(CNN) With a tell-all book raising concerns about US President Trump's mental stability, there is a renewed question asking why the most powerful man in the world is not required to pass a thorough mental health exam.

Trump defended himself as a "very stable genius" in a tweetstorm over the weekend. But his remarks have done little to quell questions swirling about why the man with the nation's nuclear codes doesn't have to undergo more rigorous mental health evaluations.

"I think it's totally legitimate to ask about that and to have that as part of the examination," said Julian Zelizer, a professor of history and public affairs at Princeton University and a CNN contributor. "I generally think that once someone's president -- or once someone's even running for office -- this should be fair game.

"This isn't about scandal, and it's not some kind of exposé. It's understanding whether someone is fit to hold the office and whether there is any problem that the president himself -- or his advisers, or the country -- should be aware of."

Trump, 71, is to undergo a medical exam on Friday by White House physician Dr. Ronny Jackson, who performed President Barack Obama's last several physicals while he was in office. White House press secretary Sarah Huckabee Sanders has promised a readout of the results as soon as information becomes available, but it's ultimately up to the president as to what information he wants to share with the public.

CNN reached out to multiple former White House physicians to learn what is routinely covered during the exams and what sort of mental health evaluations, if any, are done, but they were all unreachable or declined to comment.

The debate over a president's mental acumen is not new. Former President Jimmy Carter sought to change the criteria in the mid-1990s, saying the president should be evaluated by an outside panel of medical experts because too much is at stake.

"At this time, the determination is made by the president's personal physicians who must try to balance patient confidentiality and personal interest vis-à-vis the nation's interest. We must find a better way," Carter wrote in the December 1994 issue of the Journal of the American Medical Association.

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Carter pushed for the formation of a nonpartisan group of medical experts, not involved in the president's care, to be given the responsibility "for determining disability, thereby relieving the president's physicians from their potential conflict of interest and enabling the 25th Amendment to work prudently and smoothly."

The 25th Amendment allows for the vice president to assume the presidency if the sitting president is unable to do his job.

"The great weakness of the 25th Amendment is its provision for determining disability in the event that the president is unable or unwilling to certify to impairment or disability," Carter wrote. "In this case, the constitutional duty to act falls on the vice president and a majority of the Cabinet. In such an unhappy event, it is absolutely necessary for the vice president and Cabinet to obtain accurate and unbiased medical advice to determine whether the president is able to perform his or her duties."

Carter's press secretary declined to make the former president available for this story. In October, he said the media were harsher on Trump than on most presidents

Zelizer said it "shouldn't really be a debate" as to a president's mental fitness, because it's imperative for the nation.

"The top concern of all is the power of the president to use and exercise force, whether it's nuclear or whether it's sending troops into a region of conflict. Either is bad -- and that's an area where you want to make sure that the president's mind is working properly. So I think that should be the top issue," Zelizer said. "More importantly, it's important to the democracy: to know if the individual that holds that office is fit, mentally and physically."

Dr. Gail Saltz, a clinical associate professor of psychiatry at NewYork-Presbyterian Hospital, said an internist should definitely ask questions of a senior about memory and mental status, because they're on the front lines of early detection.

"If you're a good internist, you do ask about those issues, because they should be caught and treated," Saltz said. "It would surprise me if the White House is saying that there will be no psychiatric nor neurologic screening."

The White House said Monday that Friday's test will not include a psychiatric exam.

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Dr. Nassir Ghaemi, a professor of psychiatry at Tufts Medical Center in Boston, explored the links between leadership and mental illness in his book "A First-Rate Madness." He issues a cautionary tale on dismissing a leader over mental health issues, because some of the greatest figures in history, such as Winston Churchill and Abraham Lincoln, suffered from episodes of depression and benefited from it in times of crisis.

"People with mild manic symptoms are more creative and more resilient to stress than normal mentally healthy people," Ghaemi said on CNN's "Smerconish" on Saturday.

In his new bestseller "Fire and Fury," author Michael Wolff says that aides close to President Trump have raised concerns about his mental health and that discussions about replacing him are "alive every day in the White House." CNN has been unable to verify all of Wolff's claims.

"It's not unreasonable to say this is 25th Amendment kind of stuff," Wolff said on NBC's "Meet the Press."

Photos: U.S. presidents: Ailing in office Photos: U.S. presidents: Ailing in office John Adams 1797-1801 – study by Duke psychiatrists found John Adams would have been diagnosed with a bipolar disorder. Hide Caption 1 of 14 Photos: U.S. presidents: Ailing in office William Henry Harrison, March-April 1841 – William Henry Harrison battled with dyspepsia and indigestion. Before he had been in office a month, he caught a cold that developed into pneumonia. On April 4, 1841, he became the first president to die while in office.

Hide Caption 2 of 14 Photos: U.S. presidents: Ailing in office Abraham Lincoln, 1860-1865 – Abraham Lincoln is widely thought to have suffered from depression. Hide Caption 3 of 14 Photos: U.S. presidents: Ailing in office Ulysses S. Grant, 1869-1877 –

The study by Duke psychiatrists found Ulysses S. Grant would have been diagnosed as an alcoholic with social phobias. Hide Caption 4 of 14 Photos: U.S. presidents: Ailing in office Chester Arthur, 1881-1885 – Chester Arthur was diagnosed with Bright's disease, a fatal kidney condition, after a year in office. He did not seek a second term and died less than two years after leaving office. Hide Caption 5 of 14 Photos: U.S. presidents: Ailing in office Grover Cleveland, 1885-1889 and 1893-1897 – Grover Cleveland suffered from obesity and gout and was treated for cancer in his jaw while in office.



