The findings of this study indicated that after adjusting for the nonindependence of risk factors more than 60% of dementia cases in Jiangxi Province in China in 2018 were associated with the nine potentially modifiable risk factors assessed here. Among the nine factors, the PAFs of the five classical risk factors (low education, physical inactivity, low social contact, hearing loss, and diabetes) for dementia were incorporated in previous studies but were higher in our study (51.2%) than CMDS estimates in China (42.8%); the PAFs of the eight classical risk factors (hearing loss, low education, smoking, physical inactivity, low social contact, hypertension, diabetes, and obesity) for dementia were higher in our study (64.3%) than 10/66 estimates in China (39.0%), indicating the especially higher potential for dementia prevention in moderate aging regions of China. The burden of disability due to dementia in Jiangxi Province in 2018 was heavy, of which physical inactivity, low social contact, and hearing loss were the three most important factors. As a quantitative study to estimate the burden of dementia caused by risk factors, we evaluated the burden of dementia caused by nine risk factors using YLDs as an index. It may provide an opportunity for the general public and dementia patients to consider better management and prevention strategies with a targeted understanding, thereby reducing the risk burden. There was a higher total number of YLDs attributable to the 9 risk factors in males than in females. Males were the dominant group with dementia disability burden caused by the main risk factors. This was because females were more interested in health issues in daily life; therefore. males should pay more attention to the prevention of risk factors (22).
The PAF of low education level (lower than primary school) was 6.8%, which was lower than previous studies on the Chinese population (12-14). Studies have shown that the prevalence and incidence of dementia in older people in China are on the rise, and the sharp increase in the number of patients with dementia is most likely to occur in people with low education level (23). Attaining a higher level of education during one’s life may prevent a large number of symptoms of dementia, which may reduce the incidence and prevalence of dementia. Historically, around the 1970s in China, Jiangxi Province experienced a unique period in which many young people did not have the opportunity to receive adequate education. However, with the popularization and improvement of education, the prevalence of this factor in the population may be greatly reduced. Now and in the future, dementia disability due to low levels of education may not cause as much loss of life as in the past. Physical inactivity was the largest PAF (19.0%) contributor to dementia, resulting in the greatest number of years of disability and life lost caused by dementia among nine risk factors, which was higher than CMDS estimates (13.2%) and 10/66 estimates (5.8%) (13, 14). The results of a meta-analysis showed that physical activity, especially high levels of physical activity, had a significant protective effect on cognitive decline (24). The benefits of physical activity may accumulate throughout a person's life, improving mood and physical function. Physical inactivity was a major risk factor for dementia, and it was also related to risk factors such as diabetes, high blood pressure and obesity, which in turn were associated with an increased risk of dementia (25). Physical inactivity was very common in Jiangxi Province, possibly due to a lack of consensus on health and cultural promotion for older people, which hinders the promotion of health plans and appropriate physical activity programs. It should be noted that there are some bidirectional associations between the status of physical activity and dementia. It is possible that dementia could cause physical inactivity to occur, which suggests that the estimates of PAF for physical activity may be overestimated in our study. It was calculated by our model that low social contact was the second major reason for the heavy burden of dementia, which was caused by the higher prevalence and higher OR. Social conditions in China have changed significantly over the past two decades, and nearly 80% of the older people lack social contact in Jiangxi Province, which is higher than the overall prevalence in China (14). Social contact indicates a social connection with friends, not relatives. Frequent social contact may be independent of social and lifestyle factors. Compared with relatives, keeping in touch with friends can bring more happiness and less pressure, because friends reflect personal choices (26). Social contact has a protective effect on dementia, and more frequent contact will bring higher cognitive reserves (27). Studies have shown that building cognitive reserve capabilities allows individuals to establish and maintain social relationships, while higher cognitive capabilities can prevent dementia that may occur later (27). However, because social dysfunction is a part of dementia, as the severity of dementia increases, the time spent with others decreases, and these changes have been described in the prodromal phase (28). Therefore, similar to physical inactivity, low social contact may be a consequence rather than a cause of dementia (27).
