American football players are four times more likely than normal to die from Alzheimer's disease, new research has found.
Scientists in the US reviewed almost 3,500 retired NFL players to determine their risk of certain brain-related diseases.
They looked at the death certificates of 334 players from the sample who were no longer alive, which revealed seven had died from Alzheimer's disease, while seven had Amyotrophic lateral sclerosis (ALS).
This represented a four-fold increased risk of death from both illnesses
Dr Simon Ridley, head of research at Alzheimer's Research UK, said: "The numbers are small, but this study adds further support to the suggestion that head injury can be a risk factor for Alzheimer's and highlights the potential long-term damage posed by high impact sports.
"It would be useful to follow up these players for longer to help understand this link further.
"It is important to remember that these professional players received frequent and very high impact head trauma, more than just the odd knock on the head.
"Identifying the risk factors for Alzheimer's helps us to understand how we could prevent it.
"With over half a million people living with the disease in the UK, and these numbers rising, we desperately need more research to reveal how we can reduce our risk of this devastating disease."
Results from the study have been published in the journal Neurology online, the medical journal of the American Academy of Neurology.