You might know a tomato when you see one, but for the federal government, identifying one is harder. Is a tomato a fruit or a vegetable? In 1893, this question went all the way to the Supreme Court, which decided that, for tax purposes, at least, a tomato was a vegetable. What about milk? For a bureaucrat setting standards, a straightforward explanation such as “liquid from a cow’s udders” won’t quite cut it; according to the FDA, milk is “the lacteal secretion, practically free from colostrum, obtained by the complete milking of one or more healthy cows.”

Even for the government, however, tomatoes and milk are relatively simple to define compared with ultra-processed foods. Although many researchers agree that these creations are negatively affecting people’s health, they cannot agree on what foods do and don’t belong in that category. That’s not keeping Robert F. Kennedy Jr. from trying: After months of promising the federal government’s first formal definition of ultra-processed foods, the secretary of Health and Human Services announced on Monday that the department is finalizing one.
He stopped short, though, of sharing the exact text in either the public announcement or at a “food policy celebration” held Monday at HHS’s Washington headquarters; the definition is still being reviewed by the White House. This effort may have real upsides. Having a regulatory definition of ultra-processed foods (or UPF, as they’re often referred to) could at the very least advance inquiries into the harms of these products. But the effort also raises the question: Can the government adequately define a problem before researchers know its true cause?

So far, scientific efforts to define UPF have revealed only how fuzzy the category is. The most commonly used definition, which was developed by Brazilian researchers, takes the most expansive approach: It lumps all foods with industrial ingredients, such as sweeteners and emulsifiers, into the ultra-processed category. A classification system created by academics at the University of North Carolina at Chapel Hill, meanwhile, defines “highly processed foods” as products “processed to the extent that they are no longer recognizable as their original plant/animal source.” Yet another well-respected system, this one designed by a French analytics company, attempts to incorporate additional factors, such as a food’s salt, sugar, and fat content, as well as the “function, number, and potential health risk of additives.”

These rubrics are distinct enough from one another that flavored yogurt is an ultra-processed food under the Brazilian system and a moderately processed food under the UNC system. Under the French system, the classification depends entirely on the yogurt itself (even though a paper from the group found that 95 percent of flavored yogurts in French grocery stores were indeed ultra-processed).

Researchers disagree on how to define these foods, because the science around how and why they may be harmful is ambiguous. Epidemiological studies have found that UPF-heavy diets have consistently been associated with a range of maladies including cardiovascular disease and type 2 diabetes. But although researchers suspect that certain additives are causing dietary issues, the number of industrial ingredients or additives in Americans’ food is so large that many more may also be causing trouble. Nor do researchers know what processing steps cause everyday food ingredients to go from healthful to harmful. That means Kennedy and his department are essentially trying to define a food group that no one really understands.

Kennedy is under no illusions about how little we know about the American food supply. He has frequently criticized a decades-old loophole that allows food companies to put new additives in their food without even telling the FDA. On Monday, he announced a new proposal to fix this issue; if finalized, the rule would require companies to let the FDA know when they’re introducing a new additive. But even that won’t fully solve America’s additive problem.