"President Cleveland was one of the most compelling stories of concealment in the high office," said Jerrold Post, professor emeritus of psychiatry, political psychology and international affairs at George Washington University. "He was brushing his teeth one day and found a lump on roof of the mouth. Instead of telling the public, he smuggled his dentist, head and neck surgeon and surgical team onto a pleasure yacht, where they removed the roof of his mouth to get rid of the carcinoma. He emerged a week later complaining of a toothache." Hide Caption 6 of 14 Photos: U.S. presidents: Ailing in office Theodore Roosevelt, 1901-1909 –

The Theodore Roosevelt suffered from asthma and was blind in one eye as the result of a boxing injury in 1905. He was also deaf in one ear. The 2006 study by Duke psychiatrists applied today's diagnostic criteria to historical records and found Roosevelt would have been diagnosed with bipolar. Hide Caption 7 of 14 Photos: U.S. presidents: Ailing in office Woodrow Wilson, 1913-1921 –



While in office, "he suffered a massive stroke, but they concealed it and just said he was under the weather and no one was informed," Post said. Woodrow Wilson had a debilitating stroke in 1919 that left him partially paralyzed while in office. According to Jerrold Post, Wilson had suffered several strokes while he served as president of Princeton but never revealed his medical history to voters.While in office, "he suffered a massive stroke, but they concealed it and just said he was under the weather and no one was informed," Post said. Hide Caption 8 of 14 Photos: U.S. presidents: Ailing in office Franklin D. Roosevelt, 1933-1945 –

Franklin Delano Roosevelt was paralyzed in both legs , likely as a result of polio that struck when he was 39. But it was the cover-up of his advanced heart disease and elevated blood pressure when he ran for his fourth term that historians question. FDR died just a few months after that election. Hide Caption 9 of 14 Photos: U.S. presidents: Ailing in office Dwight D. Eisenhower, 1953-1961 – Dwight D. Eisenhower suffered from ongoing gastrointestinal problems. He was later diagnosed with Crohn's disease Hide Caption 10 of 14 Photos: U.S. presidents: Ailing in office John F. Kennedy 1961-1963 –

John F. Kennedy "probably had more diseases than any of the other presidents," said George Annas, chairman of the department of health law, bioethics and human rights at Boston University School of Public Health. Kennedy took office suffering from hypothyroidism, back pain and Addison's disease and was on a daily dose of steroids and other drugs. Hide Caption 11 of 14 Photos: U.S. presidents: Ailing in office Lyndon Baines Johnson 1963-1969 – Lyndon Johnson had serious heart disease, which he often concealed, during his years in the Senate and White House, and it was his failing health that kept him from running against Nixon in 1968. The study by Duke psychiatrists also found that Johnson would have been diagnosed as bipolar. Hide Caption 12 of 14 Photos: U.S. presidents: Ailing in office Ronald Reagan 1981-1989 – Ronald Reagan had a cancerous tumor and two feet of his colon removed in 1985, but it was his diagnosis of Alzheimer's following his presidency that have many wondering whether his performance in office was affected. Hide Caption 13 of 14 Photos: U.S. presidents: Ailing in office George H. W. Bush 1989-1993 – George H. W. Bush was diagnosed with Grave's disease while in office. According to former White House physician Connie Mariano, "There was some question when he had hyperthyroidism, Grave's disease, around the time of the Gulf War. Did that make him more hyper and aggressive? Did it affect his memory, his ability to focus? It's hard to say." Hide Caption 14 of 14

Citing people close to the president, Wolff has said the president has begun repeating three stories in conversations in less than 10 minutes, when he used to repeat stories in about a 30-minute window. Wolff told CNN's Christiane Amanpour that "100% of the people around the president believes he's incapable of carrying out the duties of office."

In full damage control, Trump and the White House beat back over the weekend, trashing Wolff and his book as fiction and tabloid garbage while defending the president's mental fitness.

"Actually, throughout my life, my two greatest assets have been mental stability and being, like, really smart," Trump said in a tweetstorm that began Saturday

I've had to put up with the Fake News from the first day I announced that I would be running for President. Now I have to put up with a Fake Book, written by a totally discredited author. Ronald Reagan had the same problem and handled it well. So will I! — Donald J. Trump (@realDonaldTrump) January 7, 2018

Secretary of State Rex Tillerson told CNN late last week that he has "never questioned" the president's mental fitness, and "I have no reason to question his mental fitness." CIA Director Mike Pompeo reiterated that message on Sunday, saying the book's assertions on the president's mental abilities "are just absurd."

"This president reads material that we provide to him. He listens closely to his daily briefing," Pompeo said on "Fox News Sunday."

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Rep. Adam Schiff, a California Democrat, gave a different assessment. "Plainly, we have a seriously flawed human being in the Oval Office," Schiff said Sunday on CNN's "State of the Union."

Trump is scheduled to have his physical Friday at Walter Reed National Military Medical Center in Maryland. Test results typically take several days before the findings are known, but what information is released and when is ultimately at Trump's discretion.