Video report by ITV News Reporter Joanna Partridge
Dementia rates could be slashed by a third by taking steps to make lifestyle changes starting from early life, scientists have concluded.
Over a third of cases (35%) of the degenerative condition - including Alzheimer's disease - are likely to have been caused by environmental factors, a panel of 24 experts found.
It identified nine major risks, some of which began from childhood and together had an accumulating effect over a lifetime, in a major new research review published by The Lancet.
It found cases could be cut by 20% through action to improve education in early years, plus more proactive treatment of hearing loss, high blood pressure and obesity in mid-life.
Another 15% of diagnoses could be prevented by supporting people to quit smoking, treating depression, increasing physical activity, managing diabetes and enhancing social contact, according to the report.
Experts have welcomed the findings - but warned that making the changes outlined in the report requires major cultural shifts that cannot always be undertaken by individuals alone.
The latest study offers hope of cutting cases of degenerative brain conditions that are often highly distressing to patients and often require intensive treatment in the later stages.
In the UK, around 850,000 people are living with dementia, most of whom have Alzheimer's.
And the number of people with dementia is on the rise.
The latest study also found that social and psychological treatment can be more helpful than anti-psychotic medication for those who suffer from dementia-related aggression or agitation.
Some forms of non-medical therapy such as group cognitive stimulation and exercise led to improvements in mental ability.
The conclusions were created by The Lancet Commission on Dementia Prevention and Care - a panel of experts who together review existing research and data.
Professor Lon Schneider, a member of the team from the University of Southern California in the US, said reducing risk could be a "powerful" tool in dealing with the diseases.
"The potential magnitude of the effect on dementia of reducing these risk factors is larger than we could ever imagine the effect that current, experimental medications could have," he said.
Dr Doug Brown, director of research at the charity Alzheimer's Society, said the news that many cases of dementia could be preventable is "cause for celebration" but reducing risk was not always simple.
Dr David Reynolds, chief scientific officer at Alzheimer's Research UK, said the link between hearing loss and dementia was an area that should be researched further.
"Strategies to help people adopt and stick to healthy habits must form part of our efforts to reduce dementia risk," he said.
He added: "While this report rightly highlights measures we can take to reduce our risk of dementia, it also serves as a reminder that even if every risk factor identified here could be eliminated, we do not yet have a surefire way to prevent dementia.
"Alongside prevention research, we must continue to invest in research to find a life-changing treatment for people with this devastating condition."