WASHINGTON – A former White House doctor says that while he regrets being unable to get President Donald Trump to exercise more, he did manage to slightly improve the president's diet.

"The exercise stuff never took off as much as I wanted it to," retired Navy Rear Adm. Ronny Jackson told The New York Times.

"But we were working on his diet. We were making the ice cream less accessible, we were putting cauliflower into the mashed potatoes."

Jackson did not say whether the president, now 73, was aware the vegetable had been added to his starchy side, the Times reported.

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Jackson joined the White House medical team in 2006 and became its director in 2011. In 2013, he became President Barack Obama's physician and kept that role under Trump.

After Trump's first physical in office, Jackson declared Trump "fit for duty" in a news conference. In a summary of his exam, he said the 6-foot-3-inch, 239-pound president's "overall health is excellent."

White House doctor:President Trump 'is in excellent health' and physical 'went exceptionally well'

Amid public questions about Trump's mental health, Jackson told reporters that tests revealed "no cognitive or mental issues whatsoever." But he said he told Trump he should try to lose 10-15 pounds.

"We talked about diet and exercise a lot," Jackson said at the time. "He's more enthusiastic about the diet part than the exercise part, but we're going to do both."

Jackson's inability to get the president to boost his exercise routine is not surprising given Trump's past opposition to fitness routines. Trump reportedly believes the human body has a finite amount of energy that can be used up by exercise and in 2015 he told The New York Times Magazine that his friends "who work out all the time" are "a disaster" and have to have frequent joint replacement surgeries.

As the oldest first-term president, Trump's lack of exercise and affinity for fast food raised concerns about his health during the 2016 campaign.

But Harold Bornstein, Trump's personal doctor prior to Jackson, declared in a December 2015 letter that if Trump won he would be "the healthiest person ever elected to the presidency." Bornstein later admitted that the hyperbolic language was not written by him but by Trump.

In March 2018, Trump announced he was appointing Jackson to take over the Department of Veterans Affairs, but Jackson withdrew his name from consideration after allegations surfaced that he had doled out opioids without prescriptions and drank alcohol while on duty. Jackson said the allegations were "completely false." An investigation into Jackson was opened by the Pentagon inspector general in June 2018 and remains ongoing.

Jackson is now running in a crowded primary field for Texas' 13th District, a congressional seat that opened up after 13-term Republican Rep. Mac Thornberry announced he would not seek reelection.

Contributing: David Jackson, Jayne O'Donnell and Savannah Behrmann, USA TODAY; The Associated Press

Jackson campaign for Congress:Former White House physician Ronny Jackson files to run for Congress in Texas