A third of dementia cases could be prevented by making environmental and life-style changes starting in childhood, scientists have claimed.
The panel of 24 international experts identified a range of modifiable risk factors they believe to be responsible for around 35% of all instances of dementia, including Alzheimer's.
Different risk factors were said to make an impact at different stages in life, having an accumulating effect.
Better education in early life and addressing hearing loss, high blood pressure and obesity in mid-life could reduce the incidence of dementia by up to 20%, the research suggests.
In later life, stopping smoking, treating depression, increasing physical activity, managing diabetes and enhancing social contact could reduce dementia rates a further 15%, according to the findings.
Professor Lon Schneider, a member of the team from the University of Southern California in the US, said: "There's been a great deal of focus on developing medicines to prevent dementia, including Alzheimer's disease.
"But we can't lose sight of the real major advances we've already made in treating dementia, including preventive approaches.
"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.
"Mitigating risk factors provides us a powerful way to reduce the global burden of dementia."
The Lancet Commission on Dementia Prevention and Care brought the experts together to review a wealth of existing research and data and make evidence-based recommendations.
Their conclusions are published in The Lancet journal and were also presented at the Alzheimer's Association International Conference in London.
Around 47 million people have dementia worldwide. That number is expected to climb as high as 66 million by 2030 and 115 million by 2050.
In the UK an estimated 850,000 people are living with dementia, most of whom have Alzheimer's.
The Lancet commission also looked at the effectiveness of non-medical treatments for people with dementia.
The experts found that psychological and social interventions were better than anti-psychotic drugs for treating dementia-related agitation and aggression.
Some forms of non-medical therapy such as group cognitive stimulation and exercise led to improvements in mental ability.
Dr Doug Brown, director of research at the charity Alzheimer's Society, said: "The revelation that over a third of dementia cases worldwide are, in theory, entirely preventable is cause for celebration.
"But to achieve even close to this kind of reduction in cases we need to consider two important challenges - firstly how risk factors like education, obesity and depression apply not just at a population level, but to individual people who all have their own unique genetic risk profiles, and secondly how we can motivate people in mid to late life to change their behaviour and adopt healthier lifestyle choices.
"Not all of the nine risk factors identified are easily modifiable, factors like poor education and social isolation are incredibly challenging to address.
"But there are easier wins, particularly cardiovascular factors like lowering blood pressure and smoking cessation."
Dr David Reynolds, chief scientific officer at Alzheimer's Research UK, said: "This comprehensive report further underlines the potential impact of action to reduce dementia risk, and the importance of public health policies aimed at helping people reduce their risk of the condition.
"Research into the links between hearing loss and dementia is at an early stage and this review points to a need for further studies to establish the reasons behind this link.
"It's not yet clear from the available evidence whether treating hearing loss could help reduce the risk of dementia, and it will be important to see this explored in future research.
"The report recommends more vigorous treatment of high blood pressure, and we would welcome moves to ensure all those who could benefit from blood pressure medication do so.
"A healthy lifestyle is also important for keeping blood pressure under control, and strategies to help people adopt and stick to healthy habits must form part of our efforts to reduce dementia risk."
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."
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