Brain scans showed three years worth of development in just three months in the children, says Imperial College researcher Professor Basant Puri.
But experts warned the study was extremely small and the current evidence on the benefits of fatty acid supplements was inconclusive.
Professor Puri carried out the tests for a Channel 5 documentary "Mind the Fat: Does Fast Food Slow Kids?".
Three boys and one girl, aged between eight and 13, who were overweight took part in the tests designed to look at the effects of junk food on young brains.
They were asked to be more active and cut down on unhealthy snacks and fizzy drinks.
At the same time, they were given two capsules a day of the VegEPA supplement, which contains an omega-3 fatty acid called EPA.
Tests done at the end of the three-month study found the children showed an increase in reading age of well over a year, their handwriting became neater and more accurate and they paid more attention in class.
