Does playing professional football lead to neurodegenerative diseases?

tl;dr It did in the past, but it’s not clear to me if it still does.

My two fields of professional (in)activity, epidemiology and football, could not be more different, but now there is a tiny overlap. A recent article [paywalled] in the prestigious New England Journal of Medicine addresses the long-standing hypothesis that playing football professionally increases the risk of developing a neurodegenerative disease later in life. This effect is well-established for more brutal sports such as boxing and American football. It has always been a decent bet that it is also the case for football, but so far we have not had hard evidence.

To look into this question, the authors compiled a database of 7676 professional football players in Scotland born before 1977. For every player, 3 non-players born in the same year and living in an area with the same Scottish Index of Multiple Deprivation (SIMD) were drawn from the general population and served as the player’s own mini-control group (so-called matched controls.) The prescription history and the cause of death (if the person died before 2017) was then obtained from medical records. The data was analysed with Cox regression and related survival analysis methods. The central result is stark: ex-footballers have been dying of neurodegenerative disease at a rate 3.5 times higher than the general population. They are also significantly more likely to have been prescribed dementia medication. What stands out to me here is not so much that the hypothesis was confirmed, but the size of the effect: a hazard ratio of 3.5 is really large and unlikely to be explained away completely by any biases in the study.

Speaking of bias, and study design in general, I have a concern. The matching of footballers to controls was done only on the basis of the birth year and SIMD of their last known postcode. The latter feels like a particularly weak proxy for lifelong exposure to risk factors. In addition, disease status is likely entwined with the last postcode, with some sick people moving to have better access to care and services, and others not moving while their healthy peers retire somewhere nice. All told, without better covariates on which to match footballers to controls, I’d have been tempted to save myself the trouble and simply draw a random control group with the same age structure from the general population.

The biggest question mark hanging over this research is whether the findings can be generalised to the modern game. It is disappointing that this is not discussed in the article nor the accompanying editorial [also paywalled.] Unavoidably, the study had to reach deep into the past for its data, but 1970s (say) Scotland was a particular time and place and, crucially, the sport was also different to what it is today. With the possible exception of Tony Pulis’ training sessions, football is no longer head-tennis played with a wet medicine ball. The professional players are also a much healthier bunch today, and they have access to high quality health care and monitoring. In a throwaway sentence, the authors suggest to conduct a prospective cohort study (i.e. to follow a sample of current players through their lives after they retire) to confirm the findings, but naturally this would take a very long time. I cannot help but wonder if a compromise solution exists, where former players from an era more resembling the modern game are enrolled. For such more recent population, more data may also be available to perform better matching to controls, and to include the relevant football-related variables in the analysis (for example the player’s usual outfield position and duration of playing career).

Putting the two objections together, I don’t think that this article provides very strong evidence that footballers playing today are at risk of developing neurodegenerative disease later in life. That said, it certainly points in this direction, and it is very encouraging that the FA and the PFA are funding research into this important question. Hopefully there is much more to come.