Trying to devise a one-size-fits-all description for ultraprocessed foods is flummoxing federal regulators.

For months, the health secretary, Robert F. Kennedy Jr., has promised to create a definition of ultraprocessed, a crucial part of the Make America Healthy Again agenda. In mid-April, he testified before Congress that the Food and Drug Administration had forwarded a definition to other agencies, including the Department of Agriculture.

But behind the scenes, officials said, the process of defining ultraprocessed foods is still very much up in the air. Agencies are struggling to agree, and it is unclear when a definition will be released.

“It’s not final until it’s final,” said Calley Means, a senior adviser to Mr. Kennedy, adding that the definition would ultimately be the result of hundreds of conversations with scientists, agency staff and other stakeholders.

For the food industry, which is already seeing demand for many of its products soften as some consumers cut back on spending while others use weight-loss drugs, the debate about what an ultraprocessed food is or isn’t has potentially far-reaching consequences.

Under one classification widely used among the scientific community, essentially any foods or drinks made with ingredients you wouldn’t find in a home kitchen are defined as ultraprocessed. If regulators adopt that sort of definition, nearly three-quarters of foods sold in the United States could be deemed ultraprocessed.

The food industry is arguing against a strict definition that would label chicken nuggets, strawberry yogurt and whole-grain tortillas as ultraprocessed.

Based on that definition, deli turkey could be categorized the same as a snack cake, the National Turkey Federation wrote in a comment letter to regulators last fall. It said that certain food additives and processing steps were critical to keep turkey fresh and that those “benefits are especially important for lower-income households, where access to nutrient-dense, high-quality protein can otherwise be limited.”

One fear that emerged in interviews with food companies, lobbyists and regulators, most of whom declined to be quoted, is that foods tagged as ultraprocessed could be restricted or eliminated entirely from the nation’s school meal programs. They are multibillion-dollar revenue streams for the companies that make sandwich breads, cereals, salsas and other foods. Others are worried that regulators could create new rules for warning labels on the packaging of foods in grocery stores.

But Mr. Kennedy, who has frequently referred to ultraprocessed foods as “poison” and has suggested restricting them from the diets of Americans, particularly children, is facing intensifying pressure from the MAHA movement, which is credited with helping elect President Trump to his second term. A Politico poll published in April showed that removing ultraprocessed foods from the American diet was a core principle for people identifying as MAHA followers.

“If we can have a federal definition that is strong and science-based, it opens the door for meaningful policy,” said Vani Hari, a health advocate who is known as the “Food Babe” online and is a prominent voice in the MAHA movement. And, she added, “anything positive that is coming out of the administration on food reform is popular with voters.”

Many scientists support a strong definition, and noted that evidence linking ultraprocessed foods to a host of chronic diseases, including obesity, Type 2 diabetes, heart disease and some types of cancer, has grown in the last decade.

Defining ultraprocessed foods is “one of the most important policy actions around food that the U.S. government has done for probably 25 years or more,” said Dr. Dariush Mozaffarian, a cardiologist and the director of the Food Is Medicine Institute at Tufts University. “Industry is going to fight tooth and nail because this is a fundamental threat to their entire model,” he added.

Dr. Mark Hyman, a physician and friend of Mr. Kennedy’s, called ultraprocessed foods the “single biggest threat to public health ever.” He characterized the food industry’s lobbying on the definition as “their usual shenanigans.” It is “obfuscating, confusing, undermining credibility of scientists,” he said.

While the food industry is trying to make its case to federal regulators, it is also facing a dizzying array of new regulations by states. Emboldened by the MAHA movement, many states are not waiting for the federal government. Instead, they are writing their own rules for food. Last year, Texas and Louisiana passed laws that required warning labels on foods that contained any of 44 additives, while California banned certain ultraprocessed foods from school lunch programs. Other states are weighing similar moves.

Dr. Mozaffarian at Tufts and some other health experts argue that the federal definition should adhere closely to a food classification system called Nova, which is used in the vast majority of the research linking ultraprocessed foods to poor health.

The Nova definition deems a wide swath of products ultraprocessed, including candy, soda and hot dogs. It also includes some foods traditionally considered to be healthy, like many whole wheat breads, peanut butters and yogurts.

The definition is focused primarily on ingredients. Peanut butter, if it contains nothing more than peanuts or salt, would not be considered ultraprocessed. But most peanut butters sold in grocery stores contain hydrogenated vegetable oils, kicking them into the ultraprocessed category, according to Nova. Likewise, plain yogurt would not be considered ultraprocessed. But when ingredients like emulsifiers and flavorings are added, the food becomes ultraprocessed.

In his testimony before Congress in mid-April, Mr. Kennedy said that once a definition was in place, his agency would move forward with a plan that would mandate placing color-coded labels on the front of food and beverage packages.

“If it’s a red light, don’t eat it,” he said during his testimony. Green, he said, would signal to consumers the food is healthy.

Mr. Kennedy also suggested that following a federal definition of ultraprocessed foods, states could restrict them from their food stamp programs — jeopardizing billions more in revenue to food companies.

In conversations with regulators and members of Congress, the food industry broadly asserts that an overly strict definition of ultraprocessed foods like the one from the Nova system could target nutritious foods, that more research is needed, and that increased regulation could result in higher prices for consumers.

Using a broad brush to categorize foods “fails to capture the important reality that all processed foods are not equal,” the cereal manufacturer WK Kellogg wrote in a comment letter last year when the F.D.A. asked about how ultraprocessed foods should be defined. The National Chicken Council wrote in another comment letter that “gummy bears and chicken nuggets in this comparison are technically ‘ultraprocessed’ by common definitions. Yet, it is clear which one has potential to contribute meaningful nutrients to a higher quality diet.”

Some of those arguments are gaining traction inside the Department of Agriculture, which oversees the nation’s school meal programs. Officials there say it is unclear if the definition will be tied to policy and, if it is, expressed concern that food manufacturers could quit the programs, leaving school officials with fewer choices for children’s lunches.

Lindsey Smith Taillie, a professor of nutrition at the University of North Carolina’s Gillings School of Global Public Health, who met with F.D.A. officials in mid-April, said there was a middle ground. She argues that regulators can still create a strong federal definition similar to Nova’s while constructing useful policies — such as for warning labels or school meals — by exempting products that are “healthy,” according to F.D.A. standards.

The F.D.A. defines a healthy food as one containing a certain amount of actual food — like fruits, vegetables or milk, as opposed to processed ingredients like cornstarch — and not too much saturated fat, sodium or added sugars.

Dr. Taillie said most ultraprocessed foods don’t meet the “healthy” definition. But a policy like this could motivate food companies to reformulate products to fit that criterion, she added.