Retired NFL players run a greater risk of developing mild cognitive impairment (MCI), a form of dementia that can lead to Alzheimer's disease, than other men of the same age who did not play football, says a study presented Monday at the Alzheimer's Association International Conference in Paris.

"It appears there may be a very high rate of cognitive impairment in these retired football players, compared to the general population in that age range," according to lead researcher Christopher Randolph, a professor of neurology at Loyola University Chicago Stritch School of Medicine.

The study renews the controversy in the United States about the danger and possible consequences of the frequent blows to the head experienced by football players.

Researchers examined 513 retired NFL players and their wives, with an average age of 61, of whom 35 percent of the players showed results that suggested MCI.

Symptoms of MCI include memory and language problems that do not interfere with people's daily lives, but can be warning signs of Alzheimer's.

Research carried out by placing accelerometers in players' helmets have found that in the course of a season, the average college football player receives more than 1,000 blows to the head of a magnitude greater than 10 g-force, 1 g-force being the normal acceleration due to gravity.

More than a quarter of those impacts are greater than 30 g-force.

The discoveries of Randolph and his team suggest that repeated head traumas after years of playing this sport can lead to irreparable damage to the brain and so lead to an early appearance of degenerative illnesses like MCI and Alzheimer's.

Randolph was cautious, however, and said that "these studies should be considered very preliminary."