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Links between BMI and dementia risk unravelled

People who have a high body mass index (BMI) are more likely to develop dementia than those with a normal weight, according to a University College London (UCL)-led study. The study analysed data from 1.3m adults living in the US and Europe. The researchers also found that people near dementia onset, who then go on to develop dementia, tend to have lower body weight than their dementia-free counterparts.

"The BMI-dementia association observed in longitudinal population studies, such as ours, is actually attributable to two processes," said lead author of the study, Professor Mika Kivimäki (UCL Institute of Epidemiology & Health). "One is an adverse effect of excess body fat on dementia risk. The other is weight loss due to pre-clinical dementia. For this reason, people who develop dementia may have a higher-than-average body mass index some 20 years before dementia onset, but close to overt dementia have a lower BMI than those who remain healthy."

"The new study confirms both the adverse effect of obesity as well as weight loss caused by metabolic changes during the pre-dementia stage."

Past research on how a person's weight influences their risk of dementia has produced conflicting results. Some findings have suggested that being obese poses a higher dementia risk, but other studies have linked lower weight to increased dementia incidence.

In this study, researchers from across Europe pooled individual-level data from 39 longitudinal population studies from the US, the UK, France, Sweden, and Finland. A total of 1,349,857 dementia-free adults participated in these studies and their weight and height were assessed. Dementia was ascertained using linkage to electronic health records obtained from hospitalisation, prescribed medication and death registries.

A total of 6,894 participants developed dementia during up to 38 years of follow-up. Two decades before symptomatic dementia, higher BMI predicted dementia occurrence: each 5-unit increase in BMI was associated with a 16-33% higher risk of this condition (5 BMI units is 14.5kg for a person 5'7" (170cm) tall, approximately the difference in weight between the overweight and normal weight categories or between the obese and overweight categories). In contrast, the mean level of BMI during pre-clinical stage close to dementia onset was lower compared to that in participants who remained healthy.

In 2015, the number of people with dementia reached almost 45m, two times more than in 1990. This study suggests that maintaining a healthy weight could prevent, or at least delay, dementia.

Abstract
Introduction: Higher midlife body mass index (BMI) is suggested to increase the risk of dementia, but weight loss during the preclinical dementia phase may mask such effects.
Methods: We examined this hypothesis in 1,349,857 dementia-free participants from 39 cohort studies. BMI was assessed at baseline. Dementia was ascertained at follow-up using linkage to electronic health records (N = 6894). We assumed BMI is little affected by preclinical dementia when assessed decades before dementia onset and much affected when assessed nearer diagnosis.
Results: Hazard ratios per 5-kg/m2 increase in BMI for dementia were 0.71 (95% confidence interval = 0.66–0.77), 0.94 (0.89–0.99), and 1.16 (1.05–1.27) when BMI was assessed 10 years, 10-20 years, and >20 years before dementia diagnosis.
Conclusions: The association between BMI and dementia is likely to be attributable to two different processes: a harmful effect of higher BMI, which is observable in long follow-up, and a reverse-causation effect that makes a higher BMI to appear protective when the follow-up is short.

Authors
Mika Kivimäki, Ritva Luukkonen, G David Batty, Jane E Ferrie, Jaana Pentti, Solja T Nyberg, Martin J Shipley, Lars Alfredsson, Eleonor I Fransson, Marcel Goldberg, Anders Knutsson, Markku Koskenvuo, Eeva Kuosma, Maria Nordin, Sakari B Suominen, Töres Theorell, Eero Vuoksimaa, Peter Westerholm, Hugo Westerlund, Marie Zins, Miia Kivipelto, Jussi Vahtera, Jaakko Kaprio, Archana Singh-Manoux, Markus Jokela

[link url="https://www.sciencedaily.com/releases/2017/11/171130133812.htm"]University College London material[/link]
[link url="http://www.alzheimersanddementia.com/article/S1552-5260(17)33811-6/fulltext"]Alzheimer’s & Dementia abstract[/link]

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