Soldiers exposed to roadside bomb blasts and athletes who have suffered repeated concussions show the same long-lasting changes to brain cells, a new study reports.

The research connects two growing — and for many sufferers, chronically debilitating — health problems in the United States. It adds to the evidence that injuries once officially termed “mild” may be anything but.

Specifically, head trauma that doesn’t knock a person unconscious may nevertheless kill some brain cells and damage the architecture of others, setting up a semi-permanent state of brain inflammation. That may lead to the foggy thinking and poor memory that thousands of soldiers and football players experience even after they’ve “recovered” and been sent back to the battlefield or playing field.

“Whether your head is accelerating because you’ve been hit by a linebacker or the blast from an exploding [improvised explosive device], the injury to the brain appears to follow a similar course,” said Lee E. Goldstein, a doctor and neuroscientist at Boston University who co-led the study, published Wednesday in Science Translational Medicine.

The abnormalities found in the brains of troops and athletes studied after death are similar to those found in the brains of mice exposed to the equivalent of a moderate-size bomb blast, an experiment the 35-member research team also conducted.

The study doesn’t immediately suggest ways to prevent or treat blast injuries in soldiers or concussions in athletes. The researchers hope that future mouse experiments may point in that direction. But the findings will almost certainly draw more attention to a problem that’s now part of the medical histories of thousands of Americans.

“The take-home message is that concussions need to be taken seriously at all ages,” said Ann C. McKee, a neuropathologist with the Veterans Affairs Boston Healthcare System. “We don’t want our kids exposed to these injuries in the pursuit of amateur athletics.”

A rare look at brains

Traumatic brain injury (TBI) has been called the “signature wound” of the Iraq and Afghanistan wars.

A key question is whether a blast alone can cause permanent brain damage, as opposed to a blast that smashes a soldier’s head against a solid object, such as a vehicle roof. By one estimate, more than 320,000 troops may have experienced some form of TBI over the past decade.

Until this study, however, there’d been almost no detailed descriptions of the brains of people who survived battlefield blast injuries.

“Surprisingly, what is happening under the microscope after a blast-related TBI hasn’t been made public, except for one previous [published] case,” said David L. Brody, a brain-injury researcher at Washington University’s School of Medicine in St. Louis.

Most soldiers exposed to roadside bombs survive; there’s no way to see what their brains look like on the cellular level after the blasts. In addition, it’s nearly impossible to find someone whose only head trauma is from an improvised explosive device. Virtually all soldiers report suffering concussions previously in their military careers or as civilians, making it hard to determine the effect of a single blast.