But when Robert F. Kennedy Jr., the health secretary, convened a group of women this month for a discussion on nutrition and other topics, a healthy-eating activist who calls herself “the Food Babe” was stunned to see President Trump’s press secretary with her eight-month-old on her lap.

While several female cabinet secretaries looked on, the press secretary, Karoline Leavitt, lamented that baby formula seems healthier in Europe than in the United States, where a recent study found that many varieties are laden with added sugars. Last week, Mr. Kennedy met with formula makers and announced a push to expand options for “safe, reliable and nutritious infant formula.”

The activist, Vani Hari, was thrilled. “It was such an amazing opportunity to see some solidification of the MAHA agenda across the different cabinets,” she said, using the initials for Mr. Kennedy’s “Make America Healthy Again” movement. She called the event “a dream come true.”

The gathering of “MAHA Moms,” as Mr. Kennedy calls the corps of influencers and activists who follow him, was one of a series of choreographed events held in recent weeks by Mr. Kennedy, who occupies a highly unusual place in Washington. The scion of a famous Democratic family, his embrace of Mr. Trump, his tendency to spin wild theories out of kernels of truth and his promotion of what critics say is quack medicine have made him one of the most polarizing figures in the cabinet, even as he has developed a loyal following of his own.

Vani Hari, who attended Mr. Kennedy’s nutrition summit at the White House, called the event “a dream come true.”Credit…Mike Belleme for The New York Times

Yet even some critics of Mr. Kennedy applaud his focus on obesity and healthy eating. He makes powerful industries and civil servants uncomfortable, holding forth from his newly powerful perch as head of the Department of Health and Human Services on an eclectic menu of topics — offering up alternative remedy ideas one day while blasting industrial food companies the next.

Now companies and the government must contend with what might be called the Kennedy factor. So far, there has been little public pushback.

The Centers for Disease Control and Prevention posted information about vitamin A on its website after Mr. Kennedy promoted it as a measles treatment, to the consternation of public health officials who want him to advocate forcefully for vaccination.

A fast-food chain announced it had “RFK-d” its French fries by ditching seed oil for beef tallow, a type of rendered beef fat that is similar to lard, despite cardiologists who say it poses risks to the heart.

Infant formula makers, who came under scrutiny amid a shortage in 2022, said simply that they look forward to working with Mr. Kennedy. And after Mr. Kennedy instructed food executives to rid the food supply of artificial dyes, he followed up with a video message on social media: “They understand that they have a new sheriff in town.”

Mr. Kennedy declined an interview request.

It is far too soon to know whether Mr. Kennedy will make a real impact or whether these early steps are more posturing than substance. The Trump administration is taking actions that would seem to undermine his goals, such as disbanding an expert committee studying how to spare infants from a deadly bacteria that contributed to the decision in 2022 to temporarily shut down an Abbott Nutrition infant formula plant.

Mr. Kennedy could run into resistance from Congress. His disdain for the refined oils made from certain plants — seed oils like canola, soy and corn — and the ultra-processed foods that contain them has alarmed Republicans including Senator Charles E. Grassley of Iowa, whose farmer constituents receive subsidies from the government to grow the plants that produce the oils.

Mr. Kennedy opposes the subsidies. Mr. Grassley publicly instructed him to “leave agricultural practice regulations to the proper agencies,” including the Agriculture Department. Mr. Kennedy said he agreed.

“That’s talk; I want to see what the action is,” Marion Nestle, an emeritus professor of nutrition, food studies and public health at New York University, said of Mr. Kennedy’s ambitions to remake the food supply. “And if the only action is getting colors out of the food supply, that’s not enough.”

Infant formula makers, who came under scrutiny amid a shortage in 2022, said that they look forward to working with Mr. Kennedy.Credit…Gabriel V. Cárdenas for The New York Times

Public health experts still have serious concerns about Mr. Kennedy, whose skepticism of vaccines has colored his response to a Texas measles outbreak. Biomedical researchers say that if he really wanted to make America healthy, he would block Elon Musk’s Department of Governmental Efficiency from targeting the nation’s scientific enterprise by reducing jobs and cutting grants.

“I think he has to take the blame for it; he’s destroying science in America,” said Dr. Walter C. Willett, a pioneer in nutrition research at Harvard’s School of Public Health.

Yet so long as he is not talking about vaccines, Mr. Kennedy’s ideas are winning cautious support in some surprising places. Dr. Willett said he agrees with Mr. Kennedy that the National Institutes of Health should rebalance its research portfolio to spend more studying ways to prevent disease. Dr. Nestle praised him for taking on the food industry.

“When President Trump announced on Twitter that he was appointing R.F.K. Jr., he used the words industrial food complex,” she said. “I couldn’t believe that. It sounded just like me, and R.F.K. sounds just like me.”

At the height of the coronavirus pandemic, Mr. Kennedy was identified as one of the top spreaders of misinformation by the Center for Countering Digital Hate, which listed him as one of the “disinformation dozen.” His Instagram account was suspended in 2021, and reinstated in 2023 when he began his presidential bid.

Now, as the health department leader, Mr. Kennedy has a much bigger platform from which he can shape American attitudes and beliefs.

Some of his assertions, public health experts say, have been just plain wrong. Mr. Kennedy, for instance, told Sean Hannity of Fox News that immunity to the measles vaccine wanes over time and thus “older people are essentially unvaccinated.”

