Middle-aged people who are defined as underweight are a third more likely to develop dementia than people of similar age with a healthy body mass index, according to a UK study results.
The findings, which come from the largest ever study to examine statistical association between BMI and dementia risk, also found that middle-aged obese people are nearly 30% less likely to develop dementia than people of a healthy weight.
Researchers analysed information from a database recorded during routine general practice over nearly 20 years, representing around 9% of the UK population.
“It’s possible that further down the line, researchers might be able to use these insights to develop new treatments for dementia”
The researchers analysed the medical records of nearly two million people with a median age of 55 years at the start of the study period, and an average BMI of 26.5 kg/m2 – just within the range usually classed as overweight.
During an average of nine years follow-up, 45,507 people were diagnosed with dementia, according to the research published in The Lancet Diabetes & Endocrinology journal.
Those who were underweight in middle age were 34% more likely to be diagnosed with dementia than those of a healthy weight, and this increased risk of dementia persisted even 15 years after the underweight was recorded.
As participants’ BMI at middle age increased, the risk of dementia reduced, with very obese people 29% less likely to get dementia than people in the normal weight range.
An increase in BMI was associated with a substantial steadily decreasing risk of dementia for BMI of up to 25 kg/m² – classed as a healthy weight. Above a BMI of 25 kg/m² – classed as overweight or obese – dementia risk decreased more gradually, and this trend continued up to a BMI of 35 kg/m² or higher.
“This mixed picture highlights the difficulty of conducting studies into the complex lifestyle risk factors for dementia”
Adjusting for confounding factors known to increase the risk of dementia, such as alcohol use or smoking, made little difference to the results, said the researchers.
Study author Professor Stuart Pocock, from the London School of Hygiene and Tropical Medicine, said: “Our results suggest that doctors, public health scientists, and policy makers need to re-think how to best identify who is at high risk of dementia.
“If we can understand why people with a high BMI have a reduced risk of dementia, it’s possible that further down the line, researchers might be able to use these insights to develop new treatments for dementia,” he said.
“The reasons why a high BMI might be associated with a reduced risk of dementia aren’t clear, and further work is needed to understand why this might be the case,” added fellow study author Dr Nawab Qizilbash, from OXON Epidemiology. “Many different issues related to diet, exercise, frailty, genetic factors, and weight change could play a part.”
Dr Doug Brown, director of research and development at the Alzheimer’s Society, said: “Previous research has suggested that being overweight in midlife increases risk of developing the condition and yet this study suggests that it may actually be protective.
“This mixed picture highlights the difficulty of conducting studies into the complex lifestyle risk factors for dementia and reinforces the need for further research so we can identify the most important risk factors,” he said.