Researchers from the New York University found that children showing physical changes due to being obese, such as raised blood pressure, higher levels of bad cholesterol and resistance to the blood sugar controlling hormone, insulin, had poorer scores on thinking tests, the Telegraph reported.
The study compared 49 children with metabolic syndrome, a collection of at least three health problems associated with obesity which can include a large waist, low good choelsterol, high blood fats, high blood pressure and insulin resistance which is a pre-cursor to type 2 diabetes.
The researchers concluded that even a few years of problems with metabolism may cause brain complications.
The study found that those classified as having metabolic syndrome showed significantly lower maths and spelling scores, as well as decreased attention span and mental flexibility.
Maths scores were ten points lower on average in the metabolic syndrome group and spelling scores were four points lower.
There was also a tendency towards lower overall IQ but memory was not affected.
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They also found differences in brain structure and volume, with the metabolic syndrome groups showing a smaller hippocampus which is involved in the learning and recall of new information, and other changes.
The children were all from similar socio-economic backgrounds, the same age and at the same school grade.
"The kids with metabolic syndrome took longer to do tasks, could not read as well and had poorer math scores," lead researcher, Dr Antonio Convit said.
"These findings indicate that kids with metabolic syndrome do not perform well on things that are very relevant to school performance," Covit said.
"The message is that just being overweight and obese is already impacting your brain," Covit was quoted as saying by the paper.
"Kids who are struggling with their weight and moving toward having metabolic syndrome may have lower grades, which could ultimately lead to lower professional achievement in the long run," Covit added.
The study was published in the journal Pediatrics.