This study used GBD 2017 data to systematically describe the incidence of AD and other dementias at the global, region, and country levels. We found that there were 73 million incident cases of AD and other dementias worldwide in 2017, which was more than double the number in 1990. However, the incidence rate of these diseases has been declining during the past 28 years, which is consistent with other reports in the literature that the incidence rate of AD and other dementias has stabilized or decreased slightly [15–19]. The present study also examined the differences in the incidence rates according to different countries, ages, and sexes. We found that the incidence rate among elderly people (older than 70 years) has increased significantly, with the incidence rate being slightly higher in females than in males. The incidence rate is higher in high-income Asia-Pacific countries, and the lowest in South Asia. We also found that the incidence rate of AD and other dementias has a significant positive correlation with the development level of a country.
The decline in the incidence rate of AD and other dementia is mainly related to improvements in education and the control of risk factors such as vascular disease. According to the viewpoint of Stern [20] regarding the concept of cognitive reserve, at any given level of brain pathology, a higher education level is associated with better cognitive function. That is, individuals who are better educated and have higher cognitive function can tolerate a greater extent of neuropathologic changes before they reach the threshold at which the early symptoms of dementia manifest [20]. Vascular disease is an important factor in the onset of AD and other dementias. Vascular brain lesions are very common in people aged ≥ 70 years, and a large proportion of dementia cases might be attributable to cerebrovascular disease [21]. Pase et al. [22] also found that better cardiovascular health, including better management of stroke risk factors, could be responsible for the observed reduction in the incidence rate of dementia.
Hamad et al. [23] pointed out that at the beginning of the 21st century in Qatar, because the public generally believed cognitive impairment to be a normal aging process, many patients and families arrived at the hospital later than the actual onset time of dementia. The results of the present study show that from 1990 to 2017, Qatar had both the fastest growth rate and the largest increase in the number of incident cases of AD and other dementias among the 195 countries or territories. Another reason might be that public awareness of these diseases is gradually improving as the level of education continues to increase, resulting in more patients visiting their doctors in time, which ultimately leads to an increase in the diagnosis rate. Another major contributing factor that the population of Qatar increased by more than 470% from 1990 to 2017.
The incidence rate of AD and other dementias in Japan showed a slight upward trend from 1990 to 2017, whereas in most other countries it was declining. This is consistent with the investigation by Ohara et al. [24] of the trend in the incidence rate of dementia in a Japanese elderly population. This trend may be related to the relatively high life expectancy of Japanese people [25], the aging of the population, and changes in the diet structure. Under increasing influence of the West, the typical diet structure in Japan has undergone great changes in recent decades, from the traditional low-fat, rice-based diet to a high-fat diet [26, 27]. Since an increased intake of fat is closely related to increases in the incidence rate of AD and other dementias [28], the diet changes may have directly led to the increased incidence rate of AD and other dementias in Japan.
Seblova et al. [29] found that the incidence rate of dementia in Sweden may have declined from 1987 to 2016. The results of the present study further support that result. The decline in the incidence rate of AD and other dementias was clear in all countries, and most obviously in Sweden. In addition to improvements in education and the control of cardiovascular and other risk factors, lifestyle changes may also play an important role, such as a decrease in the smoking rate [30, 31].
At present there is no cure for AD and other dementias, with drug treatments only being able to delay the progression of the disease, which makes primary prevention of these diseases particularly important. Norton et al. [30] pointed out that one-third of AD cases worldwide are due to intervenable risk factors such as education level, physical exercise, smoking, hypertension, and obesity [32, 33]. Shimada et al. [34] researched the association of lifestyle factors with the risk of dementia in Japanese elderly, and they found that regularly participating in activities such as conversation, shopping, driving, and gardening can reduce the risk of dementia. Therefore, managing these intervenable risk factors and encouraging the elderly to adopt active lifestyles are highly significant for the primary prevention of AD and other dementias.
Our research was subject to certain limitations. First, data in the GBD database are calculated using an algorithm based on the available data for each country, which makes the accuracy of these data highly dependent on the quality and quantity of the data used in the algorithm. This means that the data for countries with low levels of development have greater limitations, and so conclusions about countries with the highest and lowest disease levels reported here should be treated with caution. Second, the GBD database does not specifically classify the diagnosis of dementia. Therefore, we cannot calculate the long-term trends in specific types of dementia such as AD, vascular dementia, Lewy-body dementia, and frontotemporal dementia, or which type of dementia declines lead to a general trend in the decline of cognitive function.