Children’s reward circuits may signal weight gain linked to ultra-processed foods

A small study found that children who ate more ultra-processed food showed stronger reward-region responses to food images, which were associated with greater gains in body fat and BMI over the following year.

Children who ate more ultra-processed foods showed stronger activity in a brain region tied to reward and motivation when they were shown images of foods such as biscuits and chips, according to a new study published in JAMA Network Open on 23 October 2026. That heightened response was linked to greater increases in body mass index and body fat over the next year, suggesting a possible biological pathway between heavily processed diets and weight gain in childhood.

The research was small, involving 46 children aged seven to 10 in Southern California, so it cannot prove that ultra-processed foods caused the brain changes or the later weight gain. Even so, the findings are notable because the children started the study at a healthy weight, allowing researchers to look for early markers before obesity developed. The team, based at the Keck School of Medicine of USC, also found that the children consumed an average of about 68% of their daily calories from ultra-processed foods, a level close to the 67% average previously reported for children in the United States.

To measure the effect, researchers drew on data from BrainChild, a long-running study of diabetes and obesity risk that recruited families through Kaiser Permanente Southern California. The group included many Hispanic or Latino children, reflecting the population served by the health system. Each child completed detailed 24-hour diet recalls, then underwent functional MRI scans after drinking a standard glucose drink, a step intended to keep hunger levels broadly similar during the imaging session.

The strongest signal appeared in the dorsal striatum, a region involved in reward learning and motivation. According to the study, children with higher ultra-processed food intake showed greater activity there when viewing food images, and that response was associated with larger increases in body mass index, body fat percentage and waist size over 12 months. When researchers accounted for the brain response, the apparent links between ultra-processed food and later gains in BMI and body fat became smaller, although the authors did not conduct a formal mediation analysis because the sample was limited.

Kathleen A. Page, the senior author, said in a USC statement that the work goes beyond simply linking processed food intake with weight gain and points to the neurobiological mechanisms that may help explain the pattern. But the study also had important limits. The direct associations between ultra-processed food intake and later changes in BMI and body fat did not reach statistical significance, and once added sugar was taken into account, the link between ultra-processed food and the brain response also weakened.

That caution is echoed by earlier research. A 2021 prospective study in the British Journal of Nutrition, using data from more than 1,100 children in Portugal’s Generation XXI cohort, found that higher ultra-processed food intake at age four was associated with greater food responsiveness at age seven and with higher BMI by age 10. Other reviews and observational studies published in recent years have also pointed to possible links between these foods, appetite regulation and childhood obesity, though they have not settled the question of cause and effect.

Russell J. de Souza and Anthea Christoforou of McMaster University, writing an invited commentary, praised the care taken in measuring the children’s diets but warned that findings from such a small sample can be unstable and may overstate the strength of an association. They described the results as potentially useful, while stressing the need for replication in larger studies.

For parents and caregivers, the study does not mean an occasional packet of crisps or sweet snack is damaging a child’s brain, nor does it support banning processed foods altogether. It does suggest that the overall balance of a family’s diet matters, and that shifting some meals and snacks towards less processed options, such as fruit, plain yoghurt, nuts or home-cooked dishes, may be sensible when time and budget allow. The researchers noted that ultra-processed foods are often the cheapest and most convenient option, underlining how cost and access shape what families eat. Page said changing the food environment so healthier, less processed foods are easier to obtain is part of the solution.

The next step, according to the USC team, is an intervention study that would give children who regularly eat ultra-processed foods a more minimally processed diet and see whether brain responses and weight trajectories change. The BrainChild project will also continue to follow participants through puberty into young adulthood, which may help clarify whether the brain changes seen in this study are a cause of weight gain, a consequence of it, or part of a feedback loop between the two.