disability prevalence increases with advanced age , and disability in the elderly population is a major public health problem .
this is especially crucial for chinese : there are 88 million persons aged 65 and above in china alone , and the number is projected to increase to 341 million in 2050 ; importantly , disability prevalence in chinese elderly population has been increasing [ 35 ] in contrast to a declining trend in the developed countries [ 6 , 7 ] .  
there is a big gap between the scale of the problem and the quantity and quality of available information .
the first population - based report from china was published 16 years ago , and countable number of publications [ 35 , 812 ] exist in the literature .   in this study , with large population - based data from shanghai ( the biggest city in china ) and singapore ( a city country in southeast asia , with majority of the residents being descendents of immigrants from south china ) , we aimed to examine the independent contributions of physical and cognitive health to disability in chinese older adults and explore the potential influences of environment .
participants of the present study were identified from two big population - based studies : the shanghai survey of alzheimer 's disease and dementia ( ssadd ) and the singapore longitudinal ageing study ( slas ) .
the ssadd was conducted in 1987 based on a probability sample of 5055 community - dwelling chinese older adults from jing'an district of shanghai city .
the slas was conducted among a total population sample of 2808 older adults from south east region of singapore .
baseline assessments of the slas were completed from september 2003 to december 2005 .   details of the ssadd [ 8 , 13 ] and
, we selected 4639 participants from the ssadd sample and 2397 participants from the slas cohort .
all participants had complete data on demographic information , physical health , and cognitive function and obtained a mini - mental state examination ( mmse ) total score of at least 21 points .  
face - to - face interviews were conducted by psychiatrists / psychiatric nurses ( in shanghai ) or trained research nurses ( in singapore ) , and data were collected for a wide range of variables .   for the present analysis , we extracted the following variables from the databases : age , sex , functional status , chronic diseases , self - rated health status , and mmse total score .  
functional status was assessed by the participant 's level of dependency in performing 8 activities of daily living ( adl ) : eating , grooming , dressing , transferring , walking , toileting , bathing , and climbing stairs .
the participants were asked  do you have or not have any of the following illnesses or conditions at the present time ? " in the slas , a list of 14 medical conditions was covered in the interview .
the participants were asked  do you have a history of this medical problem ?   
medical conditions that were not included in the list were recorded under  any other problems .  we selected ten chronic diseases on which data were available from both samples : hypertension , diabetes , heart diseases ( in singapore : defined as any of heart attack , heart failure , or atrial fibrillation ) , stroke ( in shanghai : effects of stroke ) , kidney disorder ( in singapore : kidney failure ) , chronic obstructive lung disease ( in shanghai : emphysema / bronchitis ) , asthma , arthritis ( in shanghai : arthritis or rheumatism ) , mental illness , and cancer ( in singapore , identified from  any other problems  ) . in statistical analysis ,
the number of chronic diseases was used as an objective measure of physical health .   in singapore ,
self - rated health status was assessed using a single question :  in general , would you say your health is : excellent , very good , good , fair , or poor ?  in shanghai , the same question was asked but there were four choices : excellent , good , fair , poor .  
excellent  and  very good  together and created a new variable  self - rated health status  : 1 = excellent or very good , 2 = good , 3 = fair , and 4 = poor .
this was used as a continuous variable in multivariate analyses ( score ranged from 1 to 4 , with higher value representing poorer health status ) .  
the test has been validated in shanghai   and singapore , respectively . summed scores of mmse range from 0 to 30 , higher values denoting better cognitive functioning .
chi - square test ( for dichotomous variables ) or independent sample t test ( for continuous variables ) was used to compare the difference regarding various characteristics between the two study samples .
multiple logistic regressions were used to examine the relationship between health factors and disability , and the influence from environment .   in analysis based on the combined sample , age , sex , number of chronic diseases , self - rated health status , mmse total score , and environment ( singapore versus shanghai ) were included in the regression model .  
the same set of variables ( except environment ) was used in the stratified analysis .
the prevalence of disability was much higher in shanghai sample ( 5.0% ) than that in the singapore sample ( 1.8% ) .
however , the shanghai participants were also older , had more chronic diseases , had poorer self - rated health status , and obtained lower mmse total score as compared to their singapore counterparts ( table 1 ) .
a scrutiny of the eight adl items revealed that the difference was only significant for bathing ( 3.5% versus 0.4% ) and climbing stairs ( 3.1% versus 1.2% ) . in multiple logistic model based on the combined sample , number of chronic diseases , self - rated health status , mmse total score , and environment
the odds ratio was 1.35 ( 95% ci 1.221.50 ) for number of chronic diseases , 2.85 ( 95% ci 2.363.43 ) for self - rated health status ( continuous ) , 0.89 ( 95% ci 0.850.94 ) for mmse total score , and 0.68 ( 95% ci 0.480.96 ) for environment ( singapore versus shanghai ) , respectively .  
results from stratified analysis showed that the strength of associations between health variables and disability differed between the two samples ( table 3 ) .
for example , in the shanghai   sample , one point increase on the 4-point self - rated health status scale was associated with 3.13 times ( 95% ci 2.553.85 ) higher odds of having disability while the odds ratio was only 1.73 ( 95% ci 1.072.81 ) in the singapore sample . for mmse total score ,
the corresponding odds ratio was 0.91 ( 95% ci 0.860.96 ) in the shanghai sample and 0.83 ( 95% ci 0.730.94 ) in the singapore sample .
based on data from two population - based studies , we found that the number of chronic diseases , self - rated health status , cognitive function , and unmeasured environment factors represented by study sample were significantly associated with functional disability among chinese older adults .
the contributions of physical health [ 8 , 18 ] and cognitive function [ 19 , 20 ] to functional disability have been documented and were well replicated in our analysis .   with large population - based data , we firstly quantified the strength of differential contributions of each of self - reported physical health , self - rated health status , and cognitive function to disability in chinese elderly population .  
the findings are relevant and important for policymakers , medical practitioners , and academics .  
participants in the singapore sample were 32% less likely to have disability compared with participants in the shanghai sample .
participants from both studies were chinese older adults living in big city , and a number of important covariates were adjusted in multiple regression models .  
it is less likely that the observed difference was caused by unmeasured factors at individual level .  
however , a careful examination of the two most difficult adl tasks ( bathing and climbing stairs ) provided us with plausible explanations .   in 1987 , most families in shanghai had no bathroom and shower , and bathing was considered as a complicated task .
in contrast , the majority of slas participants lived in public houses that have standard bathroom and shower and easy - to - climb stairs .
our study provided fresh evidence on the role of environmental factors in disablement process [ 2123 ] .  
as pointed out by verbrugge and jette :  disability is not a personal characteristic , but is instead a gap between personal capability and environmental demand  disability can be diminished swiftly and markedly if the physical or mental demands of a given task are reduced  .  
our findings suggest that environment not only affects the prevalence of disability , but also modifies the strength of   associations between disability and health variables .   
strengths of our study include concurrent measuring of various health factors and simultaneous inclusion of those factors in multivariate model .  
however , no causal relationship could be drawn given cross - sectional nature of the study design .   
in conclusion , physical health and cognitive function were significantly associated with disability among chinese older adults living in asian metropolises .
all the authors contributed substantially to the design , analysis and interpretation of the data and participated in drafting or revising the paper .
 
