To use all the old metaphors, Tris Dixon’s “Damage: The Untold Story of Brain Trauma in Boxing” is a slap in the face, a punch in the gut, a kick in the groin. Given that this remarkable, long-overdue treatise on the mental and physical ravages of boxing doesn’t hold anything back, it’s fitting that Mr. Dixon’s book lands with power and precision. For boxing fans, it’s a wake-up call. To remain a fan of the sport—to cheer on the punishment that takes place in the ring, then choose to ignore its consequences—constitutes a cruel form of enabling.

Even for readers who are not aficionados of the sweet science, “Damage” hits hard. At its core is the public adoration of athletes as modern-day gladiators—or, at least, icons of physical prowess—and what we demand of them: the courage and sacrifice that we celebrate from the peanut gallery. “This is a sport in which bravery can be measured by the amount of punishment one can withstand,” Mr. Dixon tells us. Meanwhile, in the ring, “bravery” is a surefire prescription for chronic traumatic encephalopathy (CTE), a deterioration of the brain caused by repeated head trauma.

A veteran boxing journalist in the U.S. and England, Mr. Dixon tackles his subject with great compassion. He interviews neurologists, psychologists and overseers of the sport—managers, trainers and commissioners—but mostly he spends time with the fighters. Some are remarkably sanguine about the physical and mental toll they can see coming. “I’m on the back end, I know what it’s like,” says former World Boxing Organization heavyweight champion Shannon Briggs, who is still fighting as he nears his 50thbirthday. “I know I’ve got something wrong with me, I know all these punches are going to eventually catch upto me, so I’m reading about CTE and I started taking CBD,” Mr. Briggs tells the author, referring to cannabidiol, the cannabis-derived compound. “I poured the [antidepressant] pills down the toilet the day I tried cannabis.”

Like lambs to the slaughter, boxers trudge on, only to wind up on “queer street.” The most notable example is Muhammad Ali. In his prime, Ali was known for his ability to slip punches and avoid punishment, but his career was long and brutal. He fought the best and toughest fighters. And, like nearly all boxers, he stayed in the ring too long. “Age thirty is the cusp,” Ferdie Pacheco, Ali’s onetime fight doctor, is quoted saying. “Thirty-five is over the line. I don’t care how good you are, after age thirty-five you’re getting brain damage.” Ali had six fights after the age of 35; he lost three of them. The result may well have been Parkinson’s disease. Parkinson’s and other maladies—dementia; memory loss; the rapid deterioration of verbal skills and motor functions such as bowel control, breathing and walking—are all common results of CTE.

Damage

By Tris Dixon
(Hamilcar, 277 pages, $29.99)

In the past, the image of the punch-drunk fighter was a source of humor and playful ridicule, on stage and on screen. Mr. Dixon cites the example of “Slapsie” Maxie Rosenbloom, a light-heavyweight from the 1930s who had 298 professional fights. Following his boxing career, he became a comic actor in movies and on television, where he lampooned the tortured speech patterns and lumbering mannerisms of a late-career palooka. But after generations of concussions, cerebral hemorrhaging and deaths in the ring, by the early 21st century the image of a brain-damaged boxer was no longer funny. Unlike Rosenbloom, most wound up in asylums, hospitals and sanitariums.

The first major study to examine the effects of CTE on the human brain took place almost two decades ago and focused on head trauma in the NFL, not in boxing. Up until then, most observers assumed that the critical damage in boxing likely came from one major knock-out punch to the head. But the CTE study showed that the effects were cumulative. Medical professionals identified what they termed “second-impact syndrome.” When a fighter is hit in the head, the brain damage often goes undetected. “Second-impact syndrome is one of the most serious threats to brain injury, both in the long and short term,” writes Mr. Dixon. “The first hard hit has done more damage than anyone suspects and then the boxer takes a follow-up shot and life can be irreparably changed.”

The damage begins early in a boxer’s life, perhaps even before the aspiring fighter turns pro. A young brain may be more able to withstand a powerful punch, but training, with sustained periods of sparring in the gym, is often more damaging than what happens on fight night. Head gear and large gloves haven’t been shown to lessen the damage. The only way to preserve a young fighter’s health is to have certified medical professionals on hand at licensed gyms seeing to it that the boys and girls are submitting to a proper period of recovery.

It is boxing’s unwillingness or inability to address the long-term damage caused by the sport that represents this book’s most damning critique. The sport has had decades to devise systems to mitigate the crisis—Mr. Dixon outlines many in the book—but has remained willfully ignorant. The reasons for this have been detailed before: the lack of a unified governing body; the absence of a union that provides a pension, health plan and moral or financial support for retired fighters. Boxing is largely controlled by the promoters, who have shown little concern for the safety and long-term well-being of fighters.

For Mr. Dixon, “Damage” seems to be a turning point. Near the end of the book he admits that during his career in “one of the top jobs in boxing journalism” he witnessed things ringside that now make him wonder about the morality of the sport. Readers of his book may find themselves in a similar state of moral vertigo.

Mr. English’s latest book, “Dangerous Rhythms: Jazz and the Underworld,” will be published next year..