Chile already puts black stop signs on junk food. Adding one that says 'ultraprocessed' still moved 3,519 parents, most of all on crisps with no warnings at all

Since 2016, packaged food in Chile has carried black octagons, shaped like stop signs, that say a product is high in sugar, salt, saturated fat or calories. The idea has spread across Latin America. But a bag can be ultraprocessed without being high in any of those, and makers can tweak a recipe to drop a warning without making the food any less industrial. After Chile's sugar warnings arrived, for example, fewer products qualified as high in sugar while the use of non-sugar sweeteners went up. So countries are now asking whether to add a separate label for ultraprocessing: Colombia's health ministry has proposed one and Chile's Senate is debating it. A randomised trial published in Frontiers in Nutrition on 2 November tested what such a label adds on top of the warnings Chileans already know.

Researchers from the University of North Carolina at Chapel Hill and the University of Chile recruited parents of children aged 2 to 14 through an online panel in July and August 2025, with half the places reserved for people with a high-school education or less. After quality checks, 3,519 were analysed: average age 39.6, 70% women. Everyone was first given a short government-style explanation of what 'ultraprocessed' means. Then each saw three made-up bags of sour cream and onion potato crisps, all of them ultraprocessed: one with no warnings, one 'high in sodium', and one high in sodium and saturated fat. Half the parents, chosen at random, saw only those nutrient warnings, as on Chilean shelves today. The other half also saw a white octagon reading 'ultraprocessed'.

For each bag, parents said whether they thought it was ultraprocessed, how likely they were to buy it in the next week and how good or bad it would be to eat it every day. The new label raised the share who correctly called the warning-free bag ultraprocessed by 12 percentage points, and the one-warning bag by 4 points. For the bag that already carried two black octagons it made no clear difference.

It also nudged buying intentions and health ratings down for all three bags, on 1 to 5 scales: by about a third of a point for the warning-free bag, and by smaller amounts as the black octagons piled up (0.23 and 0.20 for buying, 0.17 and 0.11 for health ratings). A third of a point sounds small, but across a sample this size the ranges were tight and every one of those drops was well clear of chance.

Why would a label about processing work less well on a bag that is also salty and fatty? Part of it is probably a ceiling: a product already wearing two warnings looks unhealthy, so there is less room to fall. But the authors note that even the plain question 'is this ultraprocessed?' followed the same pattern, which suggests many parents were simply counting labels rather than reading them. Earlier interviews with Chilean mothers found exactly that habit. The authors' own unpublished count suggests that more than 40% of ultraprocessed products in Chile carry no nutrient warning at all, which is where a new label would have the most to say.

The limits matter. People answered on a screen, not in a shop, and looked at one kind of snack, so real-world buying may differ. The sample was a convenience sample of parents, not a picture of all Chileans. The bags differed slightly in colour, name and pictures as well as warnings. And telling everyone first what 'ultraprocessed' means may have strengthened the effect, although that mirrors the public information campaign that would likely come with a real label. The authors also flag a possible side effect: a shopper might pick a sugary 'natural' product over a less sugary one that is ultraprocessed only because of a sweetener. The trial was registered in advance, funded by Bloomberg Philanthropies with support from the US National Institutes of Health, and the authors declared no conflicts of interest.