| Stroke is a kind of chronic diseases with poor blood flowing to the brain and resulting in cell death.There are mainly two kinds of stroke:ischemic stroke,due to lack of blood flow;and hemorrhagic stroke,due to bleeding.Because of the high incidence,fatality,and disability rates,stroke has brought a lot of burden to the human individual,family and whole society.In China,stroke,especially ischemic stroke,was one of the most complex chronic diseases with very high morbidity and mortality.It might be beneficial and with significance in public health for ischemic stroke prevention to further clarify the important effect factors of ischemic stroke in Chinese population.So far,many domestic and international studies about the relationship between influencing factors and ischemic stroke have been reported,however,the results were inconsistent.There might be multiple possible causes for discrepant results,for instance,some of the researchers adopted the case-control or cross-sectional study design,rather than prospective cohort study design which has more advantages in confirming the cause-and-effect relationship.Besides,even though the prospective cohort study design was adopted in certain related studies,the number of study subjects recruited was small(only a few hundred to thousands).In recent years,with the rapid development of human science and technology level and the ever-growing demands of the quality of scientific research results,many authoritative epidemiologists have proposed that large scale population cohort will play a vital important role in the etiology study area of human chronic non infectious disease.The large scale cohort studies have many advantages including better study efficiency,higher repeatability,more credibility,etc.In addition,they could make up the limitation of small scale cohort studies that it is difficult to obtain cases or enough cases of rare diseases.The China Kadoorie Biobank(CKB),which began from 2004,was set up to investigate the main effect causes of common chronic diseases in the Chinese population.Totally,more than 510,000 adults were recruited from ten geographically defined regions of China.This large,well-designed study has became a very useful resource for investigating the main causes of many complex or common chronic non infectious diseases,and the results or information generated on the basis of the CKB cohort will improve our understanding of etiology for many complex diseases not only in China but also in other countries.Based on the CKB project,many researchers have carried out numerous studies focusing on the association of certain factors with several chronic diseases(including ischemic stroke),and certain research results have attracted the attention of many scholars at home and abroad and even ordinary people.For example,it might reduce the occurrence of a large number of ischemic stroke cases through effective health education and other measures to promote the daily intake of fresh fruits,moderate exercise,positive control of body weight and blood glucose level in Chinese population.However,it still has a long way to go to demonstrate the causes of ischemic stroke clearly,we need to make full use of the valuable resources of the CKB project and continually explore the etiology of ischemic stroke.Family history of stroke has been recognized throughout the world as a risk factor for stroke.Recently,only one cohort study has investigated the relationship between family history of stroke and stroke risk in Chinese population.Besides,there were still some shortcomings in this cohort study,firstly,the study sample was small(only 15,131),secondly,there was no analyses on the association of family history of stroke with risk of different types of stroke,thirdly,there was no analyses on the relationship of different types of family history of stroke to ischemic stroke risk,and finally,there was no analyses on the interaction between family history and other effect factors of ischemic stroke.Numerous studies have shown that tea is a effect factor related to the risk of stroke(including ischemic stroke).Up to now,there is no cohort study in Chinese population to analyze the relationship between tea drinking and the risk of ischemic stroke.The published related cohort studies were mainly based on European and Japanese populations.However,these cohort studies still had some limitations,for instance,the large scale prospective cohort study focusing on this research topic was still limited,and there was limited analyses on the interaction between tea and other effect factors of ischemic stroke.Besides,because there were many differences for tea production areas,main types of tea,tea production processes,and tea drinking habits of persons among different countries,the conclusions of the above cohort studies based on European and Japanese populations could not explain the relationship between tea consumption and ischemic stroke risk for Chinese populations.Therefore,in the first part of this study subject,we explored