Chris Borland has decided to retire due to concerns about the long-term effects of repetitive head trauma.

Borland notified the San Francisco 49ers of his decision on Friday.

Borland made his decision after consulting with family members, concussion researchers, friends and current and former teammates, and studying what is known about the relationship between football and neurodegenerative disease.

"I just honestly want to do what's best for my health," Borland told "Outside the Lines." "From what I've researched and what I've experienced, I don't think it's worth the risk."

More than 70 former players have been diagnosed with progressive neurological disease following their deaths, and numerous studies have shown a connection between the repetitive head trauma associated with football, brain damage and issues such as depression and memory loss.

Borland is the most prominent player to retire at the start of his career due to head trauma concerns.

Borland was expected to be a key part of the 49ers defense this season after the retirement last week of All-Pro linebacker Patrick Willis. Borland replaced Willis, 30, after six games last season; Willis had sustained a toe injury.

Borland received one vote for NFL Defensive Rookie of the Year.

"I've thought about what I could accomplish in football, but for me personally, when you read about Mike Webster and Dave Duerson and Ray Easterling, you read all these stories and to be the type of player I want to be in football, I think I'd have to take on some risks that as a person I don't want to take on." Borland was referring to former NFL greats who were diagnosed with a devastating brain disease, Chronic Traumatic Encephalopathy, or CTE, after their deaths. Duerson and Easterling committed suicide.