According to recent research, having a higher BMI (Body Mass Index) may increase the risk of developing dementia later in life.

The study, published in the Alzheimer’s & Dementia Journal in November 2017, analyzed data from 1,349,857 dementia-free adults living in the United States, United Kingdom, France, Sweden and Finland and determined that those with a higher BMI at midlife were more likely to develop dementia as seniors. The study also revealed that the opposite was true for those who were already in the pre-clinical stage and closer to dementia, where having a lower BMI was more prevalent.

“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, of the University College of London 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 higher-than-average body mass index some 20 years before dementia onset, but [those] close to overt dementia have a lower BMI than those who remain healthy.”

Researchers looked at the data of patients with no history of dementia and followed up on their medical status anywhere between 4.3 to 37.7 years, with a weighted mean follow-up at 16.1 years. Using medical records from national death and hospital admission registries, reimbursements for medical treatment of dementia and surveys on physician-diagnosed diseases, they determined that 6,894 participants developed dementia during this time.

They concluded that having a higher BMI was associated with an increased risk of dementia when weight was measured >20 years before a dementia diagnosis. The risk increased 16-33% for each five-unit rise in BMI, which equates to approximately 32 pounds in a 5-foot 7-inch person.

If weight was assessed <10 years before dementia diagnosis, the association between BMI and dementia was reversed.

The study concluded, “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.”

The authors put forward their results as an explanation for inconsistencies in some prior studies on BMI and dementia. They caution that “Further research is needed to examine underlying mechanisms for weight loss during the preclinical stage, including cognitive impairment leading to impaired self-care, reduced appetite due to decreased olfactory perception or changes in the regulation of satiety, and disturbed energy homeostasis. ”

Future studies should also examine the relationship between BMI and specific dementia related diseases, such as Alzheimer’s, vascular dementia, frontotemporal dementia, and Lewy body dementia.

According to the Alzheimer Society of Canada, an estimated 564,000 Canadians are living with dementia and it’s expected that by 2031, numbers will increase by 66% to 937,000. Health care costs associated with dementia are estimated to be over $10 billion each year.

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