The recognition of hearing loss as a risk factor for dementia was relatively new, although some studies (9, 13, 14) have previously reported on the calculation of its PAF. The estimated PAF for hearing loss in our research (9.3%) was between the CMDS estimates (11.9%) and 10/66 estimates (3.9%), which was mainly based on data from developed countries and regions. Early implementation of hearing protection will help to reduce the burden of this potential risk factor for dementia. Preliminary evidence suggests that the use of hearing aids can reduce the risk of dementia due to hearing loss (13). However, in real life, the use of intervention was often very complex because only a small number of people with hearing loss were diagnosed or treated; for example, many people were not used to or did not often use hearing aids. Compared with the 10/66 estimation for China, hypertension had a greater contribution to dementia in this study, and the burden of disease due to hypertension was also larger. In this study, the prevalence of hypertension was estimated to include older patients with hypertension who were diagnosed by doctors but not those who were not diagnosed or reported, suggesting that our results may underestimate the impact of hypertension on dementia. The OR of hypertension used in the present study was higher than that for other countries (8, 29). Nearly half of Chinese adults between the ages of 35 and 75 had high blood pressure, fewer than one-third were being treated, and fewer than one in twelve were in control of their blood pressure (30). The awareness and treatment rate of Chinese hypertension patients did not increase (12). On the one hand, hypertension is one of the most common chronic diseases, and aging is the main contributor to its development (31). On the other hand, with the rapid development of the economy, unhealthy lifestyle choices, especially excessive dietary sodium intake, are also related to the high prevalence of hypertension in China (32). It is worth noting that the PAF of smoking for dementia based on the OR in the CMDS was higher than the 10/66 estimates in China, and most of the burden of dementia caused by smoking may come from males in Jiangxi Province. Even in old age, the prevalence of smoking among males was still as high as 43.3% (females: 3.6%). The most likely mechanism of the association between smoking and dementia is cardiovascular disease (33). Atherosclerosis and cerebrovascular diseases caused by smoking in turn increase the risk of dementia (34). Cigarettes also contain neurotoxins, which also increase the risk of dementia (35). China is one of the largest tobacco consumers in the world, although interventions to reduce smoking are already being implemented throughout China (including Jiangxi Province). The public's awareness of smoking bans and tobacco control is increasing, but this has not effectively reduced the smoking rate. There are now more smokers of all ages. Having no spouse, diabetes and obesity bear relatively little burden on dementia. Compared with people living with spouses, widowed, divorced or unmarried people had a particularly increased risk of dementia (19, 36), which was similar to CMDS estimates in China. Living with a partner may imply cognitive and social challenges that can protect against cognitive impairment in later life (36). In addition, having no spouse might result in loneliness and less communication or mutual assistance (37). Adipose tissue produces several substances that are important in metabolism (adipokines) and inflammation (cytokines) and are correlated with insulin resistance and hyperinsulinemia (38). Similarly, due to unhealthy eating habits and lack of awareness of diabetes prevention, China has witnessed one of the fastest rising prevalence rates of diabetes in the world (39, 40). There were still quite a few diabetic patients who were not diagnosed in this study, which was similar to the prevalence of hypertension. More work should be done in the diagnosis and treatment of diabetes to further reduce the risk of dementia development and reduce the burden of dementia caused by diabetes. Compared with the 10/66 estimation for China (13), obesity was less prevalent in our survey, but the change to a Western diet together with physical inactivity may likely increase the prevalence of obesity in the future.
Our study has some limitations. First, the dementia status of the participants was self-reported in this study, although those with dementia were diagnosed by doctors. However, it must be acknowledged that the prevalence of dementia we estimated may be underestimated by ignoring individuals who may have dementia but were not diagnosed in hospitals. Some information about the risk factors was also self-reported, which may lead to some information being misclassified due to lack of standardization. Second, this study did not include cohort studies on the risks associated with the assessed factors. The estimate of the OR relied on secondary data, which was determined by published studies involving Chinese populations. Third, we used a questionnaire about the status of daily living ability rather than direct measures to classify the degree of disability in dementia patients, which may affect our results through measurement bias. In future research, the measurement of dementia needs to be improved by measuring care needs or instrumental activities of daily living.