A 3-year follow-up study of social, lifestyle and health predictors of cognitive impairment in a Chinese older cohort

Suzanne C Hoa, Jean Woob, Aprille Shama, Sieu Gaen Chana and Ashley LM Yuc

a Department of Community and Family Medicine, Chinese University of Hong Kong.
b Department of Medicine & Therapeutic Medicine, Chinese University of Hong Kong.
c Centre for Clinical Trials & Epidemiological Research, Chinese University of Hong Kong.

Suzanne C Ho, Department of Community and Family Medicine, The Chinese University of Hong Kong, 4/F, School of Public Health, Prince of Wales Hospital, Shatin, NT, Hong Kong. E-mail: Suzanneho{at}cuhk.edu.hk

Abstract

Background Longitudinal data on the older population in the Asian setting are limited. This paper reports the factors associated with the development of cognitive impairment (CI) in a cohort of Chinese elderly aged >=70 years.

Methods The study cohort comprising 2030 subjects aged >=70 years was assembled in 1991–1992 and followed for 36 months. Baseline information on cognitive function, as well as a number of social and health variables were obtained through face-to-face interview at the respondent's place of residence. The outcome variable was the development of CI among 988 cohort members who were initially free from CI, and who could be contacted at the 36-month follow-up. The instrument used to assess CI was based on the information/orientation part of the Clifton Assessment Procedure for the elderly (CAPE), using a cut-off point of 7.

Results Of the men, 6.7%, but 22.2% of women had CI at 3-year follow-up. The age-adjusted annual incidence of CI was 1.52% in men, and 6.37% in women. Multivariate logistic regression analysis showed that women had a 2.5-fold increased risk of having CI, compared with men. The risk increased by about 1.5-fold with every 5-year increase in age. Slow gait time, as assessed by the 16-foot walk, was a predictor of CI in both sexes (odds ratio [OR] = 1.03 per second increase, 95% CI : 1.0–1.07). Men residing in institutions had a 4.4-fold increased risk of having CI (95% CI : 1.7–11.1) compared with those residing in community, while the OR among women was 2.5 (95% CI : 1.3–4.9). Among women, no formal education increased the risk of having CI by 3.2-fold (95% CI : 1.8–5.5). Income dependency also increased the risk of CI by about fourfold, and no exercise at baseline was associated with a twofold increased risk of CI. Incident stroke during follow-up also increased the risk of CI (OR = 8.4, 95% CI : 1.2–59.4).

Conclusions Older age and female sex were independent factors associated with CI. No formal education, slow gait time and institutionalization increased the risk of CI in both sexes. While education had a stronger effect in women, institutionalization had a stronger effect in men. Financial dependency, lack of exercise and incident stroke played a significant role in women.

Keywords Predictors, cognitive impairment, Chinese elderly cohort

Accepted 27 April 2001

It is projected that by the year 2050, 58% of the world's elderly population will be residing in Asia.1 It has also been noted that people aged >=85 years constitute the most rapidly growing segment of the elderly population.2

Increased survival is often accompanied by a series of social and biological changes; possibly resulting in increased morbidity and decreased capacity for physical and cognitive functioning.3,4 A few cross-sectional studies in Asia have found increased prevalence of cognitive impairment (CI) in older age groups, but few Asian studies have examined longitudinally the risk factors associated with the onset of CI.5,6

An age- and sex-stratified cohort comprising 2030 members of the elderly population aged >=70 years was established in 1991–1992.7–10 A cross-sectional analysis of the baseline data revealed valuable information regarding risk factors associated with CI.9 This cohort was followed up at 3 years to assess a number of health outcomes and health changes. This paper attempts to investigate the extent to which age, sex and selected baseline social, health and behavioural factors are related to the development of CI over the 3-year follow-up period.

