New scientific research shows that being obese in middle and old age can reduce the risk of developing dementia.
The findings, published in the British medical journal, The Lancet, directly contradict previous research on the topic.
Those who were overweight had an 18 percent reduction in dementia, researchers found.
The analysis of nearly two million British people, in the Lancet Diabetes & Endocrinology, showed underweight people had the highest risk.
Dementia charities still advised not smoking, exercise and a balanced diet.
Dementia is one of the most pressing modern health issues. The number of patients globally is expected to treble to 135 million by 2050.
There is no cure or treatment, and the mainstay of advice has been to reduce risk by maintaining a healthy lifestyle. Yet that advice could be misguided.
The team at Oxon Epidemiology and the London School of Hygiene and Tropical Medicine analysed medical records from 1,958,191 people aged 55, on average, for up to two decades.
Their most conservative analysis showed underweight people had a 39 percent greater risk of dementia compared with being a healthy weight.
But those who were overweight had an 18 percent reduction in dementia - and the figure was 24 percent for the obese.
The researchers admit they were surprised by the findings.
"The controversial side is the observation that overweight and obese people have a lower risk of dementia than people with a normal, healthy body mass index," said lead researcher Dr Nawab Qizilbash.
"That's contrary to most, if not all, studies that have been done, but if you collect them all together our study overwhelms them in terms of size and precision."