Alkaline drinking water is water that has a greater number of oxygen ions, increasing its pH. Proponents claim that it can increase stamina, reduce aging, and prevent cancer, among other benefits. An equally vociferous opposition calls the water a scam.
Water molecules are composed of two hydrogen atoms and one oxygen. Under normal chemical processes, one in approximately 10 million of these molecules will ionize, which means splitting into hydrogen and oxygen ions. Hydrogen ions are acidic, while hydroxyl (oxygen) ions are alkaline. Water will take on the pH value of whichever ion is most abundant.
According to one alkaline drinking water advocate, the higher number of hydroxyl ions helps to neutralize the acidic hydrogen ions, which in turn reverses the aging process. It's not clear which ions "win," however, and although advocates talk about "restructuring" water electronically, it is difficult to pin down exactly how this is done. Other proponents talk about processes being used to lower the "millivoltage" of the water, in other words, lowering its potential energy. This apparently makes the water simpler and more beneficial.
On a more down-to-earth level, those who espouse the virtues of alkaline water point to the acidity of some popular beverages, such as coffee and most sodas, and say that drinking an alkaline liquid can help restore the body's pH to a more benign and beneficial balance. Some medical professionals claim that it is not possible for foods to change the acidity of anything in the body except for urine, however.
These professionals point out that the stomach is naturally acidic, which is how it is able to break down foods, while the fluids in the intestine must be alkaline to prevent damage from the acid. It is the kidneys, they point out, that regulate the pH of the bloodstream, not the outside influence of drinking water. Regardless of this information, alkaline drinking water has spawned a cottage industry ranging from machines that purport to "ionize" neutral water to squeeze bottles of water that are prescribed in very small daily amounts.