For most people the biggest risk factors for dementia are ageing and genes. But according to Dr Richard Restak, a clinical professor of neurology at George Washington University, risk factors for the condition may be noticeable much earlier than people think.

In his recently published book How To Prevent Dementia, Dr Restak said: “Experiencing the onset of dementia isn’t like falling down a flight of stairs: unpredictable, sudden, and with maximal damage in close approximation to the inciting cause.”

Instead the disease can be “gradual”. He continued: “It’s more like a slow walk in a swimming pool, starting from the shallow end and moving towards the deep end.”

A study revealed how early people may be able to spot the signs of dementia. Commonly known as “the nun study”, the research, published in 1968 by epidemiologist David Snowdon, involved 678 nuns.

One of the things Snowdon found, Dr Restak said, is how nuns described their prior jobs and lives in their autobiographical essay applications to join convents – often written when they were in their twenties – somewhat predicated instances of dementia once the nuns got older.

Snowdon found nuns who would later get dementia expressed fewer ideas in their sentences when younger than those who didn’t.

Dr Restak explained: “The best functioning nuns differed from their counterparts who had succumbed to dementia by what he termed cognitive density: many thoughts and ideas woven into few sentences and paragraphs.”

One 93-year-old nun wrote 70 years before: “After I finished the eighth grade in 1921, I desired to become an aspirant at Mankato [a convent], but I myself did not have the courage to ask permission of my parents. So Sister Egreda did in my stead.”

Then had “just finished writing a biography and engaged regularly in knitting, crocheting, card playing, and daily walking”.

Another nun, also in her 90s but showing signs of dementia wrote in her early twenties: “After I left school, I worked in the post office.”

Dr Restak said: “The first nun presents her vocation as marked by complexity, ambivalence, and perhaps even some unwillingness. She could not bring herself to mention her vocational wishes to her parents.

“The second nun, in contrast, leads with only a plain sentence concerning where she worked before entering the convent.“

The way you describe your job isn’t the only way this can appear.

But “the Nun Study adds an additional reason to believe that Alzheimer’s disease starts many years before it’s first identified by physicians and family,” added Dr Restak.