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Former Scottish professional soccer players had more than three times the risk of death from neurodegenerative diseases relative to the general population, the retrospective FIELD study showed.

Mortality with neurodegenerative disease listed as the primary cause was 1.7% in 7,700 male former pro athletes versus 0.5% in matched controls, reported Willie Stewart, MB, ChB, PhD, of the University of Glasgow, and co-authors, in the .

"This is the largest study to date looking in this detail at the incidence of neurodegenerative disease in any sport, not just professional soccer players," Stewart said in a statement. "A strength of our study design is that we could look in detail at rates of different neurodegenerative disease subtypes."

Mortality risks ranged from a 5-fold increase in Alzheimer's disease and a 4-fold increase in motor neuron disease (amyotrophic lateral sclerosis, or ALS) to doubled rates of death from Parkinson's disease, he noted.

While former pro players had higher dementia rates in this study, they had lower death rates overall. "As such, whilst every effort must be made to identify the factors contributing to the increased risk of neurodegenerative disease to allow this risk to be reduced, there are also wider potential health benefits of playing soccer to be considered," Stewart said.

These results are similar to those reported in a study of former National Football League (NFL) players that showed all-cause mortality was lower in former NFL players than in the general population, but neurodegenerative mortality was higher, observed Robert Stern, PhD, of Boston University, in an .

"Participation in elite-level sports, not surprisingly, is also associated with reductions in all-cause mortality and the risk of cardiovascular disease," but contact and collision sports have been linked to cognitive and neuropsychiatric impairment later in life and neurodegenerative disease and chronic traumatic encephalopathy (CTE) from repetitive brain trauma, he noted.

"Another study involving former NFL players included a comparison group of former Major League Baseball players and found that all-cause mortality, cardiovascular mortality, and neurodegenerative mortality were higher among the former NFL players," Stern added. "These results suggest that factors that vary in these two sports, such as body habitus and exposure to repetitive head impacts, rather than common factors such as overall physical activity could be responsible for the differences."

It seems it's "not just the 'big hits' resulting in symptomatic concussions that increase the risk of neurologic disorders later in life," he wrote. "Rather, the total duration of exposure to repetitive head impacts, including 'sub-concussive' injuries without symptoms, has been associated with neuropathology, in vivo markers of neurodegeneration, and cognitive and neuropsychiatric symptoms later in life."

In their study, Stewart and colleagues compared mortality from neurodegenerative disease in 7,676 former pro Scottish soccer players born before 1977 against 23,028 controls from the general population matched on sex, age, and degree of social deprivation. Causes of death were determined from death certificates. The researchers also looked at medications dispensed for treating dementia from national prescribing data. All former soccer players in the study were male.

Over a median of 18 years, 1,180 former soccer players and 3,807 controls died. Mean age of death was about 68 among former players and 65 among controls.

Compared with controls, all-cause mortality was lower in former players up to age 70, but higher after that. Former pro athletes had lower death rates from ischemic heart disease (HR 0.80, 95% CI 0.66-0.97; P=0.02) and from lung cancer (HR 0.53, 95% CI 0.40 to 0.70; P<0.001).

The greater neurodegenerative disease mortality among former athletes was partly reduced after adjusting for competing risks of death from ischemic heart disease and any cancer and resulted in a sub-hazard ratio of 3.45 (95% CI 2.11-5.62; P<0.001).

Mortality with neurodegenerative disease listed as a primary or contributory cause was highest in former players with Alzheimer's disease (HR 5.07, 95% CI 2.92-8.82; P<0.001) and lowest in those with Parkinson's disease (HR 2.15, 95% CI 1.17-3.96; P=0.01), compared with controls. Dementia-related medications also were prescribed more frequently to former players than to controls (OR 4.90, 95% CI, 3.81-6.31; P<0.001).

This study had several limitations, the researchers noted. Death certificates may contain reporting errors, and this analysis could not include CTE or dementia pugilistica as a possible cause of death. The study could not determine what caused increased neurodegenerative disease. The findings also cannot be applied directly to collegiate, recreational, and amateur players, they added.

Research about the neurologic consequences of soccer heading is needed, including studies of female former pro players and amateur players of both sexes, as well as prospective longitudinal studies to confirm or refute this research, Stern said. But "perhaps, however, there is already adequate evidence that repeated blows to the brain from heading in professional soccer is an occupational risk that needs to be addressed," he wrote.

Britain's Football Association, which helped fund the research, to examine potential causes of neurodegenerative deaths among soccer players and while more research is needed, "there is not enough evidence at this stage to make other changes to the way the modern-day game is played."

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