Subjects and Methods

Study subjects
Stratified disproportional random sampling in 1991–1992 assembled a cohort of 2032 elderly subjects aged >=70 years. Registration with the Old Age Allowance (OAA) Scheme, which covers over 90% of the Hong Kong elderly population, was used to define the accessible population. Eligibility for enrolment in the OAA was based on age and length of residence (>=5 years) and was independent of income. A proportional sample was also obtained from those registered for Disability Allowance. Three-hundred subjects were drawn for each of the four strata —male and female by age groups 70–74, 75–79—so the sample in these two age groups consisted of 1200 subjects. Another 150 subjects were drawn from each of the six strata—male and female by age groups 80–84, 85–90 and 90+. The total old-old sample consisted of 2032 subjects. Details of the sampling method and study participants have been reported in previous papers.7–11

Respondent follow-up was carried out as close to 3 years from the baseline interview as possible. A brief telephone interview was conducted at 18 months to update the contact telephone numbers and addresses. Trained nurses, medical doctors or senior medical students conducted face-to-face structured interviews and objective assessments at baseline and at 3-year follow-up.

In this paper, the analyses were confined to 988 subjects with no CI at baseline, and with follow-up measurements at 3 years. In all, 8 subjects were excluded because of missing mental score data at follow-up.

Definition of cognitive impairment
A number of instruments for the assessment of mental status have been found to have high sensitivity and specificity when compared with more detailed tests with a clinical assessment component (CAMDEX)12 or clinical diagnosis. These include the Clifton Assessment Procedure for the elderly (CAPE),13 Mental State Questionnaire,14 and the Mini Mental State Examination (MMSE).15 Researchers16,17 have previously reported a sensitivity and specificity ranging from 80% to 87%, and 97–99% respectively, for the Information/Orientation Subtest of CAPE with a cut-off point of 7–8 as compared to the CAMDEX.

Since the survey interview of the present study already took close to 60 minutes, we used the information/orientation part of the CAPE as a screening test so as not to overburden the respondents. As described in the previous report on the study of prevalence of CI at the baseline survey,9 12 questions were asked and a cut-off point of 7 was used.18 We noted good construct validity of this instrument in our baseline study. We used a similar procedure for the assessment of CI at the follow-up interview. Incident CI was defined as a subject who did not have CI at baseline, but was identified as having CI at the follow-up interview.

Independent variables
The independent variables included socio-demographic, medical, health factors and functional status. We chose the variables based on prior knowledge of predictors of CI from literature review, as well as our baseline cross-sectional findings.9 These baseline variables were collected with the use of a validated standardized questionnaire.7–11 Health was assessed by both subjective self-reported health, as well as by objective assessments of functional and mobility performance. The presence of specific chronic conditions over the past 12 months was assessed by asking respondents if a doctor had ever told them that they had the specified conditions. Activities of daily living (ADL) were assessed based on a modified Barthel Index.19 The assessed ability included feeding, personal grooming, dressing, chair/bed-shifting, walking, stair climbing, toileting, bathing and urinary and fecal incontinence. An ADL score of 20 out of 20 would mean that subjects did not have difficulty in any of the activities of daily living. Gait performance was assessed by a walking test. To measure the gait time, the respondents were asked to walk along an 8-foot (2.5 m) yellow line taped to the floor and back again. The time taken to complete the 16-foot (5 m) walk was assessed by a stopwatch (to the nearest second) and the mean of the time taken for two measurements was used.20

Depression was determined using the 15 items of the Chinese version of the Geriatric Depression Scale with a cut-off point of <=7.7,21 Assessment of social support was based on questions about contact with friends/relatives/neighbours and participation in community/religious activities. The questions were based on questions used in a previous study22 and those adopted from the study by Lubben.23 Subjects with a score of >=15 were considered to have good social support.

Questions about lifestyle factors in relation to smoking, drinking and practice of exercise were also asked. Smoking status was classified as current, ex- or non-smoker based on self-report.

Statistical analysis
We used the {chi}2 test and student's t-test to compare if differences in baseline characteristics existed between respondents and non-respondents of the follow-up study, between men and women, and between the respondents and deceased subjects. As baseline analysis of this cohort has shown a preponderance of CI in women, we stratified the present analyses by sex to investigate the potential gender differences in the risk factors for CI. Because of the significant confounding effect of age and education on the association between the independent variables and the onset of CI, logistic regression analyses were performed adjusting for these two factors. The significant or nearly significant variables (lower limit of 95% CI >=0.9) from the age- and education-adjusted analyses were included in the multivariate logistic regression analysis to construct the final models for the independent predictors for the onset of CI during follow-up.

