WASHINGTON — President Trump is a commander in chief who fuels himself with a steady stream of Diet Cokes, scoops of vanilla ice cream and slabs of red meat. He gets as little as five hours of sleep a night. He is not known to exercise more than the brief strolls beyond his cart on the golf course.

This, he and his aides have maintained, is the very picture of presidential stamina.

On Friday, Mr. Trump, 71, will undergo his first comprehensive physical examination as president, and the first formal check on his former doctor’s Trumpian 2015 campaign claim that he’d be the “healthiest individual ever elected” to the office. But the first year of Mr. Trump’s presidency has triggered private scrutiny over his sedentary lifestyle — friends have noticed an increase in his girth — and a public debate over erratic behavior that included the president’s recent tweet storm defending himself as a “very stable genius” after a book detailed his mercurial demeanor.

If the White House releases the same amount of information shared by other presidents, the public may soon learn more about how Mr. Trump’s lifestyle and fondness for fast food affects his health. It is less likely that the physical will yield in-depth insight into Mr. Trump’s psyche. The White House said that Mr. Trump will not undergo a psychiatric exam, and would not say if he would undergo other forms of cognitive testing that may screen for mental disorders, including dementia.

A review of annual checkups of presidents dating back to Jimmy Carter shows that there is no template to follow and no set precedent for administering physicals or reading out results. Like any other medical patient — and other presidents — what Mr. Trump ultimately reveals to the public about his health will be up to him.