Our findings suggest a relation between the degree of leisure activity and the risk of developing AD. Even when factors such as ethnic group, education, and occupation were controlled for, subjects with high leisure activity had 38% less risk of developing dementia. The effect of leisure activities on incident dementia was still present even when baseline cognitive performance, health limitations interfering with social activities, cerebrovascular disease, and depression were considered.
Among leisure activities, reading, visiting friends or relatives, going to movies or restaurants, and walking for pleasure or going for an excursion were most strongly associated with a reduced risk of incident dementia. When the overall score was used in the Cox models, a protective effect for incident dementia was seen for each additional point, implying a cumulative effect for the number of activities adopted. When the leisure items were grouped into physical, social and intellectual factors, all three factors retained their significant effect although the intellectual factor was associated with lowest RR of incident dementia.
The cognitive reserve hypothesis suggests that aspects of life experience supply a set of skills or repertoires that allow an individual to cope with progressing AD pathology for a longer time before the disease becomes clinically apparent. However the concept of a reserve must be weighed against several alternative possibilities.
Health limitations, problems or difficulties can interfere with desired social or leisure activities as evidenced by their significant association. Vascular disease has been implicated in the clinical expression of AD pathology.37
Also reports of fewer social engagements could be partially related to underlying depressive illness, and the preexistence of depression may increase the likelihood of incident dementia.38-40
The association between leisure and incident dementia remained after inclusion in the analyses of covariates for all these potential confounders.
It can also be argued that low leisure activity may represent a manifestation of early dementia rather than a premorbid risk factor. The consequence of such a premise would be that subjects with borderline dementia might have lower leisure activity as a result of early disease. We therefore repeated our analyses after eliminating subjects with scores indicating borderline dementia (CDR = 0.5) and replicated the findings seen in the complete cohort. Still, the possibility that low leisure activities reflect early disease consequences rather than a premorbid risk factor cannot be entirely excluded.
Maintaining intellectual and social engagement through participation in everyday activities seems to buffer healthy individuals against cognitive decline in later life.41-43
With regard to incident dementia, educational and occupational attainments have been extensively studied, but there are very few reports examining the influence of socially and intellectually engaged lifestyle to dementia. One case control study of AD in Japan44
(60 cases) reported that cases had less use of leisure time, hobbies, and psychosocial behaviors. Another case-control study examined the presence of intellectual, passive, and physical activities during midlife in 193 subjects with possible and probable AD. Diversity of activities and intensity of intellectual activities were reduced in patients with AD as compared with the control group.45
Only two prospective longitudinal studies have addressed the effect of social activities on incident dementia. In a survey sample of 422 elderly subjects, the relation of various indicators of socioeconomic status to incident dementia was investigated.14
Only poor quality of living accommodations was associated with increased RR of incident dementia, while indicators of social isolation and social support did not prove to be significant.
Another study evaluated social and leisure activity data in 2,040 nondemented elderly community residents from Gironde (France) and recorded incident dementia on follow-up visits.15
Traveling, doing odd jobs, and knitting were associated with lower risk of incident dementia when occupational status was controlled for. However, the analyses were not controlled for ethnic group, gender, educational attainment, cerebrovascular risk factors, or depressive symptomatology. In addition, for doing odd jobs and knitting, the effect was significant only when subjects who did the activities without difficulty were compared to subjects who did not do them because of deficiencies or disabilities. When the comparison group was subjects who were not engaged in these two activities for other reasons, no significant influence was noted. Therefore, the reasons for engaging in leisure activities, rather than the leisure activities per se, seemed to affect incident dementia in that study. In our study, medical problems interfering with desired leisure activities did not affect the association of leisure with dementia.
We also cannot dismiss the potential physiologic effects of leisure activities. Exposure to an enriched environment, defined as a combination of more opportunities for physical activity, learning, and social interaction, produces not only a host of structural and functional changes in the brain but also influences the rate of neurogenesis in adult and senescent animal models.46-48
Also, there is recent evidence that certain brain areas retain the capability to generate new neurons into adulthood, not only in rodents and primates,49
but also in humans.50
Although it may appear unlikely that aspects of life experience could impede the development of pathologic changes of AD, it has been proposed that enhanced chronic neuronal activation associated with increased brain work, increased regional cerebral blood flow, and increased glucose and oxygen metabolism may be resulting in hindering the development of the disease.7
Engagement in social, intellectual, and physical activities could also promote a physiologic process involving increased synaptic density in neocortical association cortex, acquired on the basis of stimulation.9
There are physiologic data that have provided an indirect affirmation of the reserve hypothesis. Controlling for clinical severity of dementia, regional cerebral blood flow has been shown to be lower in patients with higher educational and occupational attainments.51,52
Similar results seem to emerge for leisure: an inverse association between leisure activities and regional cerebral blood flow, when controlling for disease severity, education, and IQ, has been demonstrated in AD patients.53
Despite the results of this study, the possibility that absence of social and intellectual engagements may not be risk factors themselves but may be related to some unknown causal risk factor cannot be completely excluded. In that case, leisure activities might be a surrogate marker for the actual risk factor. Further research is necessary to clarify the exact pathophysiologic changes that underlie the mechanisms of the reserve phenomenon. However, the observed epidemiologic associations suggest the possibility that interventions that enhance life experiences and activities might reduce the risk of developing dementia.