Apparently, the culprit is sodium benzoate, a preservative which is used to prevent mold in soft drinks, and found in Coca Cola, Pepsi, Sprite, Dr. Pepper, and others.
All about sodium benzoate, from The Independent:
"Sodium benzoate has already been the subject of concern about cancer because when mixed with the additive vitamin C in soft drinks, it causes benzene, a carcinogenic substance. A Food Standards Agency survey of benzene in drinks last year found high levels in four brands which were removed from sale.
Now, an expert in ageing at Sheffield University, who has been working on sodium benzoate since publishing a research paper in 1999, has decided to speak out about another danger. Professor Peter Piper, a professor of molecular biology and biotechnology, tested the impact of sodium benzoate on living yeast cells in his laboratory. What he found alarmed him: the benzoate was damaging an important area of DNA in the "power station" of cells known as the mitochondria.
He told The Independent on Sunday: "These chemicals have the ability to cause severe damage to DNA in the mitochondria to the point that they totally inactivate it: they knock it out altogether.
"The mitochondria consumes the oxygen to give you energy and if you damage it - as happens in a number if diseased states - then the cell starts to malfunction very seriously. And there is a whole array of diseases that are now being tied to damage to this DNA - Parkinson's and quite a lot of neuro-degenerative diseases, but above all the whole process of ageing.""
It's also suspected of worsening ADHD in kids.