But food industry officials point out that many other studies have found no link between aspartame and cancer.
The rats in the study were fed various doses of aspartame throughout their lives. In female but not male rats, lymphoma and leukemia were significantly associated with daily aspartame doses as low as 20 milligrams (mg) per kilogram (kg) of body weight. And there was a trend toward these cancers at doses as low as 4 mg/kg of body weight.
To reach a dose of 20 mg/kg, a 140-pound woman would need to drink three cans of diet soda a day. A 180-pound man would need to drink four cans of diet soda a day.
And diet soda isn't the only source of aspartame. The sweetener is in thousands of products, ranging from yogurt to over-the-counter medicines.
The average person consumes about 2 or 3 mg/kg aspartame each day. However, that figure goes way up for children and young women.
The study comes from an independent research team led by Morando Soffritti, MD, scientific director of the European Ramazzini Foundation of Oncology and Environmental Sciences in Bologna, Italy.
"What I am recommending is for healthy children and women -- if they do not have diabetes -- to avoid consumer use of aspartame," Soffritti tells WebMD. "We cannot continue to use aspartame in 6,000 types of products, soft drinks, yogurt, and whatever."
Consumer Group Reacts
A consumer watchdog group, the Center for Science in the Public Interest, has called for FDA action. At a minimum, the FDA should start its own studies and warn consumers of the potential danger, says CSPI Executive Director Michael F. Jacobson, PhD.
"The U.S. government