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

Abstract

BackgroundPrevious 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.MethodsThis 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.ResultsThe 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).ConclusionLong-term high sun exposure is positively related with better cognitive functioning.

Association between Vitamin D and Cognitive Deficiency in Alcohol Dependence

“…With the discovery of vitamin D receptors and vitamin D hydroxylase in the brain, there has been an increase in the number of studies on the possible association between vitamin D and cognitive function [ 32 ]. Data obtained from studies on animals and in vitro indicate that vitamin D plays a role in brain function by regulating neurotrophic factors, neurotransmission, and influencing synaptic plasticity [ 33 , 34 ]. However, there is a lack of consensus among population-based epidemiological studies on the association between serum vitamin D deficiency and cognitive impairment [ 16 , 35 , 36 , 37 ].…”

“…The reason for this inconsistency may well lie in the fact that it is very difficult to measure an individual’s vitamin D levels in the long term. Serum concentration of 25-OH D is the accepted index of vitamin D status and its life expectancy is approximately two-three weeks, while cognitive impairment is a gradual and long-term process [ 33 ]. Cognitive screening is useful when a more detailed neuropsychological assessment cannot be performed or is inadequate.…”

Seasonality of cognitive function in the general population: the Rotterdam Study

“…Moreover, participants examined around the fall equinox were more likely to meet the criteria for mild cognitive impairment or dementia. As was previously noted [ 6 ], replication of those results is needed, especially since a peak in cognitive functioning around the fall equinox was found while findings from other studies suggested that performance likely peaks around summer solstice, corresponding to the protective effects of sunlight exposure [ 3 , 16 ].…”

“…Beside ambient temperature, exposure to sunlight is an important season-dependent factor that may drive seasonality effects. An association of sunlight exposure with cognitive functioning has been reported and may have its effect via vitamin D levels, regulations of serotonin and melatonin, and through other mechanisms involved in circadian rhythms [ 3 , 16 , 35 ]. While previous studies found associations of low vitamin D levels with poor cognition [ 36 ], it remains to be determined whether the seasonal fluctuations of vitamin D also lead to fluctuations in cognitive functioning.…”

“…reported and may have its effect via vitamin D levels, regulations of serotonin and melatonin, and through other mechanisms involved in circadian rhythms [3,16,35]. While previous studies found associations of low vitamin D levels with poor cognition [36], it remains to be determined whether the seasonal fluctuations of vitamin D also lead to fluctuations in cognitive functioning.…”