'Over 95% of people who developed dementia in this study hadn't had a brain injury, so the study does not tell us that traumatic brain injury is a definite cause of dementia.
Professor Tara Spires-Jones, from the University of Edinburgh, said the study "strongly supports the conclusion that TBI is associated with increased risk of dementia".
Researchers, led by Jesse Fann, a professor of psychiatry and behavioral sciences at UW School of Medicine, found people with any traumatic brain injury were 1.5 times more likely than their non-injured peers to develop dementia when controlling for demographic factors like age and gender. But the risk increased significantly for people with multiple brain injuries, and for people who were in their 20s at the time for their first brain injury.
A single severe traumatic brain injury raised the chances of developing dementia by more than a third.
Danish and USA researchers looked at nearly 2.8 million people over a 36-year period - 1977 to 2013.
Each year, more than 50 million people worldwide suffer a traumatic brain injury, which occurs when a bump or blow to the head disrupts normal brain function.
A traumatic brain injury can be caused by a fall, a traffic accident, a sports accident or a violent attack.
Fann said it's important to recognize that most people who sustain a single concussion do not develop dementia. TBI correlated with elevated dementia risk compared to individuals with a non-TBI fracture not involving the skull or spine (hazard ratio, 1.29).
Dr Carol Routledge, director of research at Alzheimer's Research UK, added that a healthy diet, drinking in moderation and not smoking were all things that can help maintain a healthy brain as we age. Those affected should avoid certain behaviors, researchers suggest.
The study included 36 years of follow-up, as well as access to a uniform healthcare system that tracks the number and severity of TBIs.
Those who suffered a TBI in their 30s were 37% more likely to develop dementia later in life, while those who had the injury in their 50s were only 2% more likely to get the condition. For their first T.B.I. diagnosis, 85 percent were this mild type.
There are about 10 million new dementia cases each year.
In total, 5.3 per cent of participants with dementia had a history of TBI compared with 4.7 per cent of those without the condition. Between 1999 and 2013, 126,734 people (4.5%) aged 50 or older were diagnosed with dementia.
Men with TBI histories developed dementia at a 30 percent rate, while women had just a 19 percent rate.
According to Professor Fann, "Shedding light on risk factors for dementia is one of the most important tasks in health research". He warns parents and children to be well-aware of risks of TBIs in contact sports.
That means that it's hard to apply these findings directly to many sports injuries, said Michael Hutchison, director of the concussion program at the David L. MacIntosh Sport Medicine Clinic at the University of Toronto. "If they have a history of traumatic brain injury, they should do their best to prevent further traumatic brain injuries".