 objective . we aimed to examine the independent contributions of physical health and cognitive function to disability among chinese older adults living in two asian metropolises and 
 explore the potential influences of environment . 
 
 design and participants . 
 cross - sectional analysis based on data from two population - based studies : the shanghai survey of alzheimer 's disease and dementia ( n = 4639 ) and the singapore longitudinal ageing study ( n = 2397 ) . 
 disability was defined as needing help in at least one activity of daily living . 
 
 results . 
 the prevalence of functional disability was higher in shanghai sample ( 5% ) than that in singapore sample ( 1.8% ) . 
 number of chronic diseases , self - rated health status , cognitive function ( measured by the mini - mental state examination ) , and environment ( singapore versus shanghai ) significantly contributed to functional disability independent of each other . 
 the adjusted odds ratio was 1.35 ( 95%ci   1.221.50 ) , 2.85 ( 95% ci 2.363.43 ) , 0.89 ( 95% ci 0.850.94 ) , and 0.68 ( 95% ci 0.480.96 ) , respectively . 
 the strength of associations between health variables and disability appeared to be influenced by environment . 
 
 conclusion . 
 physical health and cognitive function independently contributed to functional disability . 
 the associations are modulated by environmental factors . 
1. Introduction
2. Methods
3. Results
4. Discussion
 Conflict of Interests
 Authors' Contribution