Previous studies have shown the correlation between cardiovascular fitness and the risk of dementia in old age.
To prove their point, researchers studied data from 1.1 million young Swedish men.
The study shows that men who had poorer cardiovascular fitness were 2.5 times more likely to develop early-onset dementia later in life.
A lower IQ entailed a four times greater risk, and a combination of both poor cardiovascular fitness and low IQ entailed a seven times greater risk of early-onset dementia.
The increased risk remained even when controlled for other risk factors, such as heredity, medical history and social-economic circumstances.
"We already knew that physical and cognitive exercise reduces the risk of neurological disease. In other words, good cardiovascular fitness makes the brain more resistant to damage and disease," explained senior author professor Georg Kuhn.
People who develop early-onset dementia are often of working age and can have children still living at home, which means the consequences for both the sufferers and their families are even more serious.
"Exercising can be used both as a prophylactic and a treatment for those in the risk zone for early-onset dementia," Nyberg noted in the article published in the scientific journal Brain.