Results

Follow-up status and baseline characteristics of study subjects
In all, 1616 cohort members were initially free from CI at baseline. Among these subjects, 996 were alive at follow-up. Of the original male subjects, 155 (17.6%), and 131 (17.8%) of the female subjects were lost to follow-up. The proportions of deceased subjects were 23.1% and 17.8% respectively. The respondents and the lost-to-follow-up subjects were similar in the distribution of sex, education, baseline mental score, self-reported health status, ADL, and hospitalization experience. The lost-to-follow-up subjects were slightly older with a mean age of 78.1 years (sd = 5.89) compared with a mean age of 77.4 years (sd = 5.99) among the respondents. A statistically significantly higher proportion of the lost-to-follow-up subjects were residing in the community, and had lower social support score (39.5 % with a score of 15+) when compared with the respondents (47.6%). Subjects who had died during the follow-up period were older (mean age = 81.2, sd = 6.98), and had poorer baseline health and mental status compared with the respondents (data not shown).

Comparison of the baseline characteristics of the study subjects by gender showed that women had a statistically significant lower level of education, and higher dependency on others for major source of income. Women also had poorer self-reported health, poorer ADL score, and took longer to complete the 16-foot walk. About one-third of the women had a mental score in the range of 8–9 compared with 7% of men having this score at baseline. Also 20.7% of the women compared with 7.3% men were residing in institutions at baseline (Table 1Go).


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Table 1 Distribution of baseline characteristics by sex (n = 988)
 
Incidence of cognitive impairment during follow-up
Of men, 6.7%, and 22.2% of women had CI at 3-year follow-up. The incidences of CI for the age groups 70–74, 75–79, 80–84 and 85+ were 0.29, 1.61, 2.94 and 8.43 per 100 per year among men; and 2.79, 5.12, 10.99, and 15.24 among women. The age-standardized incidences (according to the 1991 age distribution for Hong Kong population aged >=70 years) were 1.52 per 100 for men, and 6.37 for women.

Gender differences in risk factors associated with cognitive impairment
Social demographic characteristics and lifestyle factors
Compared to those with secondary level of education, no formal education increased the risk of CI by threefold in men, and 8.6-fold in women. Further analyses were adjusted for age and education (Tables 2 and 3GoGo). Among men, being divorced, having poor social support (score <9) and residing in an institution increased the risk of having CI. Among women, income dependency on relatives or public assistance marginally increased the risk of CI.


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Table 2 Percentage distribution of baseline socio-demographic by sex and cognitive status, and odds ratios (OR) on the association between baseline variables and the risk of developing cognitive impairment during the 36-month follow-up
 

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Table 3 Percentage distribution of baseline lifestyle and health variables by sex and cognitive status, and odds ratios on the association between baseline variables and the risk of developing cognitive impairment during the 36-month follow-up
 
Among the lifestyle risk factors, we observed that female ex-smokers had an increased risk of CI, while ‘no exercise’ increased the risk of CI in both men and women. We did not observe an increased risk of CI associated with the use of prescription or non-prescription drugs.

Women with difficulty in daily activities (ADL<20) had a twofold increased risk of CI. A slow gait was associated with an increased risk of CI in both men and women. However, depression and poor vision at baseline were not associated with CI after adjustment for age and education.

Incident cerebrovascular disease during follow-up increased the risk of CI by about five folds. Incident cardiac disease also marginally increased the risk of CI in men.

Predictors of cognitive impairment
Multivariate logistic regression analyses on predictors of CI was carried out separately for men and women, as well as for all subjects with adjustment made for sex (Table 4Go). Significant as well as near significant (lower limit of 95% CI at 0.9) variables from Tables 2 and 3GoGo were included in the models separately constructed for men and women, as well as for all subjects. Table 4Go shows the significant (or marginally significant) predictors retained in the final models.


