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Dementia and obesity

Study indicates midlife obesity protects against dementia

People who were underweight in middle age were a third (34%) more likely to be diagnosed with dementia than those of a healthy weight

Middle-aged people who are underweight (BMI<20) are a third more likely to develop dementia than people of similar age with a healthy BMI, according to research published in The Lancet Diabetes & Endocrinology journal. The findings, which come from the largest ever study to examine the statistical association between BMI and dementia risk, also show that middle-aged obese people (BMI>30) are nearly 30% less likely to develop dementia than people of a healthy weight, contradicting findings from some previous research, which suggested that obesity leads to an increased risk of dementia.

"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,” said study author Professor Stuart Pocock from the London School of Hygiene & Tropical Medicine. “We also need to pay attention to the causes and public health consequences of the link between underweight and increased dementia risk which our research has established. However, our results also open up an intriguing new avenue in the search for protective factors for 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."

Researchers based at the London School of Hygiene & Tropical Medicine, and OXON Epidemiology, both in London, UK, analysed data from the Clinical Practice Research Datalink (CPRD), a large database of patient information recorded during routine general practice over nearly 20 years, representing around 9% of the UK population.

The researchers analysed the medical records of nearly two million (1,958,191) people with an average (median) age of 55 years at the start of the study period, and an average BMI26.5, just within the range usually classed as overweight. During an average of nine years follow-up, nearly fifty thousand (45,507) people were diagnosed with dementia.

People who were underweight in middle age were a third (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 (BMI>40) 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 BMI25. Above a BMI>25, dementia risk decreased more gradually, and this trend continued up to a BMI>35.

The association between BMI and dementia risk wasn't affected by the decade in which the participants were born, nor by their age at diagnosis. Adjusting for confounding factors known to increase the risk of dementia, such as alcohol use or smoking, made little difference to the results.

"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," said Dr Nawab Qizilbash from OXON Epidemiology in London, UK and Madrid, Spain, the study's lead author. "If increased weight in mid-life is protective against dementia, the reasons for this inverse association are unclear at present. Many different issues related to diet, exercise, frailty, genetic factors, and weight change could play a part."

Further research was needed to confirm the link and find an explanation for it, the researchers added.

“We don't yet know enough about the link between body weight and dementia. 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,” said Dr Doug Brown, Director of Research and Development at Alzheimer's Society. “This study of almost two million people also reports that being underweight in later years could increase risk of developing the condition by a third. 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. While the evidence on body weight and dementia is unclear, we know that people can make positive lifestyle choices to keep their brains healthy by taking regular exercise, not smoking and following a healthy balanced diet.”

Alzheimer's Disease International (ADI) projects the number of people with dementia will rise from 35.6 million in 2010 to 65.7 million by 2030 and 115.4 million by 2050.

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