One third of dementia cases could be prevented with some basic lifestyle changes and better education during childhood, researchers have found.
The nine factors which damage the brain notably include hearing loss, obesity and smoking, according to the study published in The Lancet on Thursday.
Nearly 50 million people around the world suffer from dementia and Alzheimer's, according to the latest estimates.
This figure could balloon to 132 million by 2050.
Dementia, which is caused by physical changes in the brain, leads to memory loss and hampers other mental abilities.
"Our results suggest that around 35 percent of dementia is attributable to a combination of the following nine risk factors: education to a maximum of age 11-12 years, mid-life hypertension, mid-life obesity, hearing loss, late-life depression, diabetes, physical inactivity, smoking, and social isolation," the study said.
Researchers found if people stayed in school until the age of 15, the benefits of education and socialisation would help reduce the cases of dementia by eight percent.
"Although dementia is diagnosed in later life, the brain changes usually begin to develop years before," said lead author Professor Gill Livingston, from University College London.
"Acting now will vastly improve life for people with dementia and their families and, in doing so, will transform the future of society."
The researchers said maintaining good hearing between the ages of 45 and 65 reduces the number of cases by nine percent.
Quitting smoking could reduce the number of cases by five percent, it said.
Other factors contributing to the risk include depression (four percent), physical inactivity (three percent), social isolation (two percent), high blood pressure (two percent), obesity (one percent) and type 2 diabetes (one percent).
The study said the global cost of dementia in 2015 was estimated to be $818bn, and that this figure would continue to rise.
It said nearly 85 percent of these costs were "related to family and social, rather than medical, care".
The researchers noted, however, that the study was limited.
"We have not incorporated other potential risk factors, such as diet, alcohol, living near major roads, or sleep, which could be relevant," it said. "Therefore, the potentially preventable fraction of dementia might be underestimated in our figures."
Source: News agencies