Former White House aide Omarosa Manigault-Newman questioned President Trump's physical and mental fitness in a forthcoming book about her time at The White House and claimed that he is "just this side of functionally illiterate."

In her tell-all, "Unhinged," Manigault-Newman writes: "I'd come to understand that [Trump] read at an eighth- or ninth-grade level. That's fine for some, but for the leader of the free world?"

Manigault-Newman goes on to claim that the 72-year-old Trump, whom she refers to as "Donald" throughout the book, "has only a surface-level understanding of the content he's signing into law" and adds "Donald has always relied on his charisma, his street smarts, and trusted advisers to tell him what was in the paperwork."

The White House has denounced the book, with Press Secretary Sarah Sanders calling it "riddled with lies and false accusations Friday." On Monday, Trump called Omarosa "wacky" and a "lowlife" on Twitter, adding: "People in the White House hated her. She was vicious, but not smart. I would rarely see her but heard really bad things."

"Unhinged", which is due to be released Tuesday, documents Manigault-Newman's 15-year-long working relationship with Trump -- beginning with her infamous stint on the first season of the NBC reality show "The Apprentice" and continuing through her dismissal by White House Chief of Staff John Kelly in December 2017.

Over that period, Manigault-Newman claims to have noticed a pronounced deterioration in Trump's physical and mental health. She singles out the president's now-famous interview with NBC News' Lester Holt after firing FBI Director James Comey in May 2017 -- a decision she says "not a single person in the White House" agreed with -- as a moment when "his mental decline could not be denied."

"I'd known Donald to exaggerate and boast," Manigault-Newman writes. "He'd told white lies and lies of omission, ignorance, or misunderstanding. ... But this was different. It was like he didn't know what the truth was or couldn't remember what he'd previously stated as truth."

In that interview, Trump told Holt that he "was going to fire Comey" regardless of whether Deputy Attorney General Rod Rosenstein recommended he do so. Manigault-Newman writes that White House communications aide Hope Hicks had told Trump "a dozen times" to highlight Rosenstein's recommendation that the president dismiss Comey.

"When I decided to just do it, I said to myself, 'You know, this Russia thing with Trump and Russia is a made-up story,'" Trump said at the time.

Manigault-Newman writes that she discussed her concerns about Trump's mental state with Lara Trump, the president's daughter-in-law, as well as "several high-level people in the White House" who "shut me down quickly and decisively, with warnings."

Manigault-Newman elaborated on her claims about Trump's mental agility in an interview on PBS' "Newshour" Monday night.

"Our boardrooms on 'The Apprentice' would be four, five hours long and Donald Trump was sharp. He was very perceptive. He was engaging. He had this expansive vocabulary and he very seldom took breaks," she told host Judy Woodruff. "Fast-forward to 2017 and we're in the White House and Donald Trump couldn't remember basic words or phrases."

When Woodruff asked Manigault-Newman how she knew of Trump's condition, the former White House aide answered: "Because I was in the room."

Manigault-Newman also asserts her belief that Trump "is clearly obese" and his "terrible health habits have caught up with him." Manigault-Newman specifically cites Trump's "addiction to Big Macs and fried chicken [and] daily tanning bed sessions."

On one occasion, Manigault-Newman tried to sneak an article about a study linking Diet Coke -- of which she says the president consumes "eight cans a day, at least" -- with dementia and stroke risk into Trump's briefing folder. In response, then-White House staff secretary Rob Porter told her: "Stop putting articles in the president's folder. You have to go through me first. Don't do it again."

In January of this year, White House physician Ronny Jackson proclaimed that Trump was in "excellent" overall health and said he had "absolutely no concerns about the president’s cognitive ability or his neurological function."

In her book, Manigault-Newman dismisses Jackson's diagnosis, writing that Jackson's "job depended on Trump's approval of him" and claiming that "his loyalty was rewarded" when Trump nominated Jackson as Secretary of Veterans Affairs. Jackson later withdrew his nomination after being accused of unprofessional behavior.

Fox News' Brooke Singman contributed to this report.