the relationship between family history of stroke and ischemic stroke risk on the basis of the CKB project,and in the second part of this study subject,we investigated the association of tea with the risk of ischemic stroke based on the CKB project.Part I:A nationwide prospective cohort study on the association of family history with ischemic stroke riskOne convenient way to determine the individuals with potential risks for stroke is to collect family history information.There were a lot of researches about the relationship between family history and risk of stroke.However,up to now,there were limited cohort studies exploring the effect of family history of stroke on the ischemic stroke risk.All of these cohort studies were carried out in European and Japanese populations,and the results of them were inconsistent.For example,a cohort study conducted in 8,037 Japanese participants found that,compared with individuals without family history of stroke,the relative risk(RR)and 95%confidence intervals(CI)of those with family history was 0.90(0.59-1.37).In 2010,a cohort based on Framingham study identified that the RR(95%Cl)of ischemic stroke related to family history was 3.09(1.88-5.08).The corresponding RRs(95%CIs)of other two cohort studies with larger sample sizes were 1.62(1.12-2.36)and 1.11(0.85-1.43),respectively.So far,the Chinese cohort study on the association of family history with risk of ischemic stroke is still lacking.This study was based on the China Kadoorie Biobank(CKB),a total of 467,982 individuals were included in the final analyses.Follow-up time was calculated for each participant from the date of the start of study until the date of the occurrence of ischemic stroke,loss to follow-up,or the end of follow-up(December 31,2013),whichever occurred first.The Cox proportional hazards regression model was used to evaluate HRs(Hazard ratios)and 95%CIs of ischemic stroke for family history of stroke.The multivariate adjusted HRs(95%CIs)of ischemic stroke were estimated with the following adjustments:age,sex,residential area,level of education,marital status,alcohol consumption,smoking status,physical activity,BMI,prevalent hypertension and diabetes at baseline and number of siblings.The interaction analyses were performed by means of likelihood-ratio tests,comparing models with and without the interaction items between the baseline stratifying variables and family history of stroke as a dichotomous variable,To examine the robustness of our findings,we also conducted several sensitivity analyses.Total person-years of follow-up were 3,314,798 and the median follow-up time was 7.2 years.A total of 20,528(4.33%)ischemic stroke cases were reported.The multivariable adjusted HR(95%CI)of ischemic stroke for participants with family history of stroke was 1.39(1.34-1.43)as compared with those without family history.According to the number of family members with stroke,the multivariable adjusted HRs(95%CIs)of ischemic stroke were 1.32(1.27-1.36),1.72(1.60-4.84),and 2.01(1.73-2.34)for subjects whose family history of stroke involving 1,2,and>3 members(Ptrena<0.001),respectively,in comparison with those without FHS.The multivariable adjusted HR(95%CI)of ischemic stroke was 1.41(1.33-1.49)for sibling history and 1.38(1.33-1.43)for parental history(Pheterageneity=0.532).After multivariable adjustment,the HRs and 95%CIs of incident ischemic stroke relevant to paternal history[1.36(1.31-1.42)]and maternal history[1.38(1.33-1.44)]were similar(Pheterogeneity=0.613).Men and women had similar risk of incident ischemic stroke no matter father or mother(/parents or siblings)with stroke.In the stratification analyses based on the effect factors,individuals with family history of stroke were more likely to be affected by ischemic stroke if they were smokers or with lower physical activities.Further analyses revealed significant interactions between smoking and family history on the risk of ischemic stroke.This Chinese cohort study showed that family history of stroke was an independent risk factor for developing ischemic stroke,for whatever type of family history.The relationship between sibling history and ischemic stroke was similar to that between parental history and ischemic stroke no matter for males or females.Maternal history of stroke and paternal history of stroke had no differential effects on the risk of ischemic stroke,in either men or women.The management of the health risk behaviors for reducing ischemic stroke should be highlighted,especially for the individuals with family history of stroke.Part II:A nationwide prospective cohort study on the relationship of tea to risk of ischemic strokeTea is one of the most popular beverages in the world,and the history of tea drinking dates back thousands of years.There are three main categories of tea:green tea,oolong tea,and black tea.While different processing methods produce different types of tea:green tea is non-oxidized,oolong tea is partially oxidized,and black tea is fully oxidized.China is one of