Obesity could protect against dementia: study

Middle-aged obese people have nearly 30 per cent lower risk of developing dementia than people of a healthy weight, a surprising new study has claimed.
Middle-aged people who are underweight (with a Body Mass Index [BMI] less than 20 kilogramme per square meter (kg/m2) are a third more likely to develop dementia than people of similar age with a healthy BMI, researchers said.
The findings, from the largest ever study to examine the statistical association between BMI and dementia risk, also show that middle-aged obese people (BMI greater than 30 kg/m2) are nearly 30 per cent 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.

Researchers based at the London School of Hygiene & Tropical Medicine, and OXON Epidemiology 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 per cent of the UK population.
They 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 (median) BMI of 26.5 kg/m2, 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 per cent) 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 greater than 40 kg/m2) 29 per cent 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.
The association between BMI and dementia risk was not affected by the decade in which the participants were born, nor by their age at diagnosis, researchers said.
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 study lead author Dr Nawab Qizilbash from OXON Epidemiology.

The study was published in The Lancet Diabetes & Endocrinology journal.