Contrary to current health advice, being overweight cuts the risk of dementia – according to an analysis of nearly two million British people in the Lancet Diabetes & Endocrinology.

Dementia, for which there is no cure or treatment, is expected to affect 135 million people globally by 2015 – treble the current number of patients.

Researchers 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 results show that the current advice of maintaining a healthy lifestyle may be misguided when it comes to preventing dementia.

Underweight people had a 39 per cent greater risk of dementia compared with being a healthy weight, but those who were overweight had a reduced risk percentage of 18 per cent. Obese people had a 24 per cent risk of developing dementia, according to the results.

“[Our data] is 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,” lead researcher Dr Nawab Qizilbash told the BBC.

There is no real explanation for the data at this stage and Qizilbash hastened to add: “You can’t walk away and think it’s OK to be overweight or obese. Even if there is a protective effect, you may not live long enough to reap the benefits.”

Heart disease, stroke, diabetes, some cancers and other diseases are all linked to a bigger waistline.

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