There have been a huge variety of serious concerns raised about the health implications of aspartame, the popular sugar substitute used as the primary sweetener found in Diet Coke. Potential issues raised include everything from brain tumors to cancers. The good news first: many of these concerns are unfounded. In fact, as of right now, the official stance on aspartame, according to both the European Food Safety Authority and the U.S . Food and Drug Administration is that aspartame has been studied at length and is safe for human consumption.
However, for anyone suffering from the genetic disorder phenylketonuria (frequently shortened to PKU), a component of aspartame called phenylalanine can cause really serious conditions such as brain damage, seizures, and mental retardation. Phenylalanine doesn’t just occur in diet drinks, though. It’s a natural part of many foods, including eggs, milk, and meat.
Aspartame is also considered unsafe for people who are taking certain medications, such as levodopa, neuroleptics, or monoamine oxidase inhibitors. If you’re taking a medication that you think may be contraindicated with aspartame, you should check with your doctor.
On the other hand, since it is sugar-free, Diet Coke is currently considered safe for people who have diabetes.
Some have claimed that aspartame-laden drinks actually make people gain more weight. There has been some conflicting evidence on this, but it seems that Diet Coke may not have the weight-loss effects that fans of the beverage may have desired. It appears that in one study, rats gained the same amount of weight whether they were ingesting saccharin, aspartame, or sucrose (sugar-water).
Other studies have shown that weight-gain was promoted by the use of aspartame or saccharin as compared with sucrose, although it was suspected that this might have to do with less energy being expended and that the diet drinks may have encouraged fluid retention.
So there’s no short answer to the question "Is Diet Coke bad for you?" We can assert that these drinks probably won’t help you lose weight as much as we'd all like to think, and that if you suffer from PKU, you should avoid all diet sodas. In terms of cancer, the scientific community seems to still be out on this one: there is plenty of contradictory evidence both ways. We're going to have to wait and see. But if you love Diet Coke and you're a risk-taking optimists, go ahead and do that waiting and seeing with a Diet Coke in hand.