A supplement containing fatty acids may help children do better at school.
Oily fish contains omega-3 fatty acids
Tests of VegEPA, in four overweight youngsters, showed improvement in reading, concentration, and memory.
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.
Brain scans which identified a chemical called N-Acetylaspartate (NAA) which is linked to the growth of nerve fibres in the brain also showed dramatic changes, said Professor Puri.
Although the children were encouraged to change their diet, there was no evidence they did this to any great extent, suggesting the improvements in the children were a result of the supplement.
"In three months you might expect to see a small NAA increase.
"But we saw as much growth as you would normally see in three years.
"It was as if these were the brains of children three years older. It means you have more connections and greater density of nerve cells, in the same way a tree grows more branches."
The boys in the study showed the most improvement, he added.
Omega-3 fatty acids are found naturally in oily fish such as mackerel, salmon, herring and tuna or seeds such as flax, pumpkin and hemp.
A systematic review of fish oil supplements in children published by the Food Standards Agency last year found there were too many inconsistencies in current evidence to come to any conclusion.
Professor Puri said he believed that it was EPA specifically which conferred the benefits which was why studies of fish oil supplements which also contain a fatty acid called DHA showed confusing results.
He is now planning to carry out a larger placebo-controlled study.
Professor Robert Grimble, professor of nutrition at the University of Southampton said it was vital that larger studies were done to clarify the issue.
"My view is we can't come to any clear conclusion until a proper trial is done.
"These small bits of weak data just confuse the public. The FSA looked at this very carefully and I wouldn't contradict that until we have more evidence."