citric acid and magnesium oxide won't hurt you. what you might want to watch out for are energy gels and drinks that contain sodium or potassium benzoate. These are also routinely added to soft drinks and other beverages as a preservative. In the presence of vitamin C the benzoate can break down to form benzene, a potent carcinogen. Recent studies showed many soft drinks contained high levels of benzene from benzoate breakdown. See this news article if interested:
http://news.bbc.co.uk/2/hi/health/4763528.stm . I believe this is more pronounced in some diet beverages. This may be linked to increases in esophagael cancer rates. Benzoate is simply benzene with a carboxyl group added. There is of course a reason it is a preservative.. preservatives kill organisms...hmm...
So- gu gels contain both vitamin C and sodium benzoate. Hammer gel contains potassium sorbate, not benzoate as a preservative. To my knowledge potassium sorbate is nontoxic at the levels used in food, but who knows. Anyway that is why I try to use hammer gel for training and in marathons rather than gu. You can verify the ingredients list of gu gels here
http://guenergy.com/products/gu-energy-gel/flavors-nutrition_vanilla-bean. Just thought people might find this interesting, and you might want to look at the ingredients list on that processed drink or gel before ingesting a possible source of benzene.