Though Italian researchers have found elevated rates of lymphoma, leukemia, and other cancers in rodents ingesting aspartame, most other animal studies have shown no connection between aspartame and cancer. More important, in a cohort study involving nearly 500,000 people, there was no increased risk of blood or brain cancers among aspartame users.
Likewise, most studies looking at neurological and behavioral issues haven't found adverse effects from aspartame. When a panel of scientists reviewed more than 500 studies, they uncovered no major safety problems. The review was funded by a Japanese manufacturer of aspartame, but the experts were unaware of who the funder was, and the company had no role in selecting the experts.
The panel's conclusions echo those of both the US Food and Drug Administration and the European Commission's Scientific Committee on Food, which say that a daily intake of up to 40 or 50 milligrams of aspartame per kilogram of body weight is safe for most people. Translated into plain English, this means a 150-pound adult can safely consume up to about 19 cans of diet soda a day. (Not that anyone says drinking this much is advisable.)
Still, aspartame may adversely affect certain people. One of the most common complaints is headaches, an effect detected by some (but not all) research. Also, people with a rare inherited condition called phenylketonuria (PKU) can't metabolize phenylalanine, an amino acid in aspartame. To avoid an unsafe buildup, they need to steer clear of the sweetener. (Hence that cryptic warning "Phenylketonurics: Contains Phenylalanine" on the labels of foods and beverages that contain aspartame.)
Despite the claims of some aspartame opponents, there's no solid proof that phenylalanine from normal amounts of aspartame poses a danger to the rest of us. The same goes for methanol, which is also produced when our bodies break down aspartame.
Those who are convinced aspartame is poison or an evil plot may denounce me as an ignoramus or a pawn of industry for not agreeing with them. But at least they won't be able to blame my "confusion" on aspartame-related brain problems. Personally, I can't stand the taste.