Researchers said physical exercise helps in better functioning of brain and also produces new nerve cells in adult brain. All these protect the brain from damage and diseases.

According to the study, the chance of suffering from dementia with poorer cardiovascular fitness later in life increases by 2.5 times. And for people with lower IQ, the chance of getting dementia early in life is four times more. And when both these problems combine the risk of getting early dementia is seven times more in their later life.

"Previous studies have shown the correlation between cardiovascular fitness and the risk of dementia in old age," said Jenny Nyberg, PhD, a researcher at the Sahlgrenska Academy of Gothenburg University, who headed the study.

Nyberg added, "This makes it important to initiate more research into how physical and mental exercise can affect the prevalence of different types of dementia."

The researchers analysed details of over 1.1 million 18-year-old Swedish male conscripts, who took conscription exams between 1968 and 2005. They were followed for up to 42 years.

The study is published in the scientific journal Brain.