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American Journal of Epidemiology Advance Access originally published online on February 7, 2008
American Journal of Epidemiology 2008 167(7):820-830; doi:10.1093/aje/kwm382
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American Journal of Epidemiology © The Author 2008. Published by the Johns Hopkins Bloomberg School of Public Health. All rights reserved. For permissions, please e-mail: journals.permissions@oxfordjournals.org.

ORIGINAL CONTRIBUTIONS

Is Complexity of Work Associated with Risk of Dementia?

The Canadian Study of Health and Aging

Edeltraut Kröger1,2, Ross Andel3, Joan Lindsay4, Zohra Benounissa2, René Verreault1,2 and Danielle Laurin2,5

1 Faculty of Medicine, Laval University, Quebec City, Quebec, Canada
2 Laval University Geriatrics Research Unit, Centre de recherche du CHA, Quebec City, Quebec, Canada
3 School of Aging Studies, University of South Florida, Tampa, FL
4 Department of Epidemiology and Community Medicine, University of Ottawa, Ottawa, Ontario, Canada
5 Faculty of Pharmacy, Laval University, Quebec City, Quebec, Canada

Correspondence to Edeltraut Kröger, Laval University Geriatrics Research Unit, Hôpital du Saint-Sacrement, 1050, Chemin Sainte-Foy, Québec (QC) G1S 4L8, Canada (e-mail: Edeltraut.Kroger.cha{at}ssss.gouv.qc.ca).

Received for publication August 17, 2007. Accepted for publication December 4, 2007.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 
The authors evaluated the association of complexity of work with data, people, and things with the incidence of dementia, Alzheimer's disease, and vascular dementia in the Canadian Study of Health and Aging, while adjusting for work-related physical activity. The Canadian Study of Health and Aging is a 10-year population study, from 1991 to 2001, of a representative sample of persons aged 65 years or older. Lifetime job history allowed application of complexity scores and classification of work-related physical activity. Analyses included 3,557 subjects, of whom 400 were incident dementia cases, including 299 with Alzheimer's disease and 93 with vascular dementia. In fully adjusted Cox regression models, high complexity of work with people or things reduced risk of dementia (hazard ratios were 0.66 (95% confidence interval: 0.44, 0.98) and 0.72 (95% confidence interval: 0.52, 0.99), respectively) but not Alzheimer's disease. For vascular dementia, hazard ratios were 0.36 (95% confidence interval: 0.15, 0.90) for high complexity of work with people and 0.50 (95% confidence interval: 0.25, 1.00) for high complexity of work with things. Subgroup analyses according to median duration (23 years) of principal occupation showed that associations with complexity varied according to duration of employment. High complexity of work appears to be associated with risk of dementia, but effects may vary according to subtype.

Alzheimer disease; cohort studies; dementia; motor activity; occupations; risk factors; work

Abbreviations: CI, confidence interval; CSHA, Canadian Study of Health and Aging; WPA, work-related physical activity


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 
Lifestyle factors may be important contributors to dementia (1, 2). Numerous studies, including the Canadian Study of Health and Aging (CSHA), have suggested that higher education may delay onset of dementia (3–8). Intellectually stimulating lifestyle activities such as leisure activities (9–12) and occupational characteristics (13, 14) have also been associated with dementia. Occupations with high mental demands may represent a form of mental exercise that supports brain function in older adulthood, as expressed by the "use it or lose it" hypothesis (5, 6), and may thus affect cognitive performance in older adults. People spend a considerable part of their adult life in their occupation, making intellectual stimulation inherent to occupation of particular interest. Complex work environments that reward cognitive effort and require decision making may motivate individuals to continue to develop their intellectual capacities (15, 16).

