The study conducted a comprehensive analysis of numerous factors associated with cognition in older adults, and the results revealed the influence of demographic characteristics, various environmental factors, and also uncovered some potential interactions. It was found that Hui seniors possessed higher cognitive scores (i.e., better cognitive levels) compared to Han seniors. Similar findings were confirmed in a 2019 study of Parkinson's disease patients, whose results illustrated that the Hui population had better recall domain function than the Han population.[21] Furthermore, the unique lifestyle and dietary habits of the Hui people (non-smoking and alcohol consumption, high consumption of fish, eggs and dairy foods, neighborhood interaction, and martial arts practice [22]) may also be relevant. In addition, the study similarly yielded results on demographic characteristics consistent with previous studies (negative effects of female, higher age, stroke and CVDs).[4, 23, 24]
Healthy cognitive development relies on a benign environment. This is reflected not only in the living environment such as diet, exercise, and living conditions, but also in the atmospheric environment and the social environment such as education. This study found that as individuals schooling time increases, so does their cognitive level, especially in the early years of education. The process of knowledge acquisition is also a process of brain development, and a good education means a better cognitive reserve.[25–27] A recent study explored the association between education level and [18F]-fluorodeoxyglucose positron-emission-tomography (FDG-PET) hypometabolism.[28] This corroborated the association between high education level and relatively high cognitive performance. Moreover, this effect was also reflected in occupation—more specialized workers had a better cognitive level in old age.
Alcohol consumption in China has been rising in recent years.[7] Results based on more than 40,000 Chinese seniors revealed that alcohol consumption was detrimental to the development of cognition of elderly people, as well as excessive food and tobacco consumption. This was consistent with the findings of several previous studies.[29, 30] However, further refinement of the alcohol categories found that the negative effects were more likely to come from liquor (≥ 38º) and beer. In contrast, wine showed a weak positive effect. In the context of China's unique baijiu culture, baijiu has a large audience in China, compared to the small number of people who drink wine may be the cause of this phenomenon. Alcohol affects individual cognitive function mainly through its neurotoxic effects, which may be mediated directly through damage to brain structures, or indirectly through malnutrition, metabolite toxicity, electrolyte imbalance, and adverse physiological disorders.[31] The effects brought about by ethanol on brain neurochemistry, as well as the neurochemical effects themselves, are highly dose- and time-dependent.[32] The risk of cognitive impairment and dementia is further exacerbated by high levels of ethanol intake.[33, 34]
Albeit the protective effect of alcohol on cognition cannot be dismissed in its entirety, the issue cannot be further corroborated. It is worth affirming that heavy alcohol consumption will definitely affect the benign development of individual health.[35–37] A recent study published in the Lancet Oncology confirms that even small amounts of alcohol consumption can increase the risk of cancer.[38] In conclusion, we strongly recommend that the elderly population should avoid alcohol consumption.
As for the interactions associated with alcohol consumption, although their existence has been confirmed, the results of the alcohol consumption were not statistically significant after the variable adjustment, so it is not possible to clarify the specific mechanisms of these three interactions with the available results. This section needs to be further explored.
In addition, a balanced dietary pattern and active exercise habits have been highly recommended. The Mediterranean dietary pattern (vegetables and fruits, seafood, legumes, etc.) and good exercise habits were effective in improving the cognitive status of the elderly.[39–41] Differently, we found that the high frequency of meat consumption was beneficial to the cognitive level of seniors. This may have been influenced by factors such as famine (1959–1961) and living conditions during the early years. All of the elderly population included in the study were born before the 1940s. The environment of extreme food deprivation had a profound impact on the elderly. In addition, there were also studies confirming that different dietary patterns act on different fields of cognitive function: vegetables were more beneficial for memory capacity, while meat was more helpful in slowing down the decline of attention.[42] All of these revealed the importance of a balanced nutritional intake.
Whether rural residents or living alone (farming, self-care), or high-frequency activity participation, they invariably increase the high-cognitive load of the elderly. Exercise with high cognitive load enhances the functional connectivity of the superior frontal gyrus and prefrontal cortex at rest and reduces the functional connectivity of the middle occipital gyrus and postcentral gyrus, thus delaying cognitive decline.[43] As for the urban-rural differences, the effect of education was not observed in the Chinese elderly population because of the generally low level of education received in their early life. However, earlier research has confirmed that the impact of education was more profound than the activity factor,[27] so it is essential to enhance education for all age groups. Relevant government departments should continue to implement the requirements of the Development Plan for Geriatric Education,[44] further expand the supply of geriatric education resources and strengthen the corresponding service support through measures such as geriatric universities and free open museums, in order to comprehensively improve the quality of life and living of the elderly.
A growing number of studies have shown a negative correlation between air pollutant exposure (long-term or short-term) and individual cognitive function, especially PM2.5.[45–47] The results of our study also confirmed these findings. Air pollution reduces individuals' cognitive-related semantic memory and visual construction abilities.[47, 48] It can enter the body through the respiratory tract, gastrointestinal tract and blood-brain barrier,[49] causing changes in the body's gray matter, white matter and total brain volume,[50] and further impairing cognitive functions through oxidative stress[51] and neuroinflammation.[49] In particular, people with different levels of cognitive function are affected differently by air pollution exposure—the lower the individual's basal cognitive function, the greater the impact of exposure on cognition.[45] This study also found interactions between open-air activities and SO2 emissions and PM2.5 concentrations. When pollutant concentrations (or emissions) were low, the positive effects of exercise overwhelmed the negative effects of air pollution, whereas when pollutant concentrations (or emissions) were high, the act of regular open-air activity instead impaired cognitive function. This was consistent with the results of previous studies.[45, 52] Therefore, the effects of environmental factors should also be considered when advocating exercise for seniors. Documents such as the Chinese Health Guidelines for the Elderly (2013 edition) [53] also need to be further updated.
Due to the limitation of data, we could not know the specific urban areas where the study subjects were located. Therefore, the atmospheric indicators involved in the study were provincial data and could not be further refined. This also reduced the accuracy of the study results to some extent. The types of atmospheric environmental indicators and disease data also need to be enriched. In addition, we did not obtain the relevant genetic data, which is a crucial part of the study.[54, 55]