Ultra-Processed Food Before Age 5 Predicts Anxiety and Aggression at School Age

Nearly half of what Canadian preschoolers eat every day comes from a factory, not a kitchen. Packaged snacks, flavored drinks, fast-food staples - these industrial formulations now account for close to 50% of toddlers calorie intake across Canada. For years, the concern has been physical: obesity, metabolic disease, cardiovascular risk. A large new study published in JAMA Network Open shifts the lens to the mind.

What 2,000 Canadian Children Reveal About Diet and Behavior

Researchers at the University of Toronto drew on the CHILD Cohort Study, which enrolled pregnant women between 2009 and 2012 and followed their children across four Canadian sites. The team examined dietary records from more than 2,000 children at age three, then assessed the same children two years later using the Child Behavior Checklist - a validated clinical tool for measuring emotional and behavioral wellbeing.

The pattern was consistent and dose-dependent: for every 10% increase in calories from ultra-processed foods at age three, children scored measurably higher at age five on internalizing behaviors (anxiety, fearfulness, withdrawal), externalizing behaviors (aggression, hyperactivity), and overall behavioral difficulty. Higher scores mean more reported problems. "The preschool years are critical for child development, and it is also when children begin to establish dietary habits," said Kozeta Miliku, principal investigator and assistant professor of nutritional sciences at U of T Temerty Faculty of Medicine.

Not All Ultra-Processed Foods Are Equal

When the team broke results down by category, sugar-sweetened beverages and artificially sweetened drinks showed the strongest associations with behavioral difficulties. Ready-to-eat and ready-to-heat foods - the French fries and macaroni-and-cheese aisle of childhood - ranked next. The findings held after accounting for a range of potential confounders, though as with any observational study, residual factors cannot be entirely ruled out.

The team also ran statistical simulations: what if a child swapped just 10% of ultra-processed food calories for minimally processed alternatives such as fruit or vegetables? The models suggest even that modest shift is associated with lower behavioral difficulty scores. Not a cure, but a measurable difference.

Mechanisms Are Still Being Worked Out

The biology remains unconfirmed in humans. Researchers hypothesize that ultra-processed foods tend to be low in nutrients critical for neurodevelopment, including omega-3 fatty acids, iron, and zinc. They often contain additives that some research links to gut microbiome disruption, which in turn may influence brain function through the gut-brain axis. But this study did not measure those pathways directly - it tracked diet and behavior, leaving mechanism as an open question for follow-on work.

What the study establishes is a prospective association in a large population-based cohort. The authors describe it as the first study to combine detailed prospective dietary data with standardized behavioral assessments in young children at this scale.

Structural Change, Not Just Individual Choice

Miliku is deliberate about not framing this as parental failure. "Parents are doing their best and not all families have access to single-ingredient foods, or the tools and time needed to incorporate them," she said. Ultra-processed foods are cheaper, faster, and more aggressively marketed than whole foods - conditions no individual parent created.

The recommendations point toward systems-level change: professional guidance for caregivers, public health campaigns, nutrition standards for child-care providers, and food industry reformulation. For families with options, small changes count. Adding fruit or swapping a sugary drink for water may, over time, support emotional and behavioral development. First authors were Meaghan Kavanagh, a postdoctoral fellow, and Zheng Hao Chen, a PhD student. Funding came from the Canadian Institutes of Health Research and a Temerty Faculty of Medicine pathway grant.