ArticlesBMI and risk of dementia in two million people over two decades: a retrospective cohort study
Introduction
Obesity in middle age might increase the risk of dementia at older ages,1, 2, 3 whereas obesity in old age could reduce the dementia risk.4 An understanding of the association of BMI (measured in kg/m2) with dementia is a public health priority because the number of people affected by dementia worldwide is expected to rise from 30 million in 2010, to 106 million in 2050,5 and the prevalence of obesity is also increasing worldwide.6 In England, the prevalence of obesity has almost doubled between 1993 and 2010,7 and the global burden of obesity in 2008 was estimated to be 1·46 billion overweight adults (BMI ≥25 kg/m2) and 502 million obese adults (BMI ≥30 kg/m2).8
However, the association between BMI and risk of dementia is far from clear. Several studies report that being overweight (BMI ≥25 kg/m2) in mid-life is associated with an increased risk of cognitive impairment and dementia in later life2, 9 whereas being overweight in later life might be associated with reduced dementia risk.10 This situation has been cited as another example of the so-called obesity paradox.10 Furthermore, low BMI (<20 kg/m2) is associated with an increased risk of dementia in short-term studies of elderly people10 and weight loss reportedly occurs before a diagnosis of dementia.4 Inconsistencies might arise because previous studies have been quite small with short durations of follow-up. We report our findings of the largest study so far of the association between BMI and risk of dementia.
Section snippets
Methods
We did a retrospective cohort study using routine UK primary care data from the Clinical Practice Research Datalink (CPRD). The CPRD contains patient information recorded during routine general practice, such as diagnoses, prescriptions, physiological measurements, diagnostic tests, lifestyle information, and referrals to secondary care. Data in the CPRD represent around 9% of the UK population. Data collection began in 1987 and we used data up to July, 2013, in our study.
We calculated BMI from
Results
Figure 1 shows the creation of the analysis dataset. Of 6 098 128 people in CPRD aged 40 years or older between 1992 and 2007, 2 944 587 had BMI data available. We excluded people who did not have at least 12 months of historical data, those judged to be outliers, and those with a previous history of dementia, leaving a total of 1 958 191 people (representing 18 786 640 person-years of follow-up) to include in our analysis. The median BMI was 26·4 kg/m2 (IQR 23·5–30·0), the median age at
Discussion
This cohort study in which nearly 2 million people were followed retrospectively for two decades showed an inverse monotonic association between BMI and the incidence of dementia. The association was not explained by age, sex, duration of follow-up, or available baseline covariates.
One of our key findings is that underweight people have a notably raised risk of developing dementia, and that this risk persists even 15 years after underweight is recorded. This finding might seem surprising, since
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