain specific records detailing factors such as their diets, their time spent outside, the time of day and level of sunshine, and the amount and type of clothing worn.
Participants were allowed to take multi-vitamins, but not high dose vitamin D. They could not use tanning beds or significant amounts of sunscreen, but otherwise in general were to follow their usual lifestyles. Sun-measuring wrist badges were worn. Each person’s level of skin pigmentation and vitamin D levels in the blood were measured at the start, middle and end of each season.
Vitamin D deficiency is defined by most experts as a 25-hydroxyvitamin D level of less than 50 nanomoles per liter (or 20 nanograms per milliliter). A level of 75 nanomoles or greater per liter may indicate sufficient vitamin D (2).
In this study, those with dark skin and low sun exposure in the winter had very deficient vitamin D levels of 24 nanomoles per liter, on average. Those with dark skin but high sun exposure in winter were still vitamin D deficient at 42 nanomoles per liter. Low sun exposure was defined as about 20 minutes of sun per day with 18 per cent of the skin exposed. High sun exposure was defined as more than 90 minutes per day with 35 per cent of the skin exposed. In light skinned persons, those with low sun exposure in winter had vitamin D levels in the winter that were deficient at 35 nanomoles per liter. High sun-exposed light skinned individuals on average were at 60 nanomoles per liter in winter. In summer, high sun exposed dark skinned persons achieved average levels of 60 nanomoles per liter, but those with low sun exposures in the summer continued to be vitamin D deficient at 40 nanomoles per liter. Light skinned people in the summer with low exposure were at 58 nanomoles per liter and a robust 85 nanomoles per liter if they had high sun exposure.
The average vitamin D consumed by this group of college students was 200 IU per day. The researchers then estimated the amounts of vitamin D that would optimize the levels of the subgroups in this study. In the wintertime, those with light skin would need a total of 1300 IU of vitamin D per day. Those with dark skin however, at the 38th latitude north of the equator, would need from 2100 to 3100 IU total vitamin D per day year round. By comparison, typically recommended intakes have been on the order of 400-800 IU per day. (Vitamin D toxicity is rare but has occurred with doses of 50,000 IU per day (2)).
An estimated one billion people worldwide have vitamin D insufficiency. People living near the equator in general have excellent vitamin D levels. However, even in the sunniest locations, vitamin D deficiency is common when most of the skin is kept covered from the sun. In studies in Saudi Arabia, the United Arab Emirates, Australia, India, Lebanon and Turkey, 30 to 50 per cent of children and adults have deficient vitamin D levels (2).
Source: Hall, Laura M., Michael G. Kimlin, Pavel A. Aronov, Bruce D. Hammock, James R. Slusser, Leslie R. Woodhouse, and Charles B. Stephensen. “Vitamin D intake needed to maintain target serum 25-hydroxyvitamin D concentrations in participants with low sun exposure and dark skin pigmentation is substantially higher than current recommendations.” The Journal of nutrition 140, no. 3 (2010): 542-550.
© 2010 The American Institute of Nutrition
Posted March 3, 2010.
References:
1. Demographic Differences and Trends of Vitamin D Insufficiency in the US Population, 1988-2004 Adit A. Ginde, MD, MPH; Mark C. Liu, MD; Carlos A. Camargo Jr, MD, DrPH ArchInternMed. 2009;169(6):626-632.
2. Michael F. Holick,M.D.,Ph.D, Vitamin D Defiency , The New England Journal of Medicine 2007: 357: 266-81.
3. Laura M. Hall, Michael Kimlin, et al. Vitamin D Intake Needed to Maintain Target Serum 25-Hydroxyvitamin D Concentrations in Participants with Low Sun Exposure and Dark Skin Pigmentation Is Substantially Higher Than Current Recommendations The Journal of Nutrition, January 6, 2010.
