Preventing diabetes and depression could have a dramatic impact on cutting cases of dementia, a study suggests.
Boosting levels of education and upping fruit and vegetable consumption would also have a big effect, the British Medical Journal said.
It comes as another study showed dementia patients are missing out on vital early treatments because GPs are being slow to diagnose them.
It is estimated that one million people in the UK will have dementia by 2025.
Several risk factors for the disease have been identified, including obesity, high blood pressure and high cholesterol.
But British and French researchers wanted to assess what public health interventions could have the biggest impact on reducing the burden of dementia in the population.
They took a group of 1,400 elderly people and tested them for signs of dementia after two, four and seven years.
Alongside this they recorded height, weight, education level, monthly income, mobility, dietary habits, alcohol consumption, and tobacco use and asked participants to do a reading test as a measure of intelligence.
Eliminating depression and diabetes and increasing fruit and vegetable consumption were estimated to lead to an overall 21% reduction in new cases of dementia
Increasing education would also lead to an estimated 18% reduction in new cases of dementia across the general population over the next seven years, they reported.
By contrast, removing a gene linked with the disease would only cut new cases by 7%.
The team concluded that early screening for diabetes and treatment of depression would be the most useful approach for trying to reduce the future burden of dementia.
And they added that encouraging literacy at all ages and trying to increase population intake of fruit and vegetables would also have an important effect but admitted that these aims were harder to achieve.
Further studies including younger adults are needed to test the impact of such approaches, they added.
In the second study also in the BMJ, analysis of health records of over 135,000 people in the UK found that people with dementia were three times more likely to die in the first year after diagnosis than those without the condition.
That suggests that diagnoses are being made in the later stages of the disease.
Study leader Dr Greta Rait from the Medical Research Council said: "GPs are going to be dealing with more and more dementia cases in future and primary care must get better at detection."
Rebecca Wood, chief executive of the Alzheimer's Research Trust, said the links between depression, diabetes and dementia were well known.
"Any policy that urges clear diagnosis and monitoring of these conditions could help make an impact on dementia.
"What is painfully evident from the study is the gaping hole that remains in our understanding and ability to diagnose or treat dementia effectively, a hole that can only be filled by more research."
Professor Clive Ballard, director of research at the Alzheimer's Society said a healthy lifestyle is key.
"Effective prevention of diabetes, depression and heart disease could potentially improve the lives of millions of people affected by this cruel condition and reduce the billions spent on dementia care each year."
Dr Victoria King, head of research at Diabetes UK, said there is a growing body of evidence suggesting links between diabetes and Alzheimer's disease.
"Finding ways to stop the type 2 diabetes epidemic in its tracks can only be seen as a good thing - especially as this could prevent millions of people developing the serious complications of the condition, which include heart disease, stroke, kidney failure, blindness and amputation."