ReviewDiabetes and other vascular risk factors for dementia: Which factor matters most? A systematic review
Introduction
Type 2 diabetes mellitus is a well-known risk factor for cardiovascular disease. Diabetes is also associated with cognitive decline and an increased risk of dementia in the elderly (Awad et al., 2004, Biessels et al., 2006). Type 2 diabetes develops in the context of insulin resistance and is often accompanied by other vascular risk factors such as hypertension, dyslipidaemia and obesity. The co-occurrence of these risk factors is generally referred to as ‘the metabolic syndrome’ (Reaven, 1988). There is evidence that each of the individual risk factors of the metabolic syndrome is associated with cognitive decline and dementia. As these risk factors are potential targets for therapeutic intervention it is important to examine the contribution of each of these risk factors to the risk of dementia. In this context it is also important to assess whether the effects of the individual risk factors are independent from each other.
Several studies suggest that the association between dementia and vascular risk factors varies with age. High blood pressure in middle-age, for example, is a clear risk factor for dementia in old age (Whitmer et al., 2005b). In old age, however, low blood pressure may be associated with an increased risk of dementia (Verghese et al., 2003).
In this paper we systematically reviewed studies on the risk of dementia associated with type 2 diabetes mellitus, hypertension, obesity and dyslipidaemia. We aimed to quantify and compare the association with dementia for these four risk factors, and to address the potential modifying role of age at the time of exposure.
Section snippets
Identification of studies
This systematic review aimed to include all published studies that provide an estimate of the incidence of dementia associated with type 2 diabetes mellitus or impaired glucose metabolism, hypertension, dyslipidaemia or obesity and that met the following inclusion criteria: (1) the study population was recruited at the population level; (2) the study had a longitudinal design; (3) the results were adjusted for the basic confounders age, sex as well as educational level; and (4) adjusted odds
Type 2 diabetes mellitus/impaired glucose metabolism
We included 14 studies that compared the risk of one or more types of dementia in patients with type 2 diabetes mellitus to non-diabetic persons (Table 1a). Diabetes was most commonly identified by fasting or random blood glucose levels combined with an oral glucose tolerance test. Six studies based the diagnosis of diabetes on medical history or medication use only, or did not assess blood glucose concentration in all participants (Yoshitake et al., 1995, MacKnight et al., 2002, Arvanitakis et
Discussion
This paper systematically reviewed the evidence for the association of type 2 diabetes mellitus, hypertension, dyslipidaemia and obesity with dementia. All four risk factors were associated with an increased risk of dementia, but the results of studies on diabetes and obesity were most consistent. The magnitude of the effects was comparable across the risk factors, with odds ratios for ‘any dementia’ around 1.5. For hypertension, obesity and dyslipidaemia age appeared to modulate the
Acknowledgments
This study was supported by grant 2003.01.004 from the Dutch Diabetes Research Foundation and grant 907-00-140 from the Netherlands Organisation of Scientific Research (ZonMw).
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