Why Residents in Southern China live longer than those in Northern China?


 Background: Life span of human beings is closely related to the natural geographical environment and regional lifestyle. Since 1982, the life expectancy and longevity level in southern China is always higher than northern China, and the standardized mortality rate in northern China is always higher than southern China.
Method: By calculating the contribution of four diseases to longevity in 31 provinces of China, this study found that cardiovascular disease is the main reason for the longevity disparity between northern China and southern China. The population attribution fraction of nine major risk factors of cardiovascular disease in each province were calculated based on their exposure value and relative risk.
Results: residents live in northern China eat more sodium, less vegetable and less sea products, they are more likely to be overweight. PM2.5 in northern China is much higher than in southern China. Cold temperature causes more number of death than hot temperature because of the longer cumulative risk, all these factors lead to higher CVD mortality rate, shorter life expectancy and lower life span in northern China. Diet high in sodium is the leading cause of north-south life span differential.
Conclusions: Changing diet habits, controlling air pollution can effectively reduce the loss of health and life span in northern China.


Fig.1 Northern China and Southern China
Since 1982, northern China has consistently showed lower life expectancy and 24 considerably lower longevity ratio (80+/0+, 90+/0+ and 100+/0+) than southern 25 China, while Age standard mortality rate is always higher in northern China than in 26 southern China (Table 1, the data is obtained from the demographic databases of 27 China's third, fourth, fifth, and sixth national population censuses, which were carried 28 out in 1982, 1990, 2000, and 2010 respectively. Life expectancy and centenarian 29 index in 2018 were obtained from aging office of each province.). 30 Table 1 presents that of all the five longevity indicators, residents in southern correlation between obesity and ischemic stroke, risk of ischemic stroke will be 56 increased by 30% (HR=1.30, 95%CI: 1.28-1.33) along with every 5kg/m 2 increase of 57 BMI [26] . Smoking is associated with 1.3 million cardiovascular events, accounting for 58 about one-third of male cardiovascular disease burden in China in 2011 [27] . Lacking 59 of vegetable and fruit is also an important risk factor of disease, the risk of 60 hypertension decreased with the increase of daily vegetable intake [28] . 61 Although the above factors all have important impacts on disease, health and 62 longevity, the reason of geographic distribution of each factor and the contribution to 63 north-south differential of China is obscure. southern China, we collected and mapped the provincial mortality rate data [30,31] 91 rates (Fig.3). To evaluate the correlations among the four major diseases and north-south 102 longevity differential, we generated a method to calculate the contribution, as 103 illustrated in Fig.1: Where, is contribution value of disease i to the north-south division, >0 Age-standardized mortality rate, t is total mortality rate of the four diseases.

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Comparison between northern China and southern China is presented in Table 2. We conducted principal component analyses to determine the associations of the 114 longevity indicators with the major diseases ( Table 3). The order in which factors 115 were interpreted was determined based on the magnitude of their eigenvalues. In the 116 result, the eigenvalues of the first factor (F1) are considerably higher than those of F2 117 and F3, therefore, we present the first factor (F1) of principal component analyses in 118 Table 3.  Table 2, Table3, distribution of cerebrovascular 122 disease and heart disease are positively related with north-south differential of 123 age-standardized mortality rates and longevity, because mortality rate of 124 cerebrovascular disease and heart disease is much higher in northern China than in 125 southern China. While distribution of respiratory diseases in negatively related with 126 north-south differential of age-standardized mortality rate and longevity, because

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In order to study why age standardized mortality rate of cerebrovascular disease 135 and heart disease is significantly lower in southern China than in northern China, we 136 consulted main risk factors of cardiovascular disease [32] , high sodium diet, smoking, 137 insufficient physical activity, insufficient intake of omega-3 polyunsaturated fatty 138 acids, obesity and overweighed, insufficient intake of vegetables and fruits are leading 139 risk factors of CVD. In recent years, studies found that PM2.5 exposure, high 140 temperature and low temperature are important risk factors to CVD. Therefore, we 141 select the above 9 risk factors as research objects.   (2). The per capita tobacco sales data in each province is shown in Fig.5(C).  3) Salt (sodium) intake: 163 Data on regional salt intake varied from different studies. In order to accurately 164 calculate risk of sodium, we collected salt intake data from different data soure [33,34] 165 and calculated the average value in each province. The result is illustrated in Fig.5(D).   Control and Prevention [35] . We collected the obesity rate and overweight rate of  studies have shown that the average daily temperature above 26 degree centigrade or 203 below 10 degree centigrade will lead to an increase in death and cardiovascular 204 disease incidence [36] , and the risk increases with the rise of temperature (>26 degree

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To quantitatively study the contribution of nine risk factors on provincial CVD 215 mortality, we calculated the estimation of attributable disease burden according to 216 theory and methodology in Global Burden of Disease (GBD) [37] . In GBD, the shown that both high and low temperature environment will increase the incidence of 240 CVD and mortality, and temperature has a lagging effect of CVD, high temperature 241 effects appear to last for a few days, whereas the effects of low temperature may 242 persist for up to several weeks [38] . We calculated the RR of annual high temperature 243 and annual low temperature in each province based on the RR of lag days of each 244 incidence of high temperature and low temperature (), the formula is as follows: Where, k is average temperature in a day, at low temperature, ∈ (−25°C, 9°C)， 247 at high temperature, ∈ (26°C, 33°C), n is the lag days that RR and its 95% CI 248 both >1, is relative risk of temperature k at day i of lag, ∈ (0, 30), is the 249 number of days in a year that average temperature=k.

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The standardized CVD mortality rate of nine risk factors of each province are 251 illustrated in Fig.6. The average value of attributable risk proportion of 15 northern 252 provinces and 15 southern provinces is illustrated in Table 4 and Table 5.   Longevity level in northern provinces was significantly lower than that in 261 southern provinces of China (Table 1), this is significantly correlated with CVD. Fig.2 and Xinjiang, have a high incidence of stroke, constituting a stroke belt. The stroke 269 incidence in the stroke zone was 236.2/100,000, which was significantly higher than 270 that in the areas outside the stroke zone (109.7/100,000) [39] . Because mortality rate of 271 CVD is considerably higher than that of respiratory diseases, a lower mortality rate of 275 [40] .

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Among the nine risk factors shown in Fig.5 and hypertension is the primary risk factor of CVD and all-cause mortality in the 292 global burden of disease [41] . Fig.6, Table 4, Table 5 tell us sodium exposure is leading 293 risk factor of CVD in China, which is in accordance with former studies [24,25] , and 294 sodium exposure is also the leading reason of north-south differential of CVD 295 mortality rate.

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Regions with generally higher PM2.5 concentrations are located in northern persisted for several days [46,47] , whereas the effect of high temperatures was restricted 334 to the day of the death or the immediately preceding day [48] . That is why number of 335 death caused by extreme temperature in northern China is larger than southern China. China.

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To study why death rate of CVD is much higher in northern China than in