Body mass index and the risk of dementia – what do we know and what should we do?

Abstract

Summary'Being overweight can cut risk of dementia'. The news that a health status usually associated with poor outcomes and stigma may actually be protective for one of the most feared diseases of our time was enthusiastically greeted by the media and many individuals. The reports focused on this aspect -that risk of dementia fell with increasing body mass index (BMI) -although the authors of the study focused on the findings that being underweight in middle and old age carries an increased risk of dementia. Previous findings in this area have been contradictory, and appropriate interpretation is vital in view of the public health implications if the message that obesity is protective against dementia were to be accepted.The study was a large retrospective cohort study using routinely collected UK primary care data, from the Clinical Practice Research Datalink (CPRD) database. Individuals' records were accessed if they were aged >=40 years between 1992 and 2007. Data from 1,958,191 participants were used, comprising just 32% of those in the CPRD database of the appropriate age group. The population characteristics are not presented in the paper. BMI and incident dementia data were collected between January 1992 and December 2013. First recorded BMI was used and no subsequent changes in BMI were included. Median follow-up time was 9.1 years, although the conclusions are presented as data over two decades.During the period of follow-up 45,507 people (2% of total sample) developed incident dementia. Coding of dementia was based on the use of the terms dementia, Alzheimer, Lewy body disease, Pick's disease, being used in the individual record or on death certificate.The underweight group (BMI < 20kg/m 2 ) n = 101,754, 5% of the total sample, had a 34% excess risk of dementia compared to those of a healthy weight. Incidence of dementia fell as BMI category increased adjustment for potential confounders -smoking status, alcohol use, statin use, antihypertensive use, diabetes and previous myocardial infarction -did not affect the association. CliniCal OpiniOnThe paper is interesting in its use of a large sample, and routinely collected data, to investigate an important health topic, but some methodological issues, in particular the incomplete data availability, merit caution in the interpretation of the findings.As an observational study, the results can only describe an association; causation cannot be attributed. The authors acknowledge that the association between weight and risk is complex. Flegal et al., 1 in a systematic review and meta-analysis, also identified that being overweight is associated with lower mortality than being of normal weight; however the relationship was not seen in those who were obese. On the other hand, other studies report that overweight is not associated with lower dementia risk. Kivipelto et al.2 focused on the impact of vascular risk factors in middle age and were able to adjust for sociodemographic variables not included in the current study. They found increased odds (odds ratio 2.4...

The relationship between obesity and cognitive health and decline

“…A recent retrospective cohort study of almost two million individuals aged over 40 years in the UK, reported that being underweight in middle age and old age carries an increased risk of dementia ( 109 ) . This assertion is controversial and, in contrast to the evidence of an association between obesity and dementia, may reflect the tendency to underdiagnose dementia by general practitioners at the time the data were collected, and over or under adjustment for a number of factors such as competing risk of mortality as well as selection bias, and bias in the diagnosis of dementia in those with lower BMI/age ( 111 ) .…”

Inflammatory Pathways in Overweight and Obese Persons as a Potential Mechanism for Cognitive Impairment and Earlier Onset Alzeihmer’s Dementia in the General Population: A Narrative Review

“…Possible reasons for the association include increased glucose metabolism due to increased leg muscle mass. This may prevent the pathogenic effects of increased glucose availability due to glucose uptake into muscle [136][137][138][139].…”