Mr. Kennedy appears to be largely ignoring government experts on the measles outbreak in Texas.Credit…Desiree Rios for The New York Times

That contradicts the C.D.C. website, which says measles, mumps, rubella vaccines “usually protect people for life” against measles and rubella, but mumps immunity may decrease over time. Dr. William Schaffner, an infectious disease expert at Vanderbilt University, agreed, saying, “The data continue to support that measles vaccine protects the vast majority of people lifelong.”

Last week, Mr. Kennedy proposed banning cell phones in schools, an idea with bipartisan backing. But in addition to citing children’s mental health, he made another, unsubstantiated claim: that cell phones “produce electromagnetic radiation” that can cause cancer.

So far, Mr. Kennedy also appears to be largely ignoring government experts. He has not had any in-person or virtual briefings on measles from infectious disease experts at the C.D.C., according to two people familiar with the response to the Texas outbreak. Instead, he receives written reports from the agency.

An administration official said Mr. Kennedy meets daily with “career leadership” at H.H.S., the C.D.C.’s parent agency, to discuss matters including measles.

Health officials in Texas say Mr. Kennedy’s messages have been confusing. Dr. Katharine Wells, the director of public health in the city of Lubbock, said she is having trouble persuading parents to vaccinate their children because they think “vitamin A is protective, like the vaccine.”

But Kennedy allies were thrilled when the C.D.C. added a mention of vitamin A in its measles advisory on its website. Del Bigtree, Mr. Kennedy’s former communications director, lauded the move on a recent podcast. “My God,” he said, “do you see what a small step for mankind we just made?”

Mr. Kennedy is getting quiet advice from at least one person in the public health mainstream, Dr. Jeffrey D. Klausner, a professor at the University of Southern California who spent years with the C.D.C., including work on disease prevention in South Africa. Dr. Klausner, a neighbor of Mr. Kennedy’s in Los Angeles, said he is working to identify new members of the C.D.C.’s vaccine advisory committee, a panel Mr. Kennedy says is rife with conflicts of interest.

He said Mr. Kennedy has given him just one criteria: “He wants highly credentialed, unbiased people who can look at the science objectively.”

Despite his promise of “radical transparency,” Mr. Kennedy is offering Americans a highly curated version of himself. Like Mr. Trump, he speaks to the public largely through social media and Fox News.

In a sense, Mr. Kennedy is offering a new twist on Mr. Trump’s “Make America Great Again” slogan; he wants United States customers to be able to buy Froot Loops, the colored sugary cereal, with the same ingredients — dyes made from colored blueberries and carrots, instead of chemicals — used in Canada, and French fries to be cooked like they are in Europe.

Mr. Kennedy’s crusade against seed oils has caught the eye of executives at Steak ’n Shake, which says it will now cook its fries in Mr. Kennedy’s preferred frying agent, beef tallow — even though nutrition experts say there is no evidence that tallow, a saturated fat akin to butter, is healthier than seed oils.

“He says he’s following the science,” Dr. Willett said. “If you look at the scientific evidence, that doesn’t take you to the conclusion that beef tallow is better than seed oils.”

So long as he is not talking about vaccines, Mr. Kennedy’s ideas are winning cautious support in some surprising places.Credit…Kenny Holston/The New York Times

An Indianapolis-based restaurant chain, Steak ’n Shake announced the switch this month on social media with a picture of a ball cap in Mr. Trump’s signature MAGA red that declared, “Make Frying Oil Tallow Again.” Mr. Kennedy, who otherwise appears to be no fan of French fries, traveled to a Florida Steak ’n Shake with Mr. Hannity, of Fox, and picked away at a basket of them for the cameras.

“We’re very grateful to them for R.F.K.-ing their French fries,” he said.

Ms. Hari, the healthy-eating activist, called the Steak n’ Shake announcement “an interesting example of how we can make incremental changes to the food system to make it better than it was.” She said she intends to push Mr. Kennedy to make fast-food chains post all of their ingredients online.

Mr. Kennedy’s inner circle seems to be divided into two camps: those like Mr. Bigtree, who are drawn to him because of his stance on vaccination, and those like Ms. Hari and Calley Means, an author and health care entrepreneur, whose focus is nutrition and chronic disease. Mr. Means recently joined the White House as a special government employee to help carry out Mr. Kennedy’s agenda.

Mr. Kennedy has also inspired a MAHA movement in the states. On Monday, the governor of West Virginia signed legislation banning certain food dyes from school lunches.

Last week, Mr. Means was in Arizona, along with other Kennedy allies, to speak in favor of a “Make Arizona Healthy Again” bill that would ban certain chemicals from school lunch programs and prohibit candy, soda, chips and other junk foods from being purchased with the federal nutrition dollars formerly known as “food stamps.”

Helene Leeds, who with her daughter founded Step It Up, a weight loss program, also testified, and was identified as a “MAHA Mom” by the MAHA Alliance, a group that backs Mr. Kennedy’s agenda. The moniker gave her pause.

“It’s new for me to be called that,” she said. “I mean, absolutely, I stand for health in everything that I do.” She added: “I also look at myself as a MAHA leader.”

After the MAHA Moms meeting, the White House posted video of Mr. Kennedy and some of his guests on social media stumbling over how to pronounce food ingredients like riboflavin. Mr. Kennedy posted photos with a message to the women: “You got me where I am today, and I will not let you down.”