U.S. Republican presidential candidate Donald Trump eats a pork chop at the Iowa State Fair during a campaign stop in Des Moines in 2015. (Jim Young/Reuters)

Donald Trump takes office Friday as the oldest incoming president in U.S. history — a burger-gobbling, exercise-averse 70-year-old who can expect to live 15 more years, according to actuarial data.

But unlike the fitness fanatic whom he follows into the White House, Trump apparently has never smoked tobacco. He doesn’t drink alcohol. And as a wealthy American, he has presumably spent much of his life with access to excellent health care.

Experts agree there is no reason why a healthy man in his 70s cannot carry out the demanding responsibilities of president of the United States, especially someone who has just been tested by the rigors of a 16-month campaign. Yet a person’s “healthspan” — the years he or she is healthy and free of serious disease — is a highly individual mix of genetics, nutrition, lifestyle, social support, access to care and more.

“The key thing is how any person lives with the stress,” said Gordon Lithgow, a professor of geroscience at the Buck Institute for Research on Aging in California, which studies ways to increase healthspan. “Some people absolutely thrive on the edge of stress.”

By that measure, Trump will be severely tested. Presidents, who seem to age before our eyes, die earlier than their peers, according to a 1992 book, “The Mortal Presidency: Illness and Anguish in the White House,” that looked at 32 modern leaders. Author Robert Gilbert found that even as U.S. life expectancy rose sharply in the 20th century, 21 of 32 presidents died prematurely. His study did not include the four who were assassinated.

Given that the 45th president will be exposed to extraordinary stress levels, what else could affect his health and capabilities to respond to the challenges of office?

“I think the main thing is that the future is a lot less predictable when you’re 70 than when you’re 40 or 50,” said Steve Austad, scientific director of the American Federation for Aging Research. “He could be fine 10 years after his presidency, or he could be in bad shape a year from now.”

Trump has released only limited information about the measures of his own health. In an email on Tuesday, spokeswoman Hope Hicks said there were no plans to give out more. “The President-elect is extremely healthy,” she added, “with excellent genes and great energy and stamina.”

But research shows that the chances of acquiring three diseases simultaneously rises ten-fold between ages 70 and 80, then ten-fold again during the following decade of life, said Nir Barzilai, director of the Institute for Aging Research at the Albert Einstein College of Medicine in New York.

“I think we all realize that humans age at different rates,” Barzilai said. “Seventy means nothing to me. It can be very young, and it can be very old.”

Barzilai said his first question would be how long Trump’s parents lived, particularly his mother. Eighty-five of every 100 centenarians are female, and the influence of long-lived women’s genes on their children is evident, he said.

Trump’s parents did well for people born near the turn of the last century. His mother, Mary Ann McLeod Trump, was 88 when she died. His father, Fred, died at 93 after suffering from Alzheimer’s disease for about five years.

The risk of Alzheimer’s, which eventually afflicted the second-oldest incoming president, Ronald Reagan, doubles every five years after age 60, said Valter Longo, professor of gerontology at the University of Southern California’s Leonard Davis School of Gerontology. Trump’s family history puts him at greater risk, though.

The roles of diet and physical activity have long been acknowledged in healthful aging, often in protecting against heart disease and cancer, the two biggest killers of Americans. On these issues, there is some pertinent information from the president-elect’s longtime doctor.

In a one-page letter in September, New York gastroenterologist Harold Bornstein said there was no history of “premature” cancer or heart disease in Trump’s family. He said the president-elect is 6-foot-3 and weighs 236 pounds, which qualifies him as borderline obese. Trump has acknowledged a poor diet heavy on fast food — McDonald’s hamburgers are a favorite — and admits he would like to lose some weight.

Bornstein’s letter said Trump takes a statin to lower his cholesterol. So it is difficult to judge his cholesterol level of 169, his high-density lipoprotein level of 63 or his low-density lipoprotein level of 94. All are in the normal range. Trump also takes a low dose of aspirin, the letter noted.

Trump’s blood pressure of 116 over 70 was normal, as was his blood-sugar level, Bornstein wrote. “His liver function and thyroid function tests are all within the normal range,” he added, and “his last colonoscopy was performed on July 10, 2013 which was normal and revealed no polyps.”

Trump’s latest electrocardiogram and chest X-ray were conducted in April 2016 and also were “normal,” according to Bornstein.

(Many presidential candidates have publicly shared more information about their health, but in declining to release his full medical records as a candidate, Trump was in some good Democratic company. Bill Clinton refused to do so during his 1992 and 1996 campaigns. John F. Kennedy, who traded on his seeming vitality in his 1960 race against Richard Nixon, hid his Addison’s disease, an adrenaline deficiency, during the campaign.)

Trump appears to get little exercise other than an occasional round of golf — a serious mistake, according to most health authorities. Research continues to indicate the protective benefits of physical activity for everything from declining cognition to reduced muscle strength. Tom Frieden, director of the Centers for Disease Control and Prevention, has called exercise the closest thing to “a magic bullet” known in health care.

“Exercise is the best anti-aging medicine,” said Eric Verdin, chief executive of the Buck Institute.

Famously proud of his stamina, the next president also ignores another piece of critical health advice: the American Academy of Sleep Medicine’s recommendation that adults get about seven hours of sleep per night. On the campaign trail, Trump has said he needs only about four hours.

Barzilai said that while some individuals can remain healthy on a few hours of sleep nightly, they are exceedingly rare. Most people become “metabolically compromised,” insulin-resistant and ineffective at absorbing nutrients, not to mention cranky, he said.

Trump’s early-morning Twitter posts are among his most emotional, Barzilai noted. “I would say there is evidence, at least by tweet, that Trump is not sleeping enough,” he said.

Perhaps one of Trump’s greatest health advantages is his socioeconomic status. A child of affluence, he has never had to endure poverty or anything close to it. Overwhelming evidence exists that the poor face greater risk of illness and death than the wealthy. And prior to the Affordable Care Act, which expanded health insurance to millions of lower-income Americans, they also faced greater difficulty obtaining health insurance.

“People who live and work in low socioeconomic circumstances are at increased risk for mortality, morbidity, unhealthy behaviors, reduced access to health care, and inadequate quality of care,” the CDC reported in 2011.

Trump, by contrast, has reaped the health benefits of his birth and circumstance.

“He’s rich and well-educated,” Austad said, “and those things are almost miracle drugs.”

Michael Kranish contributed to this report.