The present study suggested that the risk of cognitive impairment increased with higher RNT scores among older adults, and the robustness of the finding was confirmed through adjustment for various potential confounding variables. Additionally, individuals aged 60 ~ 79 years, junior high school and above were more prone to suffer from cognitive impairment with a high RNT score. However, the correlation between RNT and cognitive function was not significant in older adults aged 80 to 90 years, or those in elementary school and below.
To date, there have been limited endeavors to explore the correlation between RNT and cognitive function in older adults. Marchant et al. conducted a cohort study in 2016 to find that RNT was associated with decline in cognition, including global cognition, immediate and delayed memory [23]. In addition, the study found that increased level of RNT was associated with cognitive decline and neuroimaging biomarkers of Alzheimer’s disease (i.e., amyloid, tau). Another cross-sectional study found that increased level of RNT was associated with worse subjective cognition and increased memory complaints. Consistent with previous studies, our data demonstrates that higher level of RNT is related to worsecognitive function. In addition to this, this study found RNT was associated with cognitive domains except language skills. When participants were stratified by age and education level, a notable negative correlation was observed between RNT and cognitive function among older adults aged 60~69 years or junior high school and above. There are reasons why RNT does not correlate with cognitive function in older adults who are 80 to 90 years of age or have elementary school or below as follows. Increased brain aging in this age group may have altered the relationship between cognition and mood, or it may have weakened the association. The limited ability of older adults with low education level to perceive and express RNT resulted in a non-significant correlation between RNT and cognitive function.
The underlying mechanisms linking psychological disorder to cognitive function remain vague. One study found that increased level of RNT was closely related to gray and white matter structures in the brain, particularly in the dorso-lateral prefrontal cortex, anterior cingulate cortex, the arcuate fasciculus, and superior longitudinal fasciculus [31]. These regions are related to cognitive control, emotion processing and regulation [32]. Increased level of RNT may lead to changes in the brain’s structural functions related to cognitive control, leading to further cognitive decline. Cognitive debt theory suggests that psychology disorder can lead to damage to the hippocampus by increasing glucocorticoid levels and inducing inflammation and vascular disease in the brain, which impairs cognitive function [33]. RNT as a common trait of many types of psychology disorder, can be initiated and maintained without external triggers or awareness and narrows the scope of attention to repeatedly activated negative thoughts, thus provoking the individual to repeatedly experience physical and psychological distress, leading to the onset of psychology disorders, which in turn may increase the risk of cognitive impairment. As a person adopts the habits of negative thinking for a long-term, it constantly depletes the brain’s limited resources, leading to a decline in the brain’s ability to attention, executive functions, and memory [34, 35]. Older adulthood is a special stage with more pressure and stressful events. Along with the aging process, older adults will face physiological changes such as reduced self-care, frailty and the development of physical illnesses [36,37,38]. At the same time, they will experience negative stressful events such as a reduction in financial income, a decline in social status, and the death of friends and partners [39, 40]. These make older adults vulnerable to RNT, which further can have a range of negative effects on them.
Age is the biggest and uncontrollable risk factor for cognitive decline [41]. Literature has indicated that MCI incidence in China was 11.9% for older adults ages 60 to 69, 19.3% for 70 to 79, 24.4% for 80 to 89, 33.1% for 90 and above [3]. People over 80 are the fastest growing demographic around the world and they are at higher risk of developing cognitive impairment [42]. With the aging process, the physiological of the older adults gradually decline with the structure and function of the brain tissue gradual decline and the function of neural cell loss [43]. In addition, the continued accumulation of health risk factors increases the risk of chronic diseases such as hypertension, diabetes and coronary heart disease [44]. This disease led to amyloid plaque deposition through several mechanisms, such as increased oxidative stress, promoting inflammatory reaction, caused metabolic disorders. These mechanisms increase the risk of cognitive decline.
Education level is a more consistent influence on cognitive function in most studies. Older adults with lower levels of education generally have limited nutritional conditions in early childhood or limited educational resources, which may have an impact on cognitive function [45]. They are more likely to be engaged in manual occupation and lack of exercise for brain, which leads to premature degeneration of neurons in the brain, thus reducing cognitive function [46]. In addition, older adults with lower levels of education may lack such knowledge, further increasing the risk of cognitive impairment [47].
This study offered multiple strengths. Firstly, in examining the association between RNT and cognitive function, the study eliminated as many bias-inducing factors as possible to ensure more reliable results through previous research and by conducting in-depth analyses that took into account a variety of possible potential confounders. Secondly, the study investigated the relationship between RNT and cognitive function through regression and subgroup analysis suggesting a negative association between RNT and cognitive function in community-dwelling older adults. In the future, the assessment of mental health can be incorporated into the health screening of older adults to comprehensively evaluate their health status. Health professionals and carers can enhance the assessment of RNT in older adults and identify problems promptly. By developing interventions to avoid further exacerbation of psychological problems in the elderly and increased risk of other diseases such as cognitive impairment.
However, there were limitations in the present study. First, a definitive causal relationship between RNT and cognitive function could not determine in this study since this study was a cross-sectional design. Secondly, since convenience sampling method was used in this study and all the participants in our study were selected only from Wuchan District and Hongshan District in Wuhan city, which suffered short time span, small sample size, and bad representativeness. In the future, multi-center and a longer time span cohort studies on the relationship between RNT and cognitive function should be carried out to further explore the mechanisms involved. Nonetheless, these findings have implications that are crucial to interventions that promote cognitive function in older adults.