In this study, 14 diseases were selected, and the incidence of each disease was confirmed by defining ARD as a disease with a high prevalence in Koreans when the incidence increases with age. According to this, the diseases that decreased in the annual incidence from 2006 to 2019 were chronic obstructive pulmonary disease, congestive heart failure, and ischemic heart disease. Diseases with an increasing yearly incidence rate were dyslipidemia, chronic kidney disease, cataract, deafness, and Parkinson's disease. In the case of hypertension, diabetes, cerebrovascular disease, osteoporosis, osteoarthritis, and age-related macular degeneration, the incidence rate, which had gradually decreased, showed a tendency to increase after 2015. However, when the incidence rate of each disease was examined by the age of 10 years, almost all diseases, regardless of the difference in the incidence rate by year, had differences in degree; however, the incidence increased exponentially as the age increased, followed by a very high age showed a characteristic form of decreasing. There was a slight difference in the age at which the highest incidence rate of each disease was observed; however, most of the cases appeared to increase and then decrease at approximately 70 ~ 80 years of age.
Each disease included in the ARD group is caused by different causes affecting various organs, such as mutations, dysregulated homeostasis, fibrosis, and degenerative processes [9]. However, previous studies have shown that diseases belonging to ARD tend to increase approximately exponentially with age and then decrease in very old age; additionally, the slope of the rising portion of the incidence curve is similar at 6–8% per year [19, 20]. This similarity suggests that a general biological aging process dominates the pathogenesis of various diseases, which can be explained by the accumulation of senescent cells and differences in individual susceptibility to diseases [15].
Senescent cells stop dividing cells in response to various stresses and accumulate in the body with increasing age. They secrete Senescence Associated Secretion Profile (SASP) to induce inflammation or reproduce normal cells [21]. According to one study on the relationship between age and senescent cells, the turnover of senescent cells produced and eliminated rapidly occurs at a young age; however, with increasing age, the turnover slows, especially the rate of elimination [22], and this was used to develop a statistical probability model for the generation and removal of senescent cells. This is called the Saturated-Removal (SR) model, and it can be confirmed that the accumulation of senescent cells occurs because the generation of senescent cells increases by various stresses as the age increases; however, the self-removal rate slows down [23]. Nevertheless, since the number of senescent cells is different in individuals, the rate of removal of senescent cells has various distributions. If it is assumed that death occurs when senescent cells exceed the threshold, the SR model can explain the distribution of death time [22–23].
Since aging cells are associated with several ARDs, if ARDs occur when ARDs exceeds a specific disease-specific threshold, aging cells secrete SASPs that affect the 'physiological parameters' related to the occurrence of certain diseases, causing the disease to exceed the threshold. Therefore, it can be explained that the number of aging cells increases exponentially with age, and the disease increases exponentially with age [15].
Also, the decrease in the incidence at a very high age can be explained by differences in individual susceptibility to specific diseases [24–25]. Each population has a different susceptibility to disease due to the differences in genetic or environmental factors; therefore, the risk of developing a disease may appear differently. Thus, the onset of ARD will occur in individuals with a low threshold for each disease, and ARD will not occur during normal aging in a population with a high disease threshold. However, at a very high age, most people with a low threshold for the disease will have already been afflicted with the disease, and most of the remaining people have a high threshold for the disease; therefore, the probability of developing a new disease is relatively low, resulting in a decrease in the incidence.
Some limitations of this study are as follows. First, since the collected claim data were used for management purposes for insurance claims and refunds, information, such as diagnostic codes might be inaccurate, possibly affecting the incidence rate, and second, because of this, the actual disease may have been underestimated or overestimated compared to the number of occurrences. Finally, because the disease category of ARD was selected through an expert meeting based on the ARD proposed by Chang et al. and published data, there is a limitation since the DALY on the exponential increase in the incidence rate was calculated and not based on the ARD.
However, this study is significant and has meaning because it is the first to show the characteristics of ARD in the Korean population, which increases exponentially with age, and decreases at a very high age, based on a sample cohort of 1 million representative Koreans.