Michael D'Antonio is the author of the book " Never Enough: Donald Trump and the Pursuit of Success " (St. Martin's Press). The opinions expressed in this commentary are his.

(CNN) Donald Trump wants us to know that he would have "run in" to stop the Parkland high school shooter "even if I didn't have a weapon."

This from a man who required a golf cart when other world leaders chose to walk a short distance at last year's summit in Sicily, who sought multiple deferments to avoid carrying a gun in Vietnam to serve his country -- and then mocked a war hero, Sen. John McCain, for doing just that, and getting captured and tortured.

Donald Trump performed his most recent chest-pounding in the same gathering Monday — a meeting with US governors at the White House -- where he described a law enforcement officer who didn't enter the school building at the Florida school as "frankly disgusting." The slaughter of 17 students and faculty at Marjory Stoneman Douglas earlier this month has drawn nationwide calls for serious leadership on the problem of mass shootings.

Considering the man holding the office of President -- and his history -- it's hardly surprising to hear this new installment of bragging bravado. Even so, a president's pronouncements on national tragedies are, by definition, news. In this case we are once again called to consider the context, which includes both the devastating events in Parkland, Florida, and what it means when Trump speaks.

The nation and the world have long since learned the President is not shy about distorting reality (in his first year in office he lied or misrepresented the truth more than 2,000 times ), and liberally pumping up his image in a way that might embarrass another man, even in highly sensitive circumstances. He appears to be unable to consider the impact of what he says.

In the more than three years I studied his life to write his biography, I found no evidence that he possessed any physical courage and few signs that he ever made any significant effort on behalf of anyone who couldn't do something for him. (He bragged to me about giving a sick child a big check, but other than that, his record of charity is scant.)

You won't, for example, find pictures of Donald Trump hammering nails for Habitat for Humanity. He did flip rolls or paper towels to hurricane survivors in Puerto Rico, but he didn't actually use any of them to clean up anything.

The only online video of Trump in any kind of physical altercation shows him messing around with the wrestling entrepreneur Vince McMahon in an over-the-top, faked-for-TV scuffle.

Indeed, in the 1970s, when Trump was young and yet unknown, he was so frightened that -- as The New York Times noted -- he hired an armed guard to chauffeur him around the upscale precincts of Manhattan. Later in his career he kept a full-time staff of guards on hand to protect him at his office, which could not be accessed by anyone who hadn't cleared security in the lobby of the building.

Rather than supporting his boasts, the facts of Trump's life tell us that his self-glorification about Parkland was idle talk.

He doesn't know what he would do in a setting where automatic gunfire broke out and people were running scared. He is 71 and by any measure, out of shape: Whatever his intentions in hindsight, he doesn't know that he could run more than a few feet without collapsing.

To be fair, none of us -- not even those of us who are trained and have a duty to respond -- can be certain how we would act in a mass shooting. And no one who isn't certain of his or her fitness should brag about taking vigorous action when it's just as likely that trying it would create even more problems for first responders. Donald Trump might well end up one more person in need of an ambulance, should he throw himself into a situation like the one at Parkland.

He or his handlers apparently grasped this after he made his comments to the governors' meeting. The White House clarified that President Trump "was just stating that as a leader, he would have stepped in and hopefully been able to help."

Would that we had evidence — any evidence at all — from his past to confirm this as even a possibility.

Trump can be forgiven his age, and his physical condition and, given the psychology of old men, he might even be forgiven if he privately mused about what he would have done at Parkland. But his public bragging in the face of tragedy amounts to an unforgivable display of insensitivity. He insulted those who train and maintain themselves to respond in such crises, and once again degraded the public discourse with the kind of babbling more suited to a corner tavern.

Previous presidents, even those who might have had reason to declare themselves ready for dangerous duty, would never have done it. That Trump would brag in this way only affirms that he is unfit, in every way.