THE final years of a football hero were spent in a fog far from glory days, with the seasons in the sun lost to a desperate disease.

Frank Kopel, 65, died yesterday after suffering from vascular dementia since 2009. The former Dundee United full-back played 400 times for the club from 1972 to 1982 after a brief career with Manchester United.

He will be mourned by United fans as a hero of two league cup triumphs and a powerful and influential player.

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His family, particularly his wife Amanda, will grieve a once strong, vibrant and good man.

"It is a cruel world," said his brother-in-law, Colin Grant. "He was a topper of a guy."

It is an individual and family tragedy. But it also casts a chill over the football world.

There is a mystery at the heart of a series of deaths of footballers that has spawned a library of research papers and sparked an outbreak of fact, speculation and supposition.

It is this: does playing football cause dementia? More specifically does heading a football pose a health risk?

Mrs Kopel, in an interview conducted last year, would not be drawn saying only: "He didn't smoke, he ate healthily, he drank socially, but not excessively. He swam twice a day. He was a very, very fit man."

Her campaign on behalf of her husband was to raise awareness of the strain on carers looking after people with early-onset dementia, calling for free personal care for dementia sufferers under 65.

However, Mr Kopel joins a list of celebrated players who have died after suffering from dementia, Alzheimer's or degenerative brain disease.

In 1998, former Celtic player Billy McPhail lost his legal case for disablement benefit over a claim that he developed the first stages of senile dementia as a result of heading the old- fashioned footballs.

In 2002 an inquest into the death of former West Bromwich Albion striker Jeff Astle ruled he died from a degenerative brain disease caused by the same actions.

Dr Willie Stewart, a consultant neuropathologist at the Southern General Hospital in Glasgow, has produced evidence that shows playing rugby can cause early onset dementia.

He said last night that the conclusive evidence of causal link between brain damage and football could only be found if children were divided into two groups, one heading a ball and the other group refraining, with the results monitored over a significant period of time.

''There is anxiety over the numbers of footballers suffering from dementia,'' he said, making it clear he could not comment directly on Mr Kopel.

Dr Stewart added: ''It is clear that repeated injury to the brain can cause permanent damage.''

He believes one way forward would be to measure the percentage of ex-professional footballers suffering from such conditions and measure this rate against the population as a whole. He proposes strict guidelines on the management of concussion in all sports from the ''top to the bottom''.

Kirsty Yanik, a spokeswoman for Alzheimer's Scotland, could not comment directly on Mr Kopel's death but pointed out there was increasing evidence that high impact sports produced a much increased risk of vascular dementia.

The condition is caused by problems in the supply of blood to the brain and many health professionals believe a ball travelling at up to 80mph can cause such a damaging impact.

One senior engineer has described such an impact as being the equivalent of being struck by 10 bags of coal in 3/100ths of a second.

Ms Yanik also said there had been a "significant" number of former players being diagnosed with dementia.

Fraser Wishart, chief executive of PFA Scotland, the players' union, said his organisation would welcome "definitive research" on the subject, stating that there were obvious concerns that had to be addressed.

The Kopel family, the supporters of Dundee United and all football fans of a certain age were in mourning yesterday. There may also be highly uncomfortable reflections in the wider health and sporting world.