“Why do you want to live to 150?”

The question hung in the air. It was last fall, at a dinner during an invitation-only retreat in Aspen, Colo., filled with leaders in fields like foreign affairs, economics and entertainment.

Among them was a visionary of another sort — Bryan Johnson, the wealthy tech entrepreneur and so-called biohacker who has been waging a public and provocative fight against mortality.

He says he has poured millions of dollars into studying and perfecting his own body, and calls himself the most measured human being in history. His goal is to make it possible to live forever. (He is almost 49.)

At the dinner in Aspen, Mr. Johnson found himself seated next to another man with very particular views on longevity: Ezekiel J. Emanuel, who served as a health policy adviser in the Obama administration and is a critic of the modern wellness industry, of which Mr. Johnson is an extreme practitioner. Dr. Emanuel has said he would be content to live to 75. (He is now 68.)

They had reached only the salad course when Dr. Emanuel’s question cut to the heart of the matter.

“I don’t see the point; I don’t see the purpose. Why do you want to live to 150?”

Silence.

“He didn’t say anything,” Dr. Emanuel told me later. “It caught him up short.

“That was the most interesting part of the evening.”

A few months after that, Mr. Johnson would get what he called his “bad news.”

Enter the Longevity Bros

Mortality is an immutable fact of the human condition, but the desire to cheat death has endured for a very long time. According to legend and history, Gilgamesh, Chinese emperors and medieval alchemists all tried to attain immortality.

The allure is no less profound today, but the modern potions are somewhat more sophisticated and inherently appealing to a wealthy, techno-centric class of people like Mr. Johnson.

Mr. Johnson, who lives in Los Angeles, is perhaps the best known of a cadre of scientists and influencers known as “longevity bros.” They obsessively document their attempts to live cleaner and longer, known cuttingly as “longevity fixation syndrome.”

Mr. Johnson, whose slogan is “Don’t Die,” has called himself “the healthiest person on the planet.” He tries to prove the claim by documenting every blip and squiggle that measures his health — blood count, heart rate, visceral fat, nighttime erections.

He consumes little meat, but a lot of collagen peptides. He has experimented with loading up on more than 100 supplements a day. He even received a blood plasma transfusion from his teenage son, and in turn gave his own plasma to his father.

But this summer, Mr. Johnson’s efforts to achieve physical perfection ran up against the inevitability of illness.

In a social media post, he announced that he had developed autoimmune gastritis, a condition in which the body attacks certain cells in the stomach that are responsible for making acid. It leads to iron and vitamin B-12 deficiencies, which are associated with brain fog, hair loss, brittle nails, neurological complications and an elevated risk of stomach cancer.

Or, as he put it, “My stomach is eating itself.”

Announcing his diagnosis on social media, he framed it as a sort of betrayal. “I eat whole foods,” he wrote. “I exercise every day, and I don’t do bad things.”

But he also said the condition had inspired him to move to a phase of “disease resolution,” as he called it.

“It potentially captures what the future will be,” he said. “Right now, when someone gets sick, when they get a diagnosis, we’re not all that shocked. We just expect it.”

In the future, he predicted, “we may get to the point where it’s shocking if someone falls ill.”

At the same time, that dinner-party question seemed to be nagging at Mr. Johnson. He recently posted on X: “been thinking things over and wonder if I’ve taken this whole longevity thing too far.”

What had become of the World’s Healthiest Man?

I decided to ask him.

Flying Too Close to the Sun?

Mr. Johnson has done countless interviews with the news media, including The New York Times, and was the subject of a recent Netflix documentary.

He tends to give practiced and vague answers. He also bats away accusations that his real goal is to make money through his two longevity companies, Blueprint and Immortals. He’s rich enough already, he says.

In one of our video interviews, I reminded him of Dr. Emanuel’s question — why live to 150? — and he broke into a big, infectious grin. I had not seen him grin before.

“I find him to be a delightful person,” he said. “I think he disagrees with everything I stand for, which is great!”

But, I asked, what were you thinking while you sat there silently?

“As you can see,” he explained, “I’m not really an argumentative type. I usually say yes and enjoy other people’s opinions. So I was really trying to understand him. I think I maybe finished my perspective of him saying that I don’t think someone needs a reason, that life itself doesn’t need to be qualified. That answer usually leaves people very dissatisfied.

“Humans want reasons for everything, and if a reason is not paired with the idea, it’s very hard for humans to process that.”

Dr. Emanuel, though, is not without reason or ideas. And as much as he enjoyed Mr. Johnson’s company that night, he has his own thoughts about Mr. Johnson’s quest: He thinks the longevity industry has gone off the rails.

Trying to defeat aging, Dr. Emanuel and other critics of the live-forever movement say, is arrogant and narcissistic.

Dr. Emanuel supports simple — even enjoyable — biohacks like cutting out ultraprocessed foods, getting good sleep and taking up new experiences. The title of his latest book, “Eat Your Ice Cream,” speaks for itself.

“The point of life, I think, is to make the world better for your family, your community, for humanity,” Dr. Emanuel said. “We’re here for a short time, and it’s not all about us.”

I thought about my own Greek mother, Calliope Hartocollis, who is my role model for good aging.

She lived to be 97, still doing math in her head, still calling people if she could not visit, and almost to the end, still walking to the market. Not that she loved being old. She often repeated a Greek proverb with roots in ancient philosophy: “I can’t take two things, poverty and old age.” (It rhymes in the original.)

She might have liked to be immortal, but she did not pursue it. Instead she quoted the Greek poet Cavafy, whose poem says that when going home to Ithaka, it is the journey — what you find along the way — that matters.

Her ultimate strategy was acceptance. My mother, a psychologist, told me a few months before she died, that she had examined her life and was at peace with it.

In our interviews, Mr. Johnson accepted nothing, even as he knew that many want to see him fail. When talking about his autoimmune disorder, he brought up the Greek myth of Icarus, the boy with wings of wax who wanted to fly. “You know, I flew too close to the sun,” he said, explaining the criticism. “I went after a goal that could be described as hubris.”

He rejected the possibility that his illness, which is not fatal, had been caused by his longevity regimen. He said it may have been lurking for about 30 years, related to his diagnosis of hypothyroidism when he was 21.

“The thyroid and stomach are very closely linked in immunology,” he said.

(His explanation is plausible, but “there is no actual known cause of autoimmune gastritis,” said Dr. Rabia de Latour, a gastroenterologist at NYU Langone Health in New York City.)

Mr. Johnson believes that his so-called healthmaxxing is a positive, not a negative.

“If anything, I am extremely grateful that I’ve spent the past five years working on my health,” he said. “Had I not been taking care of my health with such meticulousness, it’s possible my diagnosis would be a lot worse.”

Besides, he said, his vigilance is not only for himself, but for the good of humanity.

Mr. Johnson shrugged away his earlier X post suggesting that he had maybe taken “the whole longevity thing too far.”

“I was making fun of myself,” he said. “My humor is — I’m deadpan, I’m ironic, and I’m absurd.”

Still, could there be a grain of truth to his regrets?

“If there is, it’s subconscious,” he insisted.

In hindsight, he said that he was surprised his team of medical experts had not found more to worry about.

“When you measure, you find stuff,” he said.

That answer hints at another truth: When you focus all your energy on staying alive, you have very little time to actually live.

I thought of my mother again. “There is one essential truth,” she used to say in Greek, quoting a popular song of the 1980s. “There is no immortality.”

Georgia Gee, Kirsten Noyes and Sheelagh McNeill contributed research.