For the study, researchers measured the waists of 6,583 people aged 40-45 years.
Over an average 36 years' follow-up, 16 per cent of participants were diagnosed with dementia.
Those with a large waist were 2.3 times more likely to develop the condition than people with a normal belly size.
Those who were obese and had a large waist were 3.6 times more likely to develop dementia than those of normal weight and waist size.
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