Gov. Gavin Newsom of California on Wednesday signed into law a set of rules aimed at reducing ultraprocessed foods in schools. The legislation comes as many health experts and consumers are becoming increasingly concerned about ultraprocessed foods, which make up 62 percent of the calories U.S. children consume.

Health Secretary Robert F. Kennedy Jr. has said that schools have become too reliant on ultraprocessed foods, calling them “poison.” The foods have been linked to a range of health risks including obesity, Type 2 diabetes and cardiovascular disease. But regulating them is challenging, in part because doing so requires a precise definition, which experts haven’t settled on.

The lack of a legal definition is a major issue in nutrition right now, said Lindsey Smith Taillie, a nutritional epidemiologist at the UNC Gillings School of Global Public Health.

The California law has tackled that problem by including what experts say is the first science-based legal definition of ultraprocessed foods in the United States — a template that other states or even federal lawmakers could follow. Starting July 2035, certain foods that are deemed ultraprocessed by this new definition will be banned from most K-12 schools in California.

The law may also have ripple effects in other states, since products reformulated to meet California’s standards could end up in more school cafeterias across the country.

Banning the ‘Worst of the Worst’

The Real Food, Healthy Kids Act, as the new legislation is called, defines ultraprocessed foods as products that contain one or more of certain additives — such as synthetic food dyes, flavoring agents, emulsifiers, thickeners and stabilizers — as well as high amounts of saturated fats, sodium or added sugars, or any amount of sugar substitutes like artificial sweeteners or sugar alcohols.

The research on the health effects of food additives is complex and often limited, so it’s not clear how or even if many of them cause harm. One advantage of this law is that it requires scientists to regularly sift through that research and consider how other countries have handled the same ingredients or products. Another advantage is the law’s focus on foods with high amounts of saturated fats, sodium and added sugars, which are well established as harmful to health.

Many products that food companies currently sell to schools, including certain corn dogs, breakfast sausages, cereals, flavored yogurts and packaged muffins, would be deemed ultraprocessed under the new definition.

But not all foods that fall under that definition will automatically be banned from schools. Scientists at the California Department of Public Health will have until June 2028 to determine which foods are the most concerning, targeting those to phase out from school cafeterias by 2035.

Companies behind those foods may choose to tweak their recipes — perhaps by removing certain additives or reducing the levels of added sugars or sodium — if they want to continue selling the products to schools.

The idea is to remove “the worst of the worst” ultraprocessed foods from school cafeterias, said Scott Faber, a senior official at the Environmental Working Group, a nonprofit organization that was one of the cosponsors of the legislation.

The Consumer Brands Association, which represents U.S. food and beverage manufacturers, opposed the legislation, saying that food companies may not be able to adjust all of their recipes to meet the new requirements, and that doing so may result in higher prices for schools.

Schools may also be prompted to consider cooking more meals from scratch, which is a good thing, said Amanda Harvey, the director of nutrition services at the Fresno Unified School District in California. That can be challenging, however, if the school lacks adequate resources, she added.

A State Law With Far-Reaching Potential

The new legislation could influence what happens in other states. If food companies reformulate their products to meet California’s requirements, it may prompt changes in other schools — or even grocery stores — nationwide.

“It’s a powerful signal to all food companies, regardless of whether they manufacture for schools or not,” Mr. Faber said. “Smart companies will start reformulating now.”

The law may also serve as a precedent for legislators to reduce ultraprocessed foods in school meals in other states.

Arizona, Utah and Louisiana approved laws this year that ban foods containing certain ingredients from school meals, but that legislation flagged only synthetic food dyes and a handful of specific additives.

California’s law is more comprehensive and evidence based. It includes larger categories of food additives and has requirements for nutrients, including sodium, added sugars and saturated fats, said Dr. Taillie, who advised California officials on their definition of ultraprocessed foods.

California has led the way in regulating food ingredients. It was the first state to ban Red Dye No. 3, potassium bromate, propylparaben and brominated vegetable oil, in 2023, for instance, and it banned certain food dyes in school meals in 2024. At least 30 other states have considered similar legislation since, Mr. Faber said at an event celebrating the new law on Wednesday.

Mr. Kennedy has said that the Food and Drug Administration would establish a federal definition of ultraprocessed foods by the end of 2025, which could allow the Trump Administration to address concerns with the foods. But Mr. Faber is not so optimistic.

“It seems clearer every day that states are going to lead the way,” he said.