Children ages 9 to 11 who consumed caffeine had a slightly smaller brain surface area than children who consumed none. The difference showed up at intakes as low as about one cup a week, according to a study of 9,632 U.S. children. Soda, not coffee, was the main source of that caffeine, the researchers reported in Biological Psychiatry: Cognitive Neuroscience and Neuroimaging.

The study shows an association — not cause and effect. Caffeine was self-reported, the brain scans were taken at a single point in time, and soda comes with sugar and other habits the researchers could not fully separate from caffeine. Shorter sleep explained part of the link.

For parents, the finding is a reason to look at where caffeine shows up in a child's week, especially in soda. It is not a reason to panic over an occasional drink.

The Low Threshold and the Soda Signal

The team, from the Institute of Psychiatry, Psychology & Neuroscience at King's College London, used data from the Adolescent Brain Cognitive Development (ABCD) Study, a large federally funded project that follows children at 21 U.S. sites. Children reported how many cups of coffee, caffeinated tea, and caffeinated soft drinks they usually drank each week.

Intake was low overall. On average, children reported about 1.2 cups of soda and 0.2 cups of coffee a week. Even so, higher total caffeine was linked to lower overall cortical surface area and lower surface area in 22 regions across all four lobes of the brain. The overall difference was already visible at about one cup a week compared with none.

Soda drove most of the pattern — not coffee. Soft drinks alone were linked to lower surface area in 19 regions, and the authors wrote that the findings were "largely driven by soft drink intake." Coffee was linked to a thinner cortex in four frontal regions but not to surface area. Tea showed no link with any brain measure.

"We found that even fairly low levels of intake were linked to small differences in brain structure and later mental health," senior author Dr. Divyangana Rakesh said, according to Nutrition Insight.

Sleep Sits in the Middle of the Link

Children who consumed more caffeine slept fewer hours, according to their parents' reports. Shorter sleep accounted for part of the link between caffeine, especially soda, and smaller surface area. Caffeine was still linked to brain structure after the researchers accounted for sleep.

The study also tracked mental health. Children with higher caffeine intake reported more attention, mood, and behavior problems six and 18 months later. At six months, shorter sleep and smaller surface area together partly accounted for that link. By 18 months, only total surface area did.

The researchers adjusted for family income, parental education, age, sex, and baseline mental health. The results held. Still, first author Jiayi Zheng noted that "soft drinks contain a lot of sugar and often go hand in hand with less healthy lifestyle habits and more risk-taking behaviors." In other words, the soda itself may be a marker for other differences in a child's routine.

Outside Scientists Urge Caution

Independent researchers stressed the limits. Prof. Derek Hill, professor of digital health at University College London, said "the study design has significant limitations and care must be taken in interpreting the conclusions," according to the Science Media Centre. He added that the conclusions should be considered speculative.

Dr. Charalampos Rallis, a reader in genetics at Queen Mary University of London, said the observational design "cannot establish that caffeine causes changes in children's brains" and called the link with later mental health "negligible."

The authors list their own limits too. Children may not reliably know whether a soda is caffeinated, and caffeine was reported in cups, without brand details. Parent-reported sleep likely overestimates real sleep. The one-cup threshold needs to be replicated. The children in the final sample also slept longer, drank less caffeine, and came from higher-income families than the children who were excluded. Neither author reported conflicts of interest, and ABCD is funded by the National Institutes of Health.

Practical Steps for Families

Pediatric guidance already leans cautious. The American Academy of Pediatrics advises against caffeine for children under 12 and recommends no more than 100 milligrams a day for ages 12 to 18, about one standard cup of coffee, according to Seattle Children's. Energy drinks are the biggest concern because they can contain far more caffeine than soda.

The simplest steps are label reading and timing. Many colas, some citrus sodas, iced teas and chocolate drinks contain caffeine, and the label or the manufacturer's website will say how much. Caffeine late in the day can delay sleep, and sleep was the part of this study with the clearest practical link. Swapping in caffeine-free versions, water or milk, especially after lunch, is a low-cost change.

Parents should not read this study as evidence that one soda damages a child's brain. Surface area differences between children are normal, and the study did not measure any harm a child would notice. A pediatrician is the right person to talk to if a child has ongoing sleep trouble, mood changes, or attention problems.

The authors are calling for clearer caffeine guidelines for adolescents. The next step is replication: following children over several brain scans to see whether caffeine habits come before brain changes. ABCD's future data releases make that possible.

The bottom line: a large study found that small amounts of caffeine, mostly from soda, track with slightly smaller brain surface area and shorter sleep in preteens. It can't show that caffeine is the cause, but limiting soda and protecting sleep are sensible steps either way.

What Readers Want to Know

What did the study find?

Among 9,632 children ages 9 to 11, caffeine intake, mostly from soda, was linked to slightly smaller brain surface area and shorter sleep, along with more mental health problems later.

How little caffeine was linked to a difference?

Total surface area was lower at about one cup a week compared with none. The authors say this threshold needs to be replicated.

Does this prove caffeine harms children's brains?

No. The study was observational and cannot show cause and effect. Sugar, sleep and other lifestyle factors may play a role.

Was coffee or soda the bigger factor?

Soda. It was the main caffeine source and drove most of the surface area findings. Coffee was linked only to cortical thickness in four frontal regions.

How much caffeine do pediatricians recommend for kids?

The American Academy of Pediatrics advises against caffeine for children under 12 and recommends no more than 100 milligrams a day for ages 12 to 18.

What can parents do now?

Check labels for caffeine, avoid caffeinated drinks later in the day, offer caffeine-free options, and talk to a pediatrician about sleep or mood concerns.