Repeated Traumatic Brain Injury (TBI) in athletes (so far most extensively studied in boxers and football players) both increases an individual’s risk for Alzheimer’s disease and is the classic cause of Chronic Traumatic Encephalopathy (CTE).  CTE is a largely preventable degenerative dementia resulting from repeated bouts of TBI. Even mild injuries from repetitive head trauma, often without concussion, increase the risk for CTE. A single moderate-to-severe TBI can also increase the risk of subsequent Alzheimer’s disease.  

https://journals.sagepub.com/doi/10.3233/JAD-161002?url_ver=Z39.88-2003&rfr_id=ori:rid:crossref.org&rfr_dat=cr_pub%20%200pubmed

Mendez MF. What is the Relationship of Traumatic Brain Injury to Dementia? J Alzheimers Dis. 2017;57(3):667-681. doi:10.3233/JAD-161002.