“We will make Americans healthy again,” Robert F. Kennedy Jr. has declared. A political action committee that has promoted Mr. Kennedy, President-elect Donald J. Trump’s pick for health and human services secretary, says his movement is “igniting a health revolution in America.”

But the word “again” presumes a time in the country’s past when Americans were in better health. Was there ever really a time when America was healthier?

For historians of medicine, there is a short answer.

“No,” said Nancy Tomes, a historian at Stony Brook University.

John Harley Warner, a historian at Yale, said, “It’s hard for me to think of a time when America, with all the real health disparities that characterize our system, was healthier.”

Dr. Jeremy Greene, a historian at Johns Hopkins University, asked: “Which particular era does R.F.K. want to take us back to?”

Probably not the 19th and early 20th century.

Rich men smoked cigarettes and cigars, the poor chewed tobacco. Heavy drinking was the norm.

“It was definitely a drinking culture,” said Dora Costa, an economic historian at the University of California, Los Angeles. “Drinking was a huge problem, saloons were a huge concern. Men were drinking away their paychecks. That is the reason we had Prohibition.”

And, Dr. Costa notes, American diets for most of the 19th century were monotonous.

It’s true that agriculture at the time was organic, food was locally produced and there were no ultraprocessed foods. But fresh fruits and vegetables were in short supply because they were difficult to ship and because growing seasons were so short. For the most part, Dr. Costa said, until the 1930s, “Americans were living off of dried fruits and vegetables.”

As for protein, Americans were relying on salted pork, she said, because meat was difficult to preserve. Only after the Civil War did meatpackers in Chicago begin to process meat and ship fresh beef across the country. At that point, Dr. Costa said, beef “became a large part of the American diet.”

But even though the availability of beef helped diversify diets, people did not become healthier.

Dr. Costa worked with Robert Fogel, the University of Chicago economic historian and Nobel laureate, to understand the health of a population of Americans living in the North around this period by examining the medical records of Union Army soldiers. Common conditions, like hernias, were untreatable — men had hernias as big as grapefruits, held in by trusses. Nineteen percent of those soldiers had heart valve problems by the time they were 60, compared with about 8.5 percent today.

Poor nutrition led to poor health. People were thin, often too thin. In 1900, 6.1 percent of Union Army veterans were underweight — a risk factor for various illnesses and often a marker of ill health — compared with 1.6 percent of U.S. adults today. In 1850, males at age 20 could expect to live to around 61 years. Today it is 74 years.

The start of the 20th century saw public health improvements (cleaner water, for example, and posters advising parents not to give their babies beer), but disease was rampant. There were no antibiotics and very few vaccines. When the 1918 flu struck the nation, no one knew the cause — the flu virus had not been discovered and strange folk remedies were rampant. About 675,000 Americans died. In 1929, the Great Depression began, and its economic toll over the next decade led to severe nutritional and health problems.

Health improved in the second half of the 20th century but was poor compared with that today.

Many people are nostalgic for the 1950s and 1960s, seeing those decades as a time of prosperity, when the American pharmaceutical industry pumped out new medical advances: antibiotics, antipsychotics, drugs for high blood pressure and vaccines for tetanus, diphtheria, measles and polio.

Despite that progress, those years were terrible for health, Dr. Greene said, with “a tremendous amount of heart attacks and strokes.”

Heart disease was rampant in 1950, with 322 deaths per 100,000 Americans annually from cardiovascular disease, double the rate today. By 1960, Dr. Greene said, heart disease, was responsible for one-third of all deaths in America.

In part, that was because nearly everyone smoked.

“We were among the heaviest smoking countries,” said Samuel Preston, an emeritus professor of sociology at the University of Pennsylvania.

Dr. David F. Musto, a medical historian at Yale, who died in 2010, once said in an interview that although he never enjoyed smoking, the social pressure to smoke when he was in college in the 1950s was so great that “I felt it was my duty to find my brand.”

Smoking greatly increases the risk for heart disease, the leading killer in the 1950s and 1960s.

Heart disease death rates plummeted in recent decades because smoking is much less common now, and treatment for heart disease is much more effective. Cholesterol-lowering statins, introduced in 1987, reduced the risk of heart disease. Other new medications as well as bypass surgery and stents also saved lives.

Cancer was the second leading killer in the 1950s, as it is today. But in 1950, there were 194 cancer deaths per 100,000 people. Now there are 142 cancer deaths per 100,000 people.

A decline in smoking is a leading reason, but there also has been a revolution in cancer treatment.

Until the 1990s, cancer was treated with brute force: surgery, radiation and chemotherapy. Now an array of targeted therapies are turning some cancers, once deadly, into treatable chronic diseases or even curing them.

Dr. Greene said he was not surprised by the idea of a halcyon past when people were healthier.

“There’s a long history in America of nostalgia for a past that was better than the present,” he said. “History is all about erasure — the things we don’t choose to remember.”

Today is not a sort of health utopia, of course.

Researchers are quick to acknowledge that Americans’ health is not as good as it can be. And they bemoan the huge disparities in health care in this country.

Yet the U.S. spends more on medical care than other countries — an average of $12,555 per capita, which is about twice what other wealthy countries spend.

But, historians say, the past was actually much worse.

And so, they say, the phrase “Make America Healthy Again” makes no sense.

“As a historian of health, I don’t know what ‘again’ Kennedy is imagining,” Dr. Tomes said. “The idea that once upon a time all Americans were healthy is a fantasy.”