Lifestyle factors and risk of dementia: Dubbo Study of the elderly

In a recent longitudinal evaluation of nursing home admission in Australia, 44% of placements were primarily related to dementia, while dementia was a secondary diagnosis in a further 20% of people admitted.1 Given the increased incidence of dementia with advancing age,2 and the anticipated ageing of the Australian population over the next 20–30 years,3 research on risk factors for dementia is a national and international priority.

Although vascular dementia may be associated with traditional cardiovascular risk factors,4-6 most cases of dementia are the result of Alzheimer’s disease.7 Cognitive impairment in late life is inversely related to educational attainment,8,9 and also to the presence of the ε4 allele for apolipoprotein E.10 Lifestyle factors such as leisure activity,11 physical activity9,12,13 or a modest alcohol intake9,14-16 appear to be associated with a lower risk of dementia. Intervention to increase physical activity in older people might reduce the risk or degree of cognitive impairment.17

The longitudinal Dubbo Study of the elderly included citizens free of cognitive impairment at baseline, with a mean entry age of about 70 years.18,19 This report examines risk factors for dementia over 16 years of follow-up in the cohort.

The Dubbo Study is an ongoing prospective study of a cohort of elderly people first examined in 1988–1989.18 All non-institutionalised residents of Dubbo, NSW, born before 1930 were eligible to participate. Methods and measures have been described in detail elsewhere.19 Briefly, the baseline examinations comprised demographic, psychosocial and standard cardiovascular risk assessments. The medical examination included anthropometry; blood pressure (mean of two readings, phase V diastolic, after 10 minutes’ seated rest); resting electrocardiography; peak expiratory flow (PEF) rate (best of two attempts, measured by a Wright peak flow meter); and 12-hour fasting levels of plasma lipids, lipoproteins and glucose. A questionnaire explored measures of social support; depression status; education level; cognitive function (Short Portable Mental Status Questionnaire);20 alcohol and tobacco use; medications; medical history; myocardial infarction and chest pain; physical activity (walking, gardening and sporting activity, coded as daily, weekly or less often); self-rated health; and physical disability. The study population was broadly representative of the Australian population born before 1930.18

Hospitalisation, nursing home admission and death records were monitored continuously, with postal surveys conducted every 2 years to confirm vital status. The first recorded presentation of dementia over 16 years (September 1988 – September 2004) was included in the analysis. Participants who died without a diagnosis of dementia were censored at that point. Records were coded according to the International classification of diseases, 9th revision, clinical modification (ICD-9-CM) and the 10th revision, Australian modification (ICD-10-AM). The dementia coding was taken at face value from the records. Most cases were diagnosed by a geriatric assessment team with access to computed tomography scanning. The medical record coding did not reliably distinguish between Alzheimer’s Disease and vascular dementia, and we have analysed dementia as a single category. All participants gave informed, written consent.

Our study was approved by institutional ethics committees at St Vincent’s Hospital (Sydney), the University of New South Wales and the University of Western Sydney.

Characteristics of the cohort at baseline have been presented in previous reports.19,21 Virtually none of the 2805 members of the Dubbo cohort showed evidence of significant cognitive impairment at baseline (more than three errors on Short Portable Mental Status Questionnaire). During 16 years of follow-up (involving 32 284 person-years) there were 115 initial presentations with dementia in men (9.3/100) and 170 presentations in women (10.8/100). On average, the men developing dementia were 3.5 years older at baseline than their peers without dementia. Similarly the women with dementia were 5.7 years older than their peers.

Box 1 shows age-adjusted and sex-adjusted HRs for the prediction of dementia for individual variables of potential interest. Using this “univariate” approach there was significant prediction of reduced risk of dementia by years of education, intake of alcohol and daily gardening activity. There was significant prediction of an increased risk of dementia by prior coronary heart disease, impaired peak expiratory flow (PEF), a poor self-rated health score, physical disability and depression score. Many potential predictors (including a number of additional health and psychosocial factors not shown in Box 1) failed to reach significance.

An abbreviated summary of the final multivariate Cox model for dementia is presented in Box 2. For the total cohort, any intake of alcohol or daily gardening at baseline predicted a reduced risk of dementia (34% lower risk for alcohol and 36% lower risk for gardening). Broadly similar findings were made in models separating men and women. Daily walking also predicted a 38% lower risk of dementia in men, but with no significant prediction in women. For the total cohort, impaired PEF and higher depression score at baseline predicted an increased future risk of dementia (84% higher risk for the most impaired PEF, and 50% higher risk for those with the highest depression scores).

The final model was recalculated with graded levels of alcohol intake and gardening activity in the total cohort. Compared with those drinking no alcohol, the risk of developing dementia (HR) for those taking: 1–7 standard drinks per week was 0.70 (95% CI, 0.52–0.93); for 8–14 drinks per week was 0.65 (95% CI, 0.45–0.96); and for 15–28 drinks per week 0.40 (95% CI, 0.21–0.79). Only 21% of men and 3% of women had more than 14 drinks per week. Compared with those not gardening, the HR for those gardening daily was 0.60 (95% CI, 0.44–0.82), and for those gardening weekly or less often, the HR was 0.89 (95% CI, 0.64–1.24). Fifty-six per cent of men and 41% of women gardened daily.

There are some weaknesses in our methods. We have no information on lifestyle behaviour in participants after baseline assessment. We have relied entirely on medical record linkage without further validation, but cases of dementia were diagnosed by a thorough and consistent process. We have not identified less severe cases managed in the community, thus indicating that we have probably underestimated the true incidence of dementia. Recent research from the Rotterdam Study has highlighted how difficult it is to assess the true incidence of dementia, either during short or longer term follow-up.23 However, a potential misclassification of cases suggests that we must be reasonably conservative in drawing conclusions.

The apparently protective effect of moderate alcohol intake is consistent with previous reports.9,14-16 High intakes of alcohol were not observed among the Dubbo senior citizens, while other studies have shown an adverse effect on cognitive function at higher intakes.16 Physical activity has regularly been found to be protective against the onset of dementia.9,12,13 While the protective effect of daily gardening is very strong, it may also be appropriate to consider daily gardening in a senior citizen as a leisure or mental activity.11

A recent epidemiological study from Sweden reported a significantly lower risk of dementia in people found in the top quartile of cholesterol distribution.24 This contrasts with a pilot placebo-controlled trial suggesting that cholesterol reduction with atorvastatin slowed cognitive decline in patients with mild Alzheimer’s disease.25 Our data show no association between plasma cholesterol level and risk of dementia, and this is equally true for levels of low-density lipoprotein cholesterol, high-density lipoprotein cholesterol, plasma triglycerides or lipoprotein(a) (data not presented).

Our findings suggest some public health messages for reducing the future incidence of dementia in our ageing society. Excess alcohol intake needs to be avoided, but it would appear safe and reasonable to recommend the continuation of moderate alcohol intake in those already imbibing, and the maintenance of physical, leisure and mental activity, especially daily gardening.