the largest tea-consuming countries in the world.Recently,the number of persons drinking tea has increased,and the annual consumption of tea has been rising.Because of its popularity,even small health affections of tea might have considerable public health implications not only in China,but also in other countries.In the past twenty years,the studies on the relationship between tea drinking and the risk of stroke have attracted much attention.Nevertheless,cohort studies focusing on the relationship between tea consumption and risk of ischemic stroke are still limited,and the related cohort studies are mainly conducted among European and Japanese populations.The conclusions of the above cohort studies showed that tea consumption was associated with reduced risk of ischemic stroke,however,the results of these cohort studies conducted in Europe and Japan could not explain the real association of tea with ischemic stroke in Chinese population.So far,the cohort study focusing on the relationship between tea drinking and risk of ischemic stroke in Chinese population was still lacking.This study was also based on the CKB project,a total of 481,181 individuals were included in the final analyses of this study.The main research variable of tea for analyzing in this study was a categorical variable:no drinking tea,occasionally drinking tea,and regularly drinking tea.Follow-up time was calculated for each participant from the date of the start of study until the date of the occurrence of ischemic stroke,loss to follow-up,or the end of follow-up(December 31,2013),whichever occurred first.The Cox proportional hazards regression model was used to evaluate HRs and 95%CIs of ischemic stroke for tea,with age as the underlying time scale.Analyses were stratified for survey areas(ten study areas)and 5-year age-at-risk groups.The multivariate adjusted HRs(95%CIs)of ischemic stroke were estimated with the following adjustments:age,sex,level of education,marital status,household income level,alcohol consumption,smoking status,physical activity,BMI,consumption of meat,consumption of fresh fruit,consumption of fresh vegetables,prevalent hypertension at baseline,prevalent diabetes at baseline,and family history of stroke.The interaction analyses were performed by means of likelihood-ratio tests,comparing models with and without the interaction items between the baseline stratifying variables and tea as a dichotomous variable.To examine the robustness of our findings,we also conducted several sensitivity analyses.Among all the subjects,the number of regular tea drinkers was 161,546,which accounted for about 31.96%of all participants.In the regular tea drinkers,green tea had the most drinkers and accounted for 85.43%of the total.By contrast with women(21.21%),the regular tea drinker accounted for 51.35%among men.By the time of December 31,2013,total person-years of follow-up were 3,405,091 and the median follow-up time was 7.2 years.A total of 21,427(4.45%)ischemic stroke cases were reported during the follow-up period.The main results show that,compared with non-tea drinkers,the age-and gender-adjusted HR(95%CI)for occasional tea drinkers and regular tea drinkers was 0.95(0.92-0.98)and 0.98(0.94-1,02),respectively.After multivariable adjustment,tea consumption still was associated with reduced risk of ischemic stroke,the respective adjusted HR(95%CI)for occasional tea drinkers and regular tea drinkers was 0.94(0.91-0.98)and 0.94(0.90-0.98)in comparison with individuals who never consumed tea,and the linear trend was identified(Ptrend<0.001).In order to further verify the reliability of our results about the relationship between tea and ischemic stroke risk,we also analyzed the associations of many other evaluation index of tea with ischemic stroke risk.After multivariate adjustment,the risk of ischemic stroke decreased with the increase of the frequency of tea drinking or level of tea consumption(all Ptend<0.05).Among males,compared with non-tea drinkers,the multivariable-adjusted HR(95%CI)for occasional tea consumers and regular tea consumers was 0.92(0.87-0.97)and 0.91(0.86-0.96),respectively(Ptrend<0.001).For females,the respective multivariable-adjusted HR(95%CI)for occasional tea consumers and regular tea consumers was 0.95(0.90-0.99)and 0.98(0.92-1.04).The inverse relationship between regular tea consumption and ischemic stroke was stronger in men(Pheterogeneity=0.032,Pinteractin=0.003).In this large prospective study,tea consumption was inversely associated with the risk of ischemic stroke in Chinese population.The protective effect of tea consumption on ischemic stroke risk was stronger in males and regular green tea consumers.The results of this study provided new scientific information for the prevention of ischemic stroke in Chinese population.In future studies,we still need to carry out randomized controlled trials to further verify the causal relationship between tea consumption and the risk of ischemic stroke. |
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