British researchers found removing food additives from the diet of a group of 3-year-olds caused a reduction in the children's hyperactive behavior reported by their parents. And when the food colorings and preservatives were added back into the children's diets, the parents reported an increase in hyperactivity.

Based on these parental reports of behavioral changes, researchers estimate that if the current 15% of children thought to have hyperactivity-related behavior problems were to go on an additive-free diet, the prevalence could be reduced to 6%.

Source f-ex. ific.org
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Although this theory was popularized in the 1970s, well-controlled studies conducted since then have produced no evidence that food color additives cause hyperactivity or learning disabilities in children. A Consensus Development Panel of the National Institutes of Health concluded in 1982 that there was no scientific evidence to support the claim that colorings or other food additives cause hyperactivity. The panel said that elimination diets should not be used universally to treat childhood hyperactivity, since there is no scientific evidence to predict which children may benefit.
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Fitzgerald: We looked at some of the studies done in the later part of 2005, examining the synergistic reactions between aspartame -- the synthetic sweetener that is found in thousands of products now -- and MSG, along with two common food colorings. This was a study done in England at a university in the later part of 2005. The result was found that when MSG, aspartame and these two food colorings were mixed together, they create a synergy that kills nerve cells. It causes neurological damage. We are only now at the threshold of medical science beginning to do any systematic examination of synergies.

Mike: Just to put in a real-world example of that, I would like to add that you can get that combination by drinking a Diet Pepsi and eating a bag of Doritos.

Fitzgerald: Absolutely. It is a children's meal. In fact, these researchers, at the University of Liverpool, wanted to examine what children commonly consume every day. That is why they picked aspartame, MSG and these two food colors, because they are so commonly found in all of the junk drinks and foods that children consume. The results should alarm us all.