It was previously reported that old age was an important independent predictor of cognitive decline[14], and the present study also confirmed that increasing age was associated with cognitive decline. The possible reason is the gradual decline in cognitive function due to physical deterioration with age[15]. Females exhibit a higher risk of cognitive decline, which was consistent with the findings of previous studies[16]. There are possible reasons: first, the elderly people in this study were born before the 1950s. The traditional family patriarchal concept of patriarchal family resulted in women's social status, education level, and nutritional supply being inferior to men, adversely affected women’s cognitive function[17, 18]; second, the decreased secretion of estrogen in elderly women performed less protective effect on cholinergic nerve fibers, which damaged their cognitive function[19]. Lower level of education is a risk factor for cognitive decline according to this study. Previous researches indicated that the incidence of MCI decreases significantly with the increase of education level[20], as a possible result of their high ability to accept and learn new things. And the brain was constantly stimulated to increase its cognitive reserve and synaptic connections through learning[21]. The elderly in married took a lower risk of cognitive decline, which may be related to their positive and optimistic attitudes. It’s consistent with a previous study which found being married was associated with better cognitive function[22]. The elderly lived in rural areas before the age of 16 was at greater risk of cognitive decline. This may be due to the poor education and nutritional status of older people living in rural areas during their early years, which affected the cognitive function of the elderly.
Some health behaviors associated with cognitive decline were found by the present study, for example, current smokers were more likely to experience cognitive decline than former smokers or non-smokers, consistent with previous studies[23, 24]. As the world’s largest producer and consumer of tobacco, China should viable strategies such as stronger control regulations of tobacco and education about the dangers of exposure to secondhand smoke to reduce the cognitive decline and MCI epidemic in the elderly[25]. Several previous studies have shown that limited alcohol intake in early adulthood and small to moderate alcohol intake in old age may decrease the risk of cognitive decline and dementia [26, 27]. And in consistent with previous studies, the current study found that drinking once a month and above in the past year was associated with reduced risk of cognitive decline[28]. However, contrary findings have also been reported. So the researches on mechanisms based on the observed association between alcohol and brain function are required in the future[23]. A previous study showed healthy seniors took a 90-minute nap every day as a napping regimen, so we selected 90 minutes as the critical point for judging excessive nap time[29]. The results of the present study showed findings that long naps increase the risk of cognitive decline in older adults. However, the relationship between napping and cognitive function deserves further study[29, 30]. Additionally, although we found that taking care of grandchildren was protectively against the progression of cognitive decline, as compared to not taking care of juniors, the impact of caring for grandchildren on the physical and mental health of older adults is controversial now. Some scholars have found that caring for grandchildren increased the risk of cognitive decline by increasing the mental burden and physical exertion of older adults[31]. While other scholars believed that caring for the younger generation was beneficial to the cognitive function of the elderly, because it not only increased the frequency of activities, but also promoted their physical and mental health[32].
This study showed significant associations between cognitive decline and health status, including difficulties with everyday activities, current self-rated health status and BMI. Studies previously have shown physical activity was associated with a decreased risk of cognitive impairment and provided a modest improvement in the cognition function[23, 33]. The elderly who had difficulties in everyday activities were more likely to lack physical activity due to inconvenience. So, it’s easy to understand that the cognitive function of them declines faster. The current study also found that the elderly having difficulties in everyday activities were at high risk of cognitive decline. In this study, we found that poor self-rated health was associated with cognitive decline. But self-rated health is so comprehensive and non-specific that it is difficult to interpret[34]. A previous study found that self-rated health strongly affecting instrumental abilities of daily living decline, and it may be related to cognitive decline[35]. However, it is clear that there are still many unanswered questions and significant uncertainty with respect to the relationship between self-rated health and cognitive decline, and need for more research it. The majority of participants underweight were more susceptible to cognitive decline, while overweight and obese were vital protective factors for them. The relationship between obesity and cognitive function or AD in the elderly is controversial at present[36, 37], and further prospective researches are needed to provide relevant evidence.
This study covered 150 county-level units and 450 village-level units in China, and the sample is highly representative. The results from this analysis may suggest important policy implications for the cognitive function of Chinese elderly people. However, the follow-up period since the baseline survey in this study was only 4 years, and cognitive function change is a chronic process that requires longer follow-up observation. In addition, all participants in this study were from China, and heterogeneity of samples also affects cognitive decline studies due to their differences in origins, locations and cultures. Researches which contain a wide range of populations with different cultures will be needed in the future.