Effect of sun exposure on cognitive function among elderly individuals in Northeast China

Previous findings on the relationship between serum 25-hydroxyvitamin D [25(OH)D] level and cognitive impairment have been inconsistent. One reason is that serum 25(OH)D has a relatively short half-life, but the development of cognitive impairment is a long-term process. Skin exposure to solar ultraviolet radiation is the primary source of vitamin D synthesis. For this study, we hypothesized that people with different sun exposure behaviors would have different cognitive functions.

This study included 1,192 rural-dwelling elders in People's Republic of China whose sun exposure behaviors, including the duration of outdoor activities, use of sun protection, and time of day participants engaged in outdoor activities, were collected. Their global cognitive function was assessed by Mini-Mental State Examination (MMSE). Partitioning cluster analysis was used to classify participants into groups based on their sun exposure behaviors. Ordinal logistic regression analysis was used to examine the association between sun exposure and cognitive function.

The participants were divided into high-, medium-, and low-sun-exposure groups by cluster analysis. The participants in the high-sun-exposure group had the longest sun exposure time, reported the least sun-protective behaviors, and usually spent time outdoors in the morning (8:00-11:00) and afternoon (13:00-16:00). The participants in the low-sun-exposure group had the shortest sun exposure time, had the most positive sun-protective behaviors, and mainly spent time outdoors in the early morning (6:00-8:00) and late afternoon (16:00-18:00). After adjusting for potential confounders, participants with habitually high sun exposure were more likely to have higher MMSE score (25.4) than those with habitually low sun exposure (MMSE score =24.4).

Long-term high sun exposure is positively related with better cognitive functioning.