While vitamin D inadequacy has been linked to a variety of diseases, suitable methods of improving vitamin D status, especially in Asian populations, have not been determined.
Researchers at Seoul National University set out to test three hypotheses: firstly, that relevant sun exposure or oral vitamin D supplementation would significantly increase serum 25(OH)D concentrations; secondly, that sun exposure and supplementary vitamin D would be similar in serum 25(OH)D increases; and thirdly, that the two interventions may have different effects on cardio-metabolic markers.
Seeing the light
They conducted an eight-week randomised, placebo-controlled clinical trial on 37 vitamin D-deficient adults in South Korea, comparing changes in serum 25(OH)D concentrations among the participants who underwent 20 to 30 minutes of daily sun exposure around noon, those who were given 500IU of oral vitamin D3 daily, and those in the control group.
To assess the subjects' sun exposure duration more accurately, the researchers used real-time records of their exposure. They also used a placebo to control for background sunlight effects, with the high retention rate minimising the risk of selection bias.
They subsequently reported that both sun exposure and oral vitamin D3 supplementation "effectively increased serum 25(OH)D concentrations" when compared with placebo.
In addition, the mean increase of serum 25(OH)D levels was higher among those who had been supplemented with oral vitamin D3 than those who had undergone sun exposure.
At the end of the study, 54.2% of the participants who had taken oral vitamin D3 had achieved greater serum 25(OH)D concentrations, compared to 12.2% of the participants who had undergone sun exposure, and 4.3% of the participants in the control group.
This could have been attributed to that the participants' overall low compliance with the researcher's advice regarding sun exposure, as only those who had complied adequately experienced a significant rise in serum 25(OH)D levels.
Most public health messages recommend several short periods of sun exposure during summer to ensure sufficient vitamin D production in the body: according to current guidelines, exposing 25% of the skin's surface area for five to 30 minutes between 10 AM and 3 PM two to three times weekly can meet vitamin D requirements.
However, in the current study, 20 to 30 minutes of daily sun exposure around noon during summer and fall proved ineffective in achieving an adequate vitamin D status.
The researchers wrote: "Despite our support to improve adherence, compliance to sun exposure was much lower than that to oral supplementation.
"Given the sun-avoiding culture in Korea, we suppose that adherence to sun exposure in the general population in real-life conditions would be much lower than our results.
"Therefore, for effective public adoption of sun exposure guidelines, it is essential to take into account the widespread cultural practices in Asian countries."
Population-based protocol adjustment
The researchers acknowledged that while the study showed that enhanced sun exposure, as well as 500IU of daily oral vitamin D supplementation, significantly raised serum 25(OH)D concentrations in the participants, their protocol for sun exposure was not as effective as supplementation.
They further stated that the results could be useful in facilitating a better understanding of the optimal amounts of sun exposure and vitamin D intake in Asian countries with similar climates and cultural practices.
In conclusion, they wrote: "Both sun exposure and 500IU per day of oral vitamin D3 supplementation significantly raised serum 25(OH)D levels in young Korean adults. However, the mean increase of serum 25(OH)D was greater with oral vitamin D3 supplementation than with sun exposure.
"Compliance with sun exposure advice was relatively low, and only those with adequate compliance achieved a significant increase in serum 25(OH)D levels.
"We found little beneficial effect on cardio-metabolic markers with either sun exposure or oral vitamin D3. For evidence-based public messages to ensure a sufficient vitamin D status in Asian populations, larger and longer trials with different sun exposure protocols and supplementary doses are warranted."
Source: Clinical Nutrition
"Effect of sun exposure versus oral vitamin D supplementation on serum 25-hydroxyvitamin D concentrations in young adults: A randomized clinical trial"
Authors: Hee-Kyung Joh, et al.