When we think about barriers to getting vitamin D from sun exposure as a population, we often think about indoor working and winter coupled with high latitude as major barriers to making vitamin D. And they are.
However, according to new research in the American Journal of Epidemiology, on a population level, the biggest barrier of them all might be heavy use of clothing.
Researchers from the Queensland University of Technology in Australia examined just over 1,000 Australians aged 18–75 years. These adults enrolled into the study from four different cities:
- Townsville, which is 19.3 degrees south
- Brisbane, 27.5 degrees south
- Canberra, 35.3 degrees south
- Hobart, 42.8 degrees south
The participants received UVR dosimeters to see how much UV (sun exposure) they received each day for 10 days. They were also surveyed and asked about occupation, skin color, hair color, physical activity, diet, supplement use, use of clothing and sunscreen, and other things. The researchers measured their vitamin D levels at the end of the study.
The idea was not to see what vitamin D levels were before and after the 10 days using the UVR dosimeter. Rather, the idea was to see if people had a lifestyle of receiving lots of UVR, would they have higher vitamin D levels?
Predictably, they found that higher vitamin D levels correlated with traditional factors:
- Vitamin D levels were a mean 29.4 ng/ml in Townsville, the city closest to equator, while levels were 19.4 ng/ml in Canberra and 21.7 ng/ml in Hobart, the cities much further from the equator.
- Summer levels were a mean 29.6 ng/ml compared to 20.4 ng/ml in the winter.
- Those who worked outdoors and were physically active had higher vitamin D levels.
- Those who received over 2 SEDs of sun exposure per day (equivalent to about 30 minutes of summer sun exposure) had higher levels than those who received 0-1 SEDs per day.
Furthermore, when the researchers created a model of factors that predicted vitamin D levels, based on this research, they found that clothing was the single biggest factor. How much clothing the participants wore accounted for 27% of the variance in vitamin D levels, the highest percentage for any factor. This was followed by location (20%) and season (17%).
For every 10% decrease in clothing cover, vitamin D levels increased by 2.1 ng/ml without needing to increase the duration of sun exposure of habitually exposed body parts.
This study is a reminder that we need to consciously talk about clothing when we talk about getting vitamin D from sun exposure. Often times we focus on the season, time of day and latitude. Doctors who recommend sun exposure often say to get moderate sun exposure midday. However, it’s also important to mention that we need to expose as much skin as we can (within reason, of course) in order to maximize how much vitamin D we’re making. On a population level, this study shows that clothing accounts for the greatest variance in vitamin D levels, more so than location and season.
Kimlin, M. G., Lucas, R. M., Harrison, S. L., van der Mei, I., Armstrong, B. K., Whiteman, D. C., … Sun, J. The Contributions of Solar Ultraviolet Radiation Exposure and Other Determinants to Serum 25-Hydroxyvitamin D Concentrations in Australian Adults: The AusD Study. American Journal of Epidemiology, 2014.