WASHINGTON — Better not introduce him to Roundup.

President Trump defended his unapologetic love for diet soda by posing a bonkers theory that the fizzy beverage may kill cancer in the body, Centers for Medicare & Medicaid Services Administrator Dr. Mehmet Oz revealed.

Oz, 65, recalled how Trump, 79, has made a point of showing off his candy stash and pressing his red button in the Oval Office to bring in a diet soda when meeting with the CMS boss and Health and Human Services Secretary Robert F. Kennedy Jr.

“Bobby and I usually go to the meetings together,” Oz said on Donald Trump Jr.’s “Triggered” podcast.

“Then comes the diet soda pops, which your dad argues that diet soda is good for him because it kills grass [when] it’s poured on grass, so therefore, it must kill cancer cells inside the body.”

Diet soda is viewed by the medical community as an unhealthy beverage, with some research associating it with weight gain and insulin resistance, which can lead to Type 2 diabetes.

Kennedy, 72, and Oz have been top crusaders in the Make America Healthy Again movement, seeking to inspire Americans to steer clear of processed foods and improve their dietary habits.

But their attempts to push Trump to change his own diet have fallen flat.

“We were on Air Force One the other day, and I walk in there because he wants to talk about something, and he’s got an orange soft drink on his desk,” Oz recounted, referring to Fanta. “I said, ‘Are you kidding me.’”

“So he starts to sheepishly grin. He goes, ‘You know, this stuff is good for me, it kills cancer cells.’ And then he tells me, ‘It’s fresh squeezed, So how bad could it be for you?’”

Trump Jr. laughed at his father’s wild health theory, but credited him for being energetic for a near-octogenarian.

“I think even if he’s not going to adhere to a perfect diet himself, I think he wants people to at least have the knowledge,” the Trump scion said.

Oz suggested that Trump is healthy, harking back to the 2016 campaign cycle, when the TV doctor examined the future president.

“He was in perfect health. I mean his testosterone, quite frankly, was through the roof not taking any supplements,” Oz reflected.

The 47th president is well known for his quirky medical beliefs, reportedly theorizing that exercise can be unhealthy because people are born with a finite amount of energy — a view described as Trump’s “battery” theory.

He’s also a fan of fast food, having ordered McDonald’s, a restaurant he worked at for a day in 2024, to the White House on Monday via DoorDash.

Shortly after his 2024 victory, a beaming Trump posed next to a distraught-looking Kennedy and other allies with McDonald’s meals aboard his private plane.

“He has the constitution of a deity. I don’t know how he is alive,” RFK Jr. told “The Katie Miller Podcast” earlier this year about the president’s “unhinged” diet.

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