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Table 4 Final models (by sex and combined) on predictorsa of cognitive impairment
 
Increasing age, no formal education and residing in an institution were significant or marginally significant predictors in both sexes. Women with no formal education had a 3.5-fold increased risk compared with those with some formal education. Income dependency on others, either relatives or the government, increased the risk of CI in women but not in men. While increased time to complete the 16-foot gait walk was a significant predictor in both sexes, women who did not exercise had double the risk of CI. Incident cardiovascular disease also played a significant role in the prediction of CI in women, but not in men. In the combined model for both sexes, after adjusting for the significant social and health predictors of CI, women still had a 2.5-fold increased risk of having CI compared with men.

Discussion

The population in Asia is ageing rapidly. With an age-adjusted annual incidence of 1.52% in the male, and 6.37% in the female population aged >=70 years, the increase in the numbers of elderly with difficulty in cognitive function will be substantial. Until now, there have only been limited data on cognitive function in the Asian population,5,6,9,24 while prospective data on risk factors are even more limited.24 We have attempted to investigate the predictive health, social and lifestyle factors related to the development of CI over a 3-year follow-up period.

Sociodemographic factors
Our baseline cross-sectional9 as well as longitudinal data also revealed a substantial increase in the development of CI with age, particularly among people aged >=85 years. As observed in other studies in both Caucasian and Asian populations,6,24,25 we also observed a female preponderance of CI in both our cross-sectional and longitudinal data. Yu et al. also reported a female preponderance of CI, with a female to male ratio of 3.75 in the severe category, and 2.6 in the mild category.6 Studies have found adverse social and financial factors to have adverse consequences on physical, mental and psychological health.22,26 This population of elderly female subjects was over-represented by low level of education, low income and financial dependency. The gender differences in the development of CI between women and men were thus partly due to the more adverse baseline social conditions associated with older women. However, although the female risk was attenuated after adjusting for the significant social and health predictors, the risk of developing CI still increased by 2.5-fold in women compared with men.

Many studies have found an association between low education and impaired cognitive function.5,24,27,28 Lyketsos et al.28 postulated that a minimum period of education might confer protection against CI later in life; while Stern et al.29 supported the hypothesis that higher lifetime educational attainment may influence the incidence of Alzheizmer's disease by reducing the ease of detection, or by providing a reserve against early manifestation of the mental incapacity. The reserve could be an acquired set of skills or repertoire or the result of increased synaptic density in neocortical cortex acquired as a result of stimulation.

Because of the possible link between cerebrovascular changes and CI,24,30,31 some researchers have postulated that the negative association between education and CI may be partially mediated through the association between low education and vascular risk factors.30,32 However, our data showed that education remains a strong independent predictor of CI after controlling for cardiovascular risk factors as well as other social and health predictors. In addition, the strength of association remained stronger in women than men.

As subjects with mild CI might have been disproportionately referred to institutions (which included hostels, old-age homes, as well as care-and-attention homes), it is not surprising to observe the strong association between residence in an institution and the onset of CI. The stronger effect of institutionalization on CI in men might be partly mediated through the stronger role of social support in men (as observed in the age- and education-adjusted logistic regression analysis) and the likely decline of such support with institutionalization. Moreover, institutionalization is perhaps also a marker for declining social, mental and physical health.33

Health and disease conditions
White et al.27 demonstrated the independent effect of age, sex, low education level, and problems with ADL in a prediction model on cognitive decline over a 3-year interval. Researchers found that a number of health factors including difficulties with ADL, hearing problems, presence of diseases such as diabetes, hypertension, and the use of psychotic medications were predictive of cognitive decline.34,35 As found in our baseline study, as well as in the longitudinal study conducted in Japan,24 our longitudinal data also revealed that incident cerebrovascular disease during the follow-up period was a risk factor for CI. The differential effect between men and women may be related to a higher mortality among men during the follow-up period.

A review of the published literature shows a cross-sectional relation between CI and disability, independent of demographic, medical, and lifestyle factors.33 Our data revealed an association between baseline ADL and CI, but ADL was not retained in the final model after adjusting for other independent social and mobility risk factors. Gait speed seems to be a strong predictor of many physical health and mortality outcomes.36,37 This longitudinal study also revealed that gait speed is a strong predictor of CI in both men and women. This finding would be useful for the identification of elderly subjects with increased risk of developing CI.

Lifestyle factors
Exercise and aerobic fitness help the oxygen supply to brain neurones and help maintain brain and cardiovascular health. A 3-year followup study by Broe et al.38 did not find physical activity a protective factor in the development of CI, but the sample size in their study was comparatively small, comprising 327 subjects. Yoshitake et al. observed a protective effect of physical activity in the development of dementia.27 Wagner and LaCroix39 suggested inactivity might accelerate the rates of decline of physiological functions and reserves. Intervention studies have supported the beneficial effects of exercise even in frail elderly subjects.40,41 We also found that women who did not exercise at baseline had double the risk of CI. The observed stronger protective effect of exercise in women could be due to the fact that among this rather disadvantaged female cohort, the effect of exercise can be quite substantial. The potential effects of some of the traditional exercises or activities, such as Tai Chi or ‘morning walk’ (which is already a popular form of exercise, as well as social networking) in delaying the onset of CI in the elderly population should be further investigated.

Kilander et al.32 found an association between drinking alcohol more than twice a week and better cognitive performance. We did not observe such an association, possibly due to the small number of elderly subjects who were drinkers. Smoking has been reported to be protective against dementia in earlier case-control studies.42 A recent prospective study by Doll et al.43 did not support such a protective effect. Smoking could cause minor cerebrovascular lesions contributing to CI. In our study, we observed an increased risk of CI among female ex-smokers. A separate analysis in the same cohort on the relation between smoking and mortality and vascular risk factors also observed a stronger risk in female ex-smokers.11

Limitation of study
Due to their limited concentration span and difficulty in gathering information from the elderly, and as the interview and assessment time for this comprehensive study was already rather long, we attempted to limit the length of the interview questionnaire, and adopted the CAPE with cut-off <=7 in the baseline as well as follow-up interviews. Other researchers have reported that CAPE has similar sensitivity and specificity to MMSE, and is less influenced by educational attainment.16,17 We observed good construct validity of this assessment instrument in both the baseline and follow-up findings. Although we cannot directly compare our results with those reported in other populations because of differences in instruments used, our results are in agreement with other findings on the major risk factors for CI.

Previous studies43,44 on CI have highlighted the bias towards the underestimation of CI because of the selective drop-out of subjects affected by CI, particularly in the very old. Although the baseline data among the responders and non-responders in our study were quite similar in many aspects, the latter were more likely to be older and have less social support. Both of these factors were associated with CI, particularly in men. Their selective drop-out may bias the results towards an underestimation of the onset of CI. Subjects who died during follow-up had more adverse social and health conditions, which were also predictors of CI. Thus this survival cohort may bias the results towards an underestimation of the association between certain risk factors such as cardiovascular conditions and CI.

Study implications
Our data revealed a high incidence of CI in our older population implying a substantial increase in demand on social and health services, as the population, particularly the old-old segment, is increasing rapidly. Older age and female sex were independent factors associated with CI. No formal education, slow gait time and institutionalization increased the risk of CI in both sexes. While education had a stronger effect in women, institutionalization had a stronger effect in men. Financial dependency, lack of exercise and incident stroke played a significant role in women.

Further research would be required to assess whether modification of some of these factors, such as the provision of life-long education, rehabilitation to maximize functional mobility, exercise, and control of risk factors for stroke, could ameliorate the process of cognitive deterioration.45 Vascular CI related to stroke should be detected early for preventive therapy to prevent deterioration. The protective role of social support should be further explored. We should also further explore the use of gait time as a marker for screening the high risk elderly population, so that rehabilitative measures may be provided to alleviate their decline in mental and physical functions.


KEY MESSAGES

  • Older age and female sex were independent factors associated with cognitive impairment (CI).
  • Slow gait time and institutionalization increased the risk of CI in both older men and women, but institutionalization seems to have a stronger effect in men.
  • No formal education had a significant effect in the development of CI in women.
  • Financial dependency, lack of exercise and incident stroke played a significant role in women.

 

Acknowledgments

The study was supported by the Croucher Foundation and the Hong Kong Health Services Research Grant#411009. The authors would like to thank Ms Carol Mui and Ms Daisy Fung for typing the manuscript, and all the elderly subjects who participated in the study.

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