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A simple blood test could help predict someone’s risk of Alzheimer’s disease in their forties – before any symptoms appear.

Scientists have discovered a warning sign for the condition that appears decades before any memory loss.

This marker is extreme inflammation – an overreaction of the immune system often caused in middle age by weight gain, high blood pressure or diabetes.

For the study, led by Johns Hopkins University in the US, more than 1 600 participants aged 45 to 65 had blood tests for five signs of inflammation.

They then had brain scans and memory tests an average of 24 years later. Those who tested positive for three or more signs of inflammation had indications of Alzheimer’s disease later in their lives. They also performed less well in memory tests.

The study, published in the journal Neurology, offers hopes of an early test for Alzheimer’s, which affects 850 000 people in the UK.

Lead author Dr Keenan Walker said: ‘These results suggest that inflammation in mid-life may be an early contributor to the brain changes that are associated with Alzheimer’s disease and other forms of dementia.’

Inflammation is thought to cause Alzheimer’s because it changes the structure of brain cells and kills them off.