1. Stroke mortality and risk factor levels in China during the mid-1980s: an ecological analysis of 67 counties Background Studies over the last 30 years have reported substantial variation in stroke mortality rates in different parts of mainland China. The reason for this variation is poorly understood. Objectives To describe the variation in county stroke mortality rates for 1986-88 in mainland China. To investigate the extent to which this variation can be explained by differences in the levels of known risk factors for cardiovascular disease at an ecological level. Methods County selection. In the early 1980s, 69 counties were selected from a total of about 2,400 largely rural counties in China. The counties were chosen to represent the full range of mortality rates for seven major types of cancer. Most counties had populations of a few hundred thousand, but a few were substantially larger or substantially smaller. Mortality survey 1986-88. For each county, the numbers and underlying causes of the deaths were obtained by a retrospective review undertaken in 1989 specifically for this project. Stroke mortality data was collected for 67 of the 69 counties selected for the study. Biochemistry, Diet and Lifestyle survey 1989. Within each county, two smaller administrative areas, ‘xiangs’, were randomly selected, and within each xiang, either one village or two adjacent villages (depending on their size) were then randomly selected as the actual sites of the interviews and other data collection. The average results for the two-xiang pairs were used to estimate the average values for each county. Within each study site, 60 households were randomly selected from an official registry of residences. One individual per household aged 35-64 (in approximately equal numbers for age groups 35-44, 45-54, and 55-64) was randomly selected, and these individuals and their households were the focus of data gathering. Statistical methods . Age-standardised mortality rates were calculated as the unweighted average of the component five-year mortality rates. Pearson’s correlation co-efficients with 95% confidence intervals were calculated for sex-specific correlations of county stroke mortality rates and the county level values of risk factor variables. Where the correlation coefficient was statistically significant (p<0.05), the r 2 value and regression coefficient (β) was calculated (see Figure 2 for examples). Results Stroke mortality in men varied from 95 to 656 deaths/100,000 men/yr: a ~ 7-fold difference (Figure 1). There was a positive correlation with county latitude (r:0.27, p:0.03) but not with longitude (p:0.62). There was no statistically significant correlation (2p<0.05) with any of the other selected risk factors for cardiovascular disease (Table 1). Stroke mortality in women varied from 60 to 814 (433 excluding Longxian, an outlier) deaths/100,000 women/yr: a >7-fold difference (Figure 1). There was a positive correlation with county latitude (r:0.53, p:<0.001) but not with longitude (p:0.32). A statistically significant correlation was found with the following risk factors: systolic and diastolic blood pressure, green vegetable consumption, BMI, HDL cholesterol and household income (Table 1). However, only a small proportion (<20%) of the total variation in stroke mortality (as measured by r 2 ) was explained by any single risk factor. 1 ST4 Public Health, Oxford Deanery; 2 CTSU, Oxford; 3 Chinese Centre for Disease Control, Beijing; 4 Chinese Academy of Medical Sciences, Beijing; 5 Institute of Biomedical Sciences Academia Sinica, Taipei; 6 Tumour Institute, Chinese Academy of Medical Sciences, Beijing; 7 Division of Nutritional Sciences, Cornell University, USA Ben Lacey 1,2 , Gary Whitlock 2 , Zhengming Chen 2 and others 2-7 Correspondence to: Dr Ben Lacey, CTSU, University of Oxford; Email: ben.lacey@ctsu.ox.ac.uk Figure 1: Maps of mainland China with study counties designated by bullets, coloured by level of age-standardised stroke mortality rates (35-69 years of age) for 1986-88, by sex Table 1: Pearson’s correlation coefficient of 67 county age-standardised stroke mortality rates (35-69 years of age) for 1986-88 with county level values of selected variables from the Biochemistry, Diet and Lifestyle surveys 1989, by sex Figure 2: Scatterplots of county age-standardised stroke mortality rates (35-69 years of age) for 1986-88 vs county level values of selected variables from the Biochemistry, Diet and Lifestyle Surveys 1989, by sex. Conclusion This study found almost uniformly high rates of stroke mortality compared to Western populations. In addition, the variation in rates was substantial and poorly explained by differences in any single risk factor between counties. Ecological studies, such as this, have significant limitations and the extent to which variation in stroke mortality is attributable to combinations of differences in the prevalence of risk factors is ideally investigated by prospective studies. Nevertheless, this study adds weight to the hypothesis that variation is due to more than differences in blood pressure (as argued by some). It is, also, possible that the presence of major avoidable cause, or causes, still awaiting discovery!