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Obesity in children may be linked to brain structure

Wednesday 11 December 2019

"Obese children have a thinner region of their brain which controls decision-making," reports Mail Online.

Researchers examined brain scans of more than 3,000 children aged 9 and 10 years old in the US. They looked at the thickness of their brain in areas called the cortex, and compared these to each child's body mass index (BMI).

The researchers found that 18 out of 66 cortex areas, mainly in the very front of the brain (prefrontal cortex), were significantly thinner in children with a higher BMI.

Some previous studies have suggested that children with higher BMI have thinner brain cortex areas, although results have been mixed.

The children in this study carried out a series of tasks designed to test their brain function around memory and decision-making. The researchers wanted to see whether thickness of the cortex explained links between BMI and this kind of brain function.

Children with a higher BMI did less well on 3 of the 4 tasks. Thickness of the cortex seemed to explain some of the variation for 1 of the tests.

The researchers say, however, that they cannot be sure that obesity caused thinner prefrontal cortex, and that this caused poorer test performance. Other factors such as the children's environment and metabolism could be involved.

Where did the story come from?

The study was carried out by researchers at the University of Vermont and Yale University in the US. It was published in the peer-reviewed journal JAMA Pediatrics. We have no information about the funding of the study.

Mail Online reported the study with reasonable accuracy but did not provide all the information to help put it in context. For example, it said that 18 brain regions were thinner, but did not say that 66 areas were measured.

The headline claim that a thinner prefrontal cortex "may make them less likely to say no to junk food" is stretching the researchers' conclusions, although the study does say that it could "contribute to poor dietary decision-making".

What kind of research was this?

This was a cross-sectional observational study. This type of study can spot patterns such as links between factors, but because the data is taken only at 1 point in time, it cannot tell us, for example, whether children had a thinner prefrontal cortex before or after they had a higher BMI. This, as well as other limitations, means we cannot tell whether 1 factor, such as obesity, causes another.

What did the research involve?

Researchers scanned the brains of 4,524 children aged 9 and 10, in 21 sites across the US, using structural magnetic resonance imaging (sMRI). Children were examined and had their weight and height measured. They also took part in tests designed to measure their memory, thinking and decision-making.

Researchers did not include children who had learning disabilities or substance misuse problems, were unable to see or hear, had been born prematurely, had birth complications or head injury. They also excluded children who had certain physical or mental health conditions, including diabetes, autism, muscular dystrophy and neurological conditions.

The researchers looked first for associations between the thickness of 66 brain regions and children's BMI. They then looked for associations between BMI and results of tests of memory and thinking. Finally, they looked to see whether differences in thickness of brain regions could explain variations between test results according to BMI.

They took account of the following potential confounding factors:

  • age
  • sex
  • ethnic background
  • intracranial volume (the size of the inside of the skull)
  • whether children were left- or right-handed
  • whether children had begun to go through puberty

What were the basic results?

After excluding children without full brain scan data and those with poor-quality brain scan images, researchers analysed 3,190 children, 51% of them boys.

Children with a higher BMI had a thinner cortex region overall. When researchers looked at the results by cortex region, 18 of the 66 regions studied were thinner among children with a higher BMI. The link was strongest for regions in the prefrontal cortex of the brain, thought to be responsible for complex tasks such as planning and decision-making.

Only 2,418 of the children scanned had full results from their memory and decision-making tests. For these children, researchers found that higher BMI was linked to lower scores for 3 out of 4 tests of memory and decision-making, but not for the 4th test.

When the researchers looked at the possible influence of prefrontal cortex thickness, they found that it could explain some of the link between BMI and 1 of the 4 test results. This test (list sorting) was a measure of working memory – the ability to keep certain facts held in the mind while carrying out a task.

How did the researchers interpret the results?

The researchers said that although the results could not demonstrate a causal relationship, "these findings suggest that BMI is associated with alterations in prefrontal cortex development and diminished executive function, such as working memory."

They add: "Deficits in working memory may in turn contribute to poor dietary decision-making. Once established, these associations may become mutually reinforcing and contribute to ongoing health issues that persist into adulthood".

Conclusion

This study, while interesting for researchers, does not really tell us anything that we can put into practice to help children stay at a healthy weight. We already know that children who are overweight or obese may grow into adults with weight and other health problems.

This study may point to some of the reasons why children who are overweight find it difficult to reach a healthy weight. But research into links between weight, brain structure and cognitive function is in the early stages.

The study is limited. It cannot tell us whether the children had thinner prefrontal cortex because they were overweight, or whether a thinner prefrontal cortex prompted them to put on weight.

It also cannot tell us whether poorer performance on tests was because of differences in the prefrontal cortex, or because of other factors associated with being overweight, such as metabolic function or environment.

Because the brain scans, tests and measurements were carried out only once, we cannot tell which happened first, or whether they changed over time.

What we do know is that it is easier for people to maintain a healthy weight as adults if they have been a healthy weight during childhood.

Eating a healthy, balanced diet and being active are important ways to achieve a healthy weight. Find out more about a healthy weight.

Analysis by Bazian
Edited by NHS Website