The occupational complexity hypothesis has been operationalized by using three dimensions of complexity, complexity of work with data, people, and things, which have been derived from occupational characteristics based on the Dictionary of Occupational Titles (17–19). Previous studies based on these measures have suggested that complexity of work may promote stable cognitive function (20–22) and reduce the risk of dementia (13, 23). Leisure physical exercise has been associated with a decreased risk of dementia and Alzheimer's disease in a number of longitudinal studies (4, 24), but few explicitly examined the association between intellectual occupational demands and cognitive outcomes in old age while taking physical activity at work into account. To our knowledge, the association between mental together with physical characteristics of work and dementia or Alzheimer's disease has not been studied in longitudinal cohorts.

This study evaluated the association between complexity of work with data, people, and things in the principal occupation, while adjusting for work-related physical activity (WPA), and the incidence of dementia, Alzheimer's disease, and vascular dementia. In addition, we explored whether effects of occupational characteristics vary according to duration of principal occupation, since dementia develops over a long period of time and duration of principal lifetime occupation may vary considerably between study participants (13).


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 References
 
CSHA is a large, longitudinal study of dementia in older Canadians from 1991 to 2001, and study methods have been described in detail elsewhere (25, 26). Briefly, representative samples of people aged 65 years or older living in the community or in institutions were drawn from 36 urban and surrounding rural areas in all Canadian provinces in 1991, resulting in 10,263 participants (CSHA-1), of whom 9,008 were living in the community. Information on personal and lifestyle risk factors was collected at baseline by using a self-administered questionnaire, including lifetime occupational history. Occupations were coded according to the 1980 Canadian Standard Occupational Classification.

The standardized clinical examination comprised a two-stage, extensive diagnostic process. First, the Modified Mini-Mental State examination (27–29) was used to screen for cognitive impairment; a cutoff point of 77/78 was used. Second, all subjects who scored below this cutoff were administered standardized medical and neuropsychological evaluations, which were used to diagnose dementia according to Diagnostic and Statistical Manual of Mental Disorders, Third Edition, Revised, criteria (30). The diagnosis of Alzheimer's disease followed criteria of the National Institute of Neurological and Communicative Disorders and Stroke–Alzheimer's Disease and Related Disorders Association (NINCDS-ADRDA) (31, 32), and the International Statistical Classification of Diseases and Related Health Problems, Tenth Revision, definition (31) and modified Hachinski Ischemic score (33) were used for vascular dementia.

All subjects evaluated in CSHA-1 were contacted again in 1996/1997 (CSHA-2) and in 2001/2002 (CSHA-3). Subjects who agreed to participate in the follow-ups were reevaluated. The same two-stage CSHA-1 diagnostic process was used, except diagnostic criteria were now updated by using the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition. Diagnosis of vascular dementia was based on criteria from the National Institute of Neurological Disorders and Stroke–Association Internationale pour la Recherche et l'Enseignement en Neurosciences (NINDS-AIREN) (34), and a cutoff of 90 was used for the screening with the Modified Mini-Mental State examination in CSHA-3 (35). The three phases of CSHA received approval from the institutional ethics committees of all participating centers; subjects or family representatives gave written consent for participation in each phase.

Assessment of exposure
Among all occupations indicated in a subject's occupational history, the one of the longest duration was defined as the principal occupation. Similarly to a procedure developed for a Swedish population (23), the Canadian Standard Occupational Classification codes were matched to codes from the 1970 US Census occupational classification (36). Matching was performed independently in Canada (E. K.) and the United States (R. A.), and, after the first round, there was agreement on 432 of 502 Standard Occupational Classification codes (86.1 percent). The codes on which the two raters disagreed were discussed until a consensus was reached. Scores for occupational complexity with data, people, and things were then applied to the occupational categories. Since these complexity scores had already been defined for the US Census codes, no further matching was required. Scores were constructed so that lower ones indicated higher complexity and higher ones indicated lower complexity, and they reflected those from the fourth edition of the Dictionary of Occupational Titles (18), where occupations were rated by means of extensive on-site observations by qualified job analysts (36). Median reliability estimates of these scores are 0.85 (range: 0.84–0.90) for complexity of work with data and 0.87 (range: 0.80–0.91) for complexity of work with people (37). The reliability for complexity of work with things is weaker at 0.46 (range: 0.25–0.65) (37). As an example, the Standard Occupational Classification occupation "commodities sales clerk" is assigned scores of 1.2 for complexity of work with data, 5.1 for complexity of work with people, and 6.7 for complexity of work with things, indicating that the occupation mostly involves "coordinating" data, "persuading" people, and "feeding/offbearing" things.

For each of the 502 Standard Occupational Classification codes, the level of WPA was classified by a two-round Delphi panel of five panelists (E. K., D. L., J. L., and two experts in kinesiology), similarly to a method used by King et al. (38). Panelists were presented with the Standard Occupational Classification codes and the corresponding occupational titles and descriptions (39) and rated each occupation as of low, high, or uncertain WPA. After the first round, all occupations for which agreement was below 80 percent were resubmitted for a second round. Occupations for which agreement was below 80 percent hereafter were considered of uncertain WPA. WPA level was rated high for 46 occupations, low for 163 occupations, and uncertain for 293 occupations (40).

Study sample
Among the initial study cohort, 9,008 subjects were living in the community at baseline; 910 either had dementia or screened positive on the Modified Mini-Mental State examination but did not undergo clinical evaluation. Thus, 8,098 subjects were considered free of dementia. No information on risk factors was available for 938 subjects, leaving 7,160 eligible for follow-up. Among them, 5,037 subjects participated in at least one follow-up. Some of these subjects were excluded because principal occupation could not be determined (n = 938) or no Standard Occupational Classification code could be assigned to the available occupational information (n = 57). For 485 participants, the principal occupation was that of a homemaker. Because occupational complexity is defined for gainful employment only, homemakers were excluded, as in similar studies (22, 23, 41). Among the remaining 3,557 subjects were 440 incident dementia cases, including 299 with Alzheimer's disease and 93 with vascular dementia, during a median follow-up of 9.3 years (interquartile range: 4.8–9.9 years).

Statistical analyses
Complexity scores were divided into three categories (low, intermediate, high), reflecting occupational groups characterized by tasks of similar complexity for the three respective scores (table 1). Sociodemographic characteristics within the study sample were compared across these categories. Two-sided p values were obtained by using chi-square tests for categorical and analysis of variance or the Kruskall-Wallis test for continuous variables. The association between occupational characteristics and incident dementia, Alzheimer's disease, or vascular dementia was assessed by using Cox proportional hazards regression models with age as the time scale (42). Age at onset of disease was determined as the midpoint between the last follow-up without and the next follow-up with dementia. Deceased or lost subjects were censored at the moment of their last follow-up.


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  		TABLE 1. Proportions (%) of subjects from the Canadian Study of Health and Aging (1991–2001) with principal occupations, by level of occupational complexity (n = 3,557)

 
In all analyses, low level of complexity and low level of WPA were the reference categories. Basic models included the three dimensions of complexity, sex, and education in years. Additional models were derived, adjusting for WPA and several possible confounders including leisure physical activity (at least some type of physical exercise such as walking, or none); physically active hobbies (at least one hobby such as gardening, home repair, car repair, or painting performed often, or none); ever smoking; any alcohol consumption; history of diabetes mellitus (yes/no), hypertension (yes/no), or cardiovascular disease (yes/no); and family history of dementia (yes/no). Information on these variables came from the baseline questionnaire. Subgroup analyses were performed according to median duration of the principal occupation.

Modification by the two strata of duration of principal occupation was tested within Cox regression models comprising the four occupational characteristics and adjustment for sex and education by using interaction terms. To test modification for education, we used interaction terms for the interaction of complexity with data, people, and things with the presence of 1) postsecondary education (13 years or more; yes/no) and 2) basic education (9 years or more; yes/no). Modification by sex was tested in a similar way. All statistical tests were two sided, with {alpha} = 0.05 considered significant. Statistical analyses were performed by using SAS 9.1 software (SAS Institute, Inc., Cary, North Carolina).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 References
 
The study sample of 3,557 subjects was compared with the sample of those eligible for follow-up but excluded from analyses (n = 3,603) because they were lost to follow-up, mainly because of death (n = 2,123), had to be excluded for lack of employment information (n = 995), or were homemakers (n = 485). Among those excluded from analyses, median age at baseline was 76 years (interquartile range: 71–82 years) and median years of education were 10 (interquartile range: 8–12 years), whereas the study sample was a median age of 73 years (interquartile range: 68–78 years; p < 0.01) and had a median of 11 years of education (interquartile range: 8–13 years; p < 0.01). Given the exclusion of homemakers, the proportions of women among those ineligible for analyses and those in the study sample were different: 67.0 percent versus 51.2 percent (p < 0.01). If homemakers had been included in the study sample, the proportion of women would have been 57 percent.

The distributions of complexity scores were skewed to various degrees, especially for complexity of work with data. Proportions of the study sample exposed to each of the three levels of complexity regarding data, people, or things are shown in table 1. Table 2 shows the levels of complexity regarding data, people, and things and of WPA for the five most frequent principal occupations. Scores for complexity of work with data, people, and things were correlated with each other: correlation coefficients were positive for the correlation between complexity with people and complexity with data (0.66) and negative, but smaller, for the correlation between complexity with things and complexity with data or people (–0.08 and –0.34, respectively). Complexity scores were also correlated with level of WPA, with negative correlations between WPA and high complexity with data (–0.26) and people (–0.47). The correlation between WPA and high complexity with things was positive (0.18). The level of WPA was low for 48.1 percent, high for 10.5 percent, and uncertain for 41.5 percent of the study sample.


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  		TABLE 2. The five most frequent occupations in the study population (n = 3,557) from the Canadian Study of Health and Aging (1991–2001) and their respective levels of occupational characteristics

 
Baseline age differed significantly across categories of occupational complexity with people or things, but not with data (table 3). Fewer women (44 percent) than men (56 percent) had high levels of complexity with data, but more women (54 percent) than men (46 percent) had high levels of complexity with people. The proportions of women and men with high levels of complexity with things differed little (0.8 percent). The proportions of women who developed dementia, Alzheimer's disease, and vascular dementia during follow-up were, respectively, 12.6 percent, 9.6 percent, and 2.5 percent compared with 12.2 percent, 7.8 percent, and 3.4 percent of men (p = 0.71 for dementia, p = 0.06 for Alzheimer's disease, and p = 0.12 for vascular dementia). Mean number of years of education decreased from the highest to the lowest category of occupational complexity with data and people. Regarding occupational complexity with things, mean number of years of education was slightly higher in the intermediate category than in the highest and the lowest categories. WPA level varied across the categories of occupational complexity, and, among those with high levels of WPA, lower proportions had high complexity with people but higher proportions had high complexity with things. Median duration of the principal occupation was 23 years (interquartile range: 14–34 years).


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  		TABLE 3. Sociodemographic characteristics and level of work-related physical activity of study subjects from the Canadian Study of Health and Aging (1991–2001) according to categories of occupational complexity

 
In univariate analyses, high and intermediate levels of occupational complexity with data and people were significantly associated with a decreased risk of dementia. The association between high complexity of work with people and Alzheimer's disease was also significant. There was no significant association of occupational complexity with vascular dementia. A high level of WPA was associated with an increased risk of dementia, Alzheimer's disease, and vascular dementia (table 4).


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  		TABLE 4. Relative risks for the association between occupational complexity and incidence of dementia, Alzheimer's disease, or vascular dementia (n = 3,557) in subjects from the Canadian Study of Health and Aging (1991–2001)

 
According to models comprising all three dimensions of complexity, WPA, and adjustment for sex and education, those subjects with high occupational complexity of work with people or things had a decreased risk of dementia when compared with those with low occupational complexity in the respective dimension (table 4). Dementia risk was decreased by 34 percent for complexity with people (p = 0.04). For complexity with things, the decrease was 27 percent and of borderline significance (p = 0.05). For Alzheimer's disease, risk decreases were similar for high complexity with people but were not significant. Further adjustment for additional confounders did not noticeably change these risk estimates (table 4). In the fully adjusted model, the risk of vascular dementia was decreased with high complexity with people (hazard ratio = 0.36, 95 percent confidence interval (CI): 0.15, 0.90) and, with borderline significance, with complexity with things (p = 0.05). Complexity with data was not associated with dementia, Alzheimer's disease, or vascular dementia.

Association with occupational characteristics varied between two subgroups formed according to duration of the principal occupation: median duration and below versus more than median duration. Modification of risk ratios for dementia by duration of occupation was significant for the interactions between high complexity with people or things; for Alzheimer's disease, the interaction was significant between high complexity with data and duration. In the fully adjusted model, those who held their principal occupation for more than 23 years had a significantly decreased risk of dementia if their occupation involved high complexity with people or things, compared with those with low complexity in these dimensions (table 5). This risk was lower by 64 percent (p = 0.001) and 55 percent (p = 0.001) for high complexity of work with people and things, respectively. Risk reductions were similar, with 69 percent (p = 0.003) and 52 percent (p = 0.02), respectively, for the association between high occupational complexity with people or things and Alzheimer's disease. There were no significant associations with vascular dementia. High complexity with data increased risk of dementia and Alzheimer's disease (hazard ratio = 1.77, 95 percent CI: 1.02, 3.08 and hazard ratio = 2.83, 95 percent CI: 1.36, 5.90, respectively) but not vascular dementia.


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  		TABLE 5. Relative risks for the association between occupational complexity and incidence of dementia, Alzheimer's disease, or vascular dementia, according to median duration of principal occupation, in subjects from the Canadian Study of Health and Aging (1991–2001)

 
However, among those with 23 years or less in their principal occupation, no dimension of occupational complexity was associated with dementia, Alzheimer's disease, or vascular dementia. There was no effect modification by sex for dementia but a significant modification of risk of Alzheimer's disease for complexity with data: the increased risk of Alzheimer's disease observed for high complexity with data was limited to men (hazard ratio = 2.60, 95 percent CI: 1.11, 6.06 for men and hazard ratio = 0.74, 95 percent CI: 0.41, 1.33 for women). We did not find an effect modification for education.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
References
 
These results show that high occupational complexity with people or things may decrease the risk of dementia, Alzheimer's disease, and vascular dementia, after adjustment for WPA and other confounders. Subgroup analyses indicate that the effects of occupational characteristics may vary according to duration of occupation. Specifically, high complexity of work with people and things was protective of dementia and Alzheimer's disease for individuals who held their principal occupation for more than 23 years, whereas high complexity of work with data increased the risk of dementia and Alzheimer's disease in this subgroup. Conversely, no dimension of complexity was associated with dementia or Alzheimer's disease for those who held their principal occupation for 23 years or less.

Maintenance of cognitive reserve (43, 44) by mental stimulation may be an underlying mechanism explaining the relation of higher complexity of work with people and things to lower risk of dementia found in our study. According to this hypothesis, intellectually demanding activities at work may provide a type of exercise contributing to more sophisticated cerebral networks in old age (45, 46) or make it possible to tolerate dementia neuropathology for a longer period during disease progression (7, 14). A study by Stern et al. (14) tested work complexity within the concept of cognitive reserve by using factor scores reflecting substantive complexity, interpersonal demands, management requirements, and physical demands. Patients who held jobs involving high interpersonal and physical demands had relatively greater deficits in cerebral blood flow in the parietal area when compared with patients who held low-demand jobs.

These results may indicate a delay in clinical expression of Alzheimer's disease attributable to the protective effect of occupational demands on the brain. In a study of 171 Alzheimer's disease patients in the United States, those whose main occupation involved high substantive complexity and high complexity of work with data and people showed relatively fast rates of cognitive decline, suggesting that their brain structures may have been affected by the disease to a greater extent at disease onset than the brain structures of patients with less complex occupations (41). Results of a case-control analysis of the population-based Swedish Twin Registry yielded results similar to ours: high occupational complexity with people was found to be associated with a reduced risk of dementia or Alzheimer's disease (23). In addition, in a US case-control study, mental occupational demands were found to be lower and physical occupational demands higher for cases than for controls, which also seems to be in accordance with our results (13).

Another possible mechanism, particularly for the protective effect of occupational complexity with people, may be through social networks, which are believed to protect cognitive health (47–49), or the effect of socioeconomic position across the life course on cognition in late life (50). The significantly increased risk of dementia and Alzheimer's disease associated with high complexity with data in the long-duration group is intriguing, but for Alzheimer's disease it was limited to men. This finding leads to the question of whether occupations involving this type of complexity may have adverse effects on brain functioning, possibly because of elevated levels of work-related stress or distress or lack of social networks or social interactions (51). A greater likelihood of adverse occupational exposures in low-status occupations may also explain some of our findings, since such occupations usually have low levels of complexity with people (52–54).

Some limitations affect this study. Cases of dementia may have been missed because of lack of sensitivity of the screening with the Modified Mini-Mental State examination; however, cutoffs are considered relatively high (25). Although considerable care was taken, exposure to occupational characteristics may have involved some misclassification. The reliability of the score for complexity with things may appear weak. The scale used to rate complexity with things is dominated by functions dealing with the relation of workers to machines (55). The difficulty of assigning ratings to occupations including tasks in which machines are unimportant may explain this weaker reliability. Misclassification of exposure to complexity with things, if nondifferential, could have led to an absence of association, but we observed an association. This result may possibly be due to occupational characteristics for which we could not account, or it might be due to chance.

As in similar longitudinal studies on dementia, CSHA suffered from loss to follow-up due to death. Although the study sample differed significantly from the cohort eligible for follow-up regarding age, sex, and education, we do not assume that selection bias explains the observed associations. It may be argued that early stages of cognitive loss prevent people from functioning in occupations with high complexity. However, in this study, high complexity with people or things more strongly reduced risk of dementia and Alzheimer's disease for those having held their principal occupation for more than 23 years. In this latter group, 75 percent already held their principal occupation by age 33 years. Furthermore, 93 percent of the survival cohort in CSHA had retired from gainful employment when entering the study, and exclusion from analyses of those still working at the beginning of the study (n = 399) did not noticeably change the results. Finally, residual confounding may have affected our results because of the categorization of the complexity scores into three levels; however, these scores were not distributed normally.

In conclusion, high complexity of work with people and things may protect against dementia and its subtypes. This effect may be stronger with prolonged exposure, but high complexity with data may increase risk for those being exposed to it for most of their working life. It will be important to examine these observations in studies carried out in other settings, including detailed assessments of occupational complexity, physical effort at work, and work and lifestyle characteristics such as social networks, job strain, and other occupational exposures.


   ACKNOWLEDGMENTS
 
This research received financial support from the Fonds de Recherche en Santé du Québec (research fellowship for D. L.), the Canadian Institutes of Health Research, the Réseau Quebecois de Recherche sur le Vieillissement, the Formation interdisciplinaire en Recherche Santé et Vieillissement, and the Chaire de Gériatrie of Laval University (doctoral fellowships for E. K.).

The authors thank Drs. Normand Teasdale and Olivier Hue for their participation in the Delphi panel.

Conflict of interest: none declared.


   References
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 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 

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