The phosphoric acid (found in colas and Dr. Pepper and its clones) is both hard on your kidneys, which have to get rid of the excess, and may upset the calcium into bone mechanism by binding with calcium in the digestive tract in a way that traps the calcium and keeps your body from using it. In addition, the calcium-phosphorus connection doubles your risk of kidney stones (you really don’t want those). This is what my husband’s nephrologist tells us (and tells us again, because my husband really doesn’t want to give up his Coca Cola. He doesn’t have much choice, though, because I won’t buy it and he can’t drive.).
The brown coloration also puts a mild strain on the kidneys because they have to filter it out. This is no biggie for normal, healthy kidneys but can be a big problem for people like my husband who is operating on 30% of normal kidney function.
Thus, if you have to drink soda opt for one of the clear ones with citric acid instead of phosphoric acid.
To quote Wikipedia (yes, I know, but some bits are very good, well-written, and accurate summaries):
“Phosphoric acid, used in many soft drinks (primarily cola), has been linked to lower bone density in epidemiological studies…The paper cites significant statistical evidence to show that women who consume cola daily have lower bone density. Total phosphorus intake was not significantly higher in daily cola consumers than in nonconsumers; however, the calcium-to-phosphorus ratios were lower…The study does not examine the effect of phosphoric acid, which binds with magnesium and calcium in the digestive tract to form salts that are not absorbed…Cola consumption has also been linked to chronic kidney disease and kidney stones through medical research. This study differentiated between the effects of cola (generally contains phosphoric acid), non-cola carbonated beverages (substitute citric acid) and coffee (control for caffeine), and found that drinking 2 or more colas per day more than doubled the incidence of kidney disease.”
( http://en.wikipedia.org/wiki/Phosphoric_acid#Biological_effects_on_bone_calcium_and_kidney_health )