It lists nine key risk factors including lack of education, hearing loss, smoking and physical inactivity.
The study is being presented at the Alzheimer's Association International Conference in London.
By 2050, 131 million people could be living with dementia globally.
There are estimated to be 47 million people with the condition at the moment.
"Although dementia is diagnosed in later life, the brain changes usually begin to develop years before," said lead author Prof 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 report, which combines the work of 24 international experts, says lifestyle factors can play a major role in increasing or reducing an individual's dementia risk.
How potentially modifiable factors contribute to the risk of dementia
- mid-life hearing loss - 9%
- failing to complete secondary education - 8%
- smoking - 5%
- depression - 4%
- physical inactivity - 3%
- social isolation - 2%
- high blood pressure - 2%
- obesity - 1%
- type 2 diabetes - 1%
These - which are thought to be modifiable risk factors - add up to 35%. The other 65% of dementia risk is beyond the individual's control.
It examines the benefits of building a "cognitive reserve", which means strengthening the brain's networks so it can continue to function in later life despite damage.
Failure to complete secondary education was a major risk factor, and the authors suggest that individuals who continue to learn throughout life are likely to build additional brain reserves.
Another major risk factor is hearing loss in middle age - the researchers say this can deny people a cognitively rich environment and lead to social isolation and depression, which are among other modifiable risk factors for dementia.
Another key message from the report is that what is good for the heart is good for the brain.
Not smoking, doing exercise, keeping a healthy weight, treating high blood pressure and diabetes can all reduce the risk of dementia, as well as cardiovascular disease, and cancer.
The researchers say they did not have enough data to include dietary factors or alcohol in their calculations but believe both could be important.
Dr Doug Brown, director of research at Alzheimer's Society, said: "Though it's not inevitable, dementia is currently set to be the 21st Century's biggest killer. We all need to be aware of the risks and start making positive lifestyle changes."
Dr David Reynolds, chief scientific officer at Alzheimer's Research UK, said: "Alongside prevention research, we must continue to invest in research to find a life-changing treatment for people with this devastating condition."
Nine lifestyle changes can reduce dementia risk, study says
20 July 2017
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One in three cases of dementia could be prevented if more people looked after their brain health throughout life, according to an international study in the Lancet.
