A study of more than 2,500 retired NFL players found that those who had at least three concussions during their careers had triple the risk of clinical depression as those who had no concussions.
Those who recalled one or two concussions were one and a half times more likely to be diagnosed with depression, said Kevin Guskiewicz, research director of the University of North Carolina's Center for the Study of Retired Athletes.
"The findings of this study are not simply relevant to 50-, 55-year-old, 60-year-old retired athletes," but to those currently playing, said Guskiewicz, lead author of the study published on Thursday in Medicine & Science in Sports & Exercise, the journal of the American College of Sports Medicine.
Repeated concussions may be even more dangerous for children, said Dr Gerry Gioia, director of the pediatric neuropsychology program and the Safe Concussion Outcome Recovery & Education program at Children's National Medical Center in Washington.
"Kids, generally speaking, do worse than adults with generalized injuries" like concussions, he said.
He also noted that many of the retired players were on the field before the NFL began a concussion management program in the mid-1990s and before studies sponsored by the NFL and US college authorities prompted new helmet designs.
"It will be interesting to see in 40 years what happens to these current players who have better management," Gioia said.
Dr Amparo Gutierrez, a child psychiatrist before becoming associate professor of clinical neurology at Louisiana State University Health Sciences Center in New Orleans, said the study's biggest problem is that it relies on athletes' memory, unverified by doctors' records. But she said its chance of accuracy is improved by its response rate: 2,552 of the 3,683 retired players who were sent surveys completed and returned them.
The study did not say when this was done; Guskiewicz said he has been collecting data since about 2001.
He said 595 players remembered at least three concussions -- an incident in which a blow to the head caused a change in mental status and at least one additional symptom such as headache, nausea, vomiting, fatigue, blurred vision, or concentration or memory problems.
He said 884 remembered one or two concussions, and 1,513 at least one. About half said at least one knocked them out or left them unable to remember what had happened.
The 269 who reported that a doctor had later diagnosed them with depression included about 20 percent of those who reported at least three concussions, nearly 10 percent of those who had one or two concussions, and almost 7 percent of those without concussions.
An earlier study at UNC found retired athletes' chronic pain also may contribute to depression. Guskiewicz said his study analyzed its potential effect and those of numerous other potential causes and factors, including arthritis, stroke, cancer, age and mild cognitive impairment -- a middle area between the changes of normal aging and more serious mental problems.
"We controlled for all these factors, and, after doing that, we still found this link between concussion history and a diagnosis of depression at some point in life," Guskiewicz said.