Despite much research into the causes of dementia, many unanswered questions remain.
The current evidence demonstrates that multiple factors, including age, genes, environment and lifestyle factors, contribute to whether a person develops dementia.
However, up to half of the total dementia burden in Australia may be due to modifiable risk factors.
Biomedical risk factors
Risk factors for dementia, as for heart disease and other chronic diseases, are classified as modifiable and non-modifiable. The scientific and policy literature documents significant associations between the modifiable risk factors for some dementias and those for other major chronic diseases, including heart disease, stroke, chronic obstructive pulmonary disease, diabetes and some cancers. The WHO has recently reviewed the evidence on risk factors and provided risk reduction of cognitive decline and dementia guidelines for preventive interventions where the evidence is considered strong enough. Sufficient evidence was found in relation to the following risks. Risk factors for some dementias are similar to those for other major chronic diseases, including heart disease, stroke, COPD, diabetes and some cancers.
Physical activity has many health benefits and is a key modifiable factor involved in the development of many chronic diseases including dementia. There is evidence that the highest levels of physical activity are the most protective and may have direct beneficial effects on brain structures. Indirectly, it is suggested that the impacts of physical activity on brain health arise from the underlying impacts on cardiovascular risk factors including hypertension, insulin resistance, high cholesterol and other biological mechanisms. Therefore, maintaining physical activity levels and muscle mass, strength and function throughout the life course may reduce the prevalence of dementia and prevent (or slow) cognitive decline among the Australian population.
Tobacco is still the leading cause of preventable deaths globally, incurring billions of dollars on healthcare and welfare costs. Smoking is a major risk factor for a number of chronic conditions including heart disease, respiratory diseases, and many cancers. A large body of evidence highlights associations between smoking in mid-life and subsequent disorders in later life including cognitive decline and dementia. There is good evidence for the effectiveness of smoking cessation interventions in reducing all of these health risks.
Healthy eating is an important contributor to optimal health across the life course as well as in the prevention of chronic diseases, including those that increase the risk of dementia. Evidence suggests that diet may be involved both directly and indirectly in the development of dementia through its effect on other risk factors. A range of high-quality studies have concluded that high levels of adherence to a Mediterranean diet, including consumption of fruit, vegetables, fish, nuts, olive oil and other unsaturated fats, whole grains and coffee may be associated with better cognitive performance and reduced incidence of dementia.
Evidence is increasing about the complex relationship between alcohol use and cognitive health and dementia. Previous reviews point to a possible beneficial effect of light to moderate alcohol use on cognitive health. However, even moderate drinking has been associated with detrimental effects on brain structure, and heavy drinking is detrimentally related to dementia risk, whatever the dementia type. There is increasing evidence in support of the contention that heavy alcohol use (>12g per day) is associated with increased dementia risk. Alcohol use disorders are characterised by the harmful and chronic consumption of alcohol. People who consume harmful amounts of alcohol are three times more likely to develop younger onset dementia. Alcohol use disorders are also associated with a range of associated risk factors for dementia, including smoking and poor nutrition and physical inactivity.
Overweight and obesity are direct risks for a wide range of chronic diseases including Type 2 diabetes and cancer. They are also indirect risks for other cardiovascular risk factors including high cholesterol and hypertension. There is evidence that obesity at mid-life increases the subsequent risk of dementia.
Diabetes mellitus management
Late life diabetes has been linked to an increased risk of cognitive decline and dementia. In addition, secondary health problems arising from diabetes including kidney disease, eye disease, hearing impairments and CVD have all been associated with increased risk of dementia. There is only inconsistent evidence yet for a direct impact on cognitive outcomes from glucose-control interventions. There is evidence that treating the cardiovascular comorbidities associated with diabetes may mediate risks for dementia.
Hypertension in mid-life has been associated with an increased risk of dementia later in life. The evidence for a direct impact on blood pressure reduction in mid or later life on subsequent cognitive decline or dementia is not yet established. However, it is clear that reducing hypertension is highly beneficial in reducing cardiovascular disease and premature mortality and therefore on improving the overall health of the older population.
Dyslipidaemia (high cholesterol) is an important modifiable risk factor, linked to a third of the burden of heart disease globally as well as premature mortality and disability. A number of epidemiological studies have found a link between high cholesterol and dementia, although this has not yet been demonstrated definitively. Other studies have identified an association between the control of cholesterol and a reduction of dementia risk.
A summary of the WHO recommendations in relation to each of these risks is provided at Appendix 1.
The WHO has not provided guidelines for the management of depression and mid-life hearing loss although a number of studies, including the Lancet Commission, have identified these as additional, modifiable risks for cognitive impairment and dementia.
Social risk factors
The World Health Organization guidelines do not address the influence of broader social determinants on dementia risk. Many other studies have, however, explored and described an inverse association between socioeconomic position and chronic conditions such as cardiovascular disease (CVD) and dementia. The biological and other pathways that lead to these conditions are known although the ways in which they exert their cumulative impact is not well understood. The social gradient in CVD persists even after adjustment for health behaviours and clinical indicators, suggesting that other processes are involved.
Immune responses to chronic ‘social stress’ may explain some of the socioeconomic gradient in CVD and other inflammation-associated conditions, including dementia. Providing a measure of support for this argument, there is some evidence that social interventions, particularly the promotion of higher educational attainment, may exert a protective impact. People who leave high school before year 8 have a 2.2-fold higher risk of dementia in later life and those leaving between years 8 and 11 have 1.5 times the risk of individuals who complete high school. There is increasing evidence that loneliness and isolation are associated with poorer cognitive function among older adults. Interventions to reduce social isolation, promote social engagement and increase levels of physical activity among older people reduce dementia risk. Interventions that foster social connections may be particularly beneficial for individuals with low levels of education. In addition, hearing loss is associated with both social isolation and the incidence of dementia.
The Lancet Commission suggests that the protective effect of education and social engagement is related to variations in levels of ‘cognitive reserve’ (brain resilience), which is enhanced by early-life experiences, including education and intellectual stimulation. There is evidence that higher socioeconomic status during gestation and early childhood has a protective association with late-life dementia risk.