Eleven- and twelve-year-olds who walked more had fewer mental health symptoms a year later, with no sign of the benefit levelling off — and slightly thicker tissue in two brain regions that partly explained the link.

Among 5,515 American adolescents who wore a Fitbit for three weeks around the age of 11 or 12, those who walked the least had the most mental health symptoms a year later. Those averaging about 8,000 steps a day had fewer. Those reaching 12,000 or more had the fewest of all — and the relationship showed no sign of flattening out at the top of the range the researchers could measure.

The study, published in JAMA Network Open by a team at King’s College London’s Institute of Psychiatry, Psychology & Neuroscience working with the University of Massachusetts Amherst, drew on the US Adolescent Brain Cognitive Development Study, a long-running project running across 21 sites that combines wearable data, brain imaging and repeated psychological assessment.

What was measured, and when

The design has a feature that matters for interpretation: the step counts came first and the mental health assessment came a year later. Symptoms were measured with the Child Behaviour Checklist, a standard instrument covering anxiety, low mood and behavioural problems, completed by both the adolescents and their parents.

The researchers also found a structural correlate. Adolescents with the highest step counts had slightly thicker brain tissue in two regions involved in movement and in emotion regulation, and that difference statistically accounted for part of the association between walking and mental health.

“This study gives us some of the clearest evidence yet that even an everyday, low-cost, accessible measure of physical activity like daily step count is tied to how adolescent brains and mental health develop,” said Dr Divyangana Rakesh, Senior Lecturer in Neuroscience and Psychology at King’s College London and the study’s senior author. “What’s striking is that we don’t see this benefit levelling off in the observed range, even among the most active young people in our sample.”

Qingyang Li, the study’s lead author, was more cautious about the brain finding. “We’re starting to understand parts of the brain that might be involved in the link between daily steps and young people’s mental health,” Li said. “It’s preliminary, but it gives us a clearer idea of where to look next.”

No effect on schoolwork

One result ran against a common assumption. The study found no link between step count and academic performance. Arguments for school physical activity often lean on claimed academic benefits; on this measure, in this cohort, there were none to find. The mental health association stood on its own.

What an observational study can and cannot say

The researchers are explicit that the design cannot establish that walking more improves mental health. The association survived adjustment for earlier mental health problems, which is a meaningful check — it argues against the simplest version of reverse causation, in which already-struggling adolescents simply move less. But adjustment is not elimination. Adolescents who were doing better in ways the Child Behaviour Checklist did not capture may have been both more active and less likely to develop symptoms, and no statistical control can fully rule that out.

The direction of causation is genuinely uncertain here, and plausibly runs both ways. Depression and anxiety reduce activity; activity may protect against them. A year’s separation between measurements helps, but an observational study with one baseline and one follow-up cannot resolve a loop.

The brain finding deserves similar care. The differences in cortical thickness were small, and cortical thickness in early adolescence is a moving target — it changes substantially with normal development, and differences between individuals are influenced by a long list of factors. That a small structural difference statistically mediates part of an association is a hypothesis about mechanism, not a demonstration of one.

A further limitation constrains the headline claim. The researchers note that few adolescents in the sample took extremely high step counts. The observation that benefit “doesn’t level off” therefore applies to the range the data actually covered; it says nothing about whether 20,000 steps would be better than 12,000, and it should not be read as an argument for pushing children toward ever-higher targets.

The practical reading

Dr Amanda Paluch, an Associate Professor of Kinesiology at the University of Massachusetts Amherst, framed the finding in terms of increments rather than targets. “It is not an all-or-nothing situation,” she said. “Adolescents don’t need to be elite athletes to benefit.” Her reading of the data is that “1,000 to 2,000 higher daily step increments, from any starting point, was associated with better brain structure and mental health outcomes, and that beneficial association keeps building from there.”

That framing is the useful one, because it converts a population-level association into something a family might act on without turning it into a prescription. An extra thousand steps is a walk to school, or a detour on the way home.

“Adolescence is a key period for brain development and mental health, so it’s encouraging to see that physical activity’s benefits extend to both,” Paluch said. “The goal is finding realistic, sustainable ways to move more, through everyday opportunities like walking to school, active play, and activities kids actually enjoy, so these benefits are accessible wherever an adolescent is starting from.” The release does not state her role in the study.

Step counts have a practical advantage over most measures of physical activity: they require no judgement, no recall and no self-report. A pedometer counts what happened. For research into adolescent mental health, where self-report is unavoidable on the outcome side, having an objective measure on the exposure side is worth something — even when, as here, the design still cannot tell you which way the arrow points.

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Notes

1. Li, Q., et al. “Physical Activity and Adolescent Brain Structure, Mental Health, and Academic Performance.” JAMA Network Open, 1 October 2026. DOI: 10.1001/jamanetworkopen.2026.36973.
2. King’s College London. “Higher daily step counts linked to fewer mental health problems in early adolescence, study finds.” EurekAlert!, 1 October 2026. https://www.eurekalert.org/news-releases/1146196.