As illustrated in Table 3, expect those of cerebrovascular disease and heart disease, the mortality rates of all other reasons in Shanghai and South China were higher than those in Beijing and North China, respectively. Thus, the difference of CVD mortality rates is the fundamental reason for the difference of the longevity level between North and South China.
To evaluate the correlations among the four major diseases and nonagenarian ratio, we generated a Spearman’s rank correlation matrix (Table 4). The correlation coefficients between the nonagenarian ratio and CVD were significant and negative, whereas the correlation coefficients between the nonagenarian ratio and illnesses such as cancer and respiratory diseases were not significant.
Based on this matrix, we conducted correlation and principal component analyses to determine the associations of the nonagenarian ratio with the major diseases (Table 5). The order in which factors were interpreted was determined based on the magnitude of their eigenvalues. In Table 5, the eigenvalues of the first factor (F1) are considerably higher than those of F2 and F3, and F1 showed a high positive loading for CVD.
The proportion of nonagenarians in northern provinces was significantly lower than that in southern provinces (Table 1), possibly due to the significant and positive correlation with CVD. Fig. 2 and Table 3 illustrate that among the four major diseases, cancer is randomly distributed in China. Additionally, the mortality rate of respiratory diseases in South China is generally higher than that in North China. Conversely, the mortality rate of CVD in South China is generally lower than that in North China, which is consistent with other studies 9: Nine provinces in North China, namely Heilongjiang, Jilin, Liaoning, Inner Mongolia, Hebei, Beijing, Ningxia, Tibet, and Xinjiang, have a high incidence of stroke, constituting a stroke belt in China. The stroke incidence in the stroke zone was 236.2/100,000, which was significantly higher than that in the areas outside the stroke zone (109.7/100,000). Considering that the mortality rate of CVD is considerably higher than that of respiratory diseases, a lower mortality rate of CVD in South China led to higher life expectancy and longevity ratio.
Based on the 2010 national census data for China, the provincial gravity center evolution map for the proportion of population from over 65 years to over 100 years was drawn (Fig. 3).
Fig. 3 shows that the provincial gravity centers of the proportion of aging population and longevity population move southward as the age increases, and this shift does not reverse. As the age increases, the gravity centers of the proportion of aging population in China move along the same path as that of the gravity centers of the lowest temperature during winter. The speed is gradually accelerating southward, as illustrated in Table 6.
According to CVD reports in China in 2015, 2016, 2017, and 201810 , some influencing factors for CVD (e.g., temperature, salt consumption, selenium intake, air pollution, overweight, and obesity) may present north–south differences.
High sodium intake is internationally recognized as the main cause of hypertension. In fact, 25.2% of the CVD deaths can be attributed to high-sodium diets, ranking first among all diseases; the disease burden attributable to high-sodium diets in North China is considerably more serious than that in South China; the top 10 provinces with high disease burden due to high-sodium diets were all located in the north11.
Extreme temperatures have a significant impact on mortality in elderly people12. The associations of high and low temperature with death differ, and more cold-related deaths have been reported than heat-related deaths13. Furthermore, the effects of low temperatures may persist for several days14, whereas those of high temperatures are restricted to the day of the death or to the next day for all CVD cases; the hot-day effect was five times less severe than the cold-day effect 15 . In Shenzhen, there was a significant increase in CVD mortality rate over low-temperature days compared with that over high-temperature days16. North China is considerably colder than South China during winter, possibly leading to higher CVD mortality rate. However, in summer, North China also faces heat waves.
In medical cases and clinical reports of 133,957 people, selenium intake was negatively correlated with CVD mortality risk in males and females 17 , and the distribution of selenium in the soil of China gradually increases from north to south18. Furthermore, air pollution is becoming a threat to public health. In several studies, PM2.5, SO2, and NO2 were positively correlated with hypertension and CVD, and PM10 and PM2.5 gradually decrease from north to south in China. The government provides free or heavily subsidized coal for indoor heating during the winter to cities north of the Huai River but not to those in the south; excess coal burning pollutes the air in the north; furthermore, the short lifespans are almost entirely caused by elevated rates of CVD mortality19,20.
In addition, obesity and overweight are two of the most common causes of CVD. These are highly prevalent in north and northeastern areas of China21. A report on risk factor surveillance of chronic disease in China also showed that obesity and overweight are more prevalent in North China than in South China, and top one-third provinces with high obesity and overweight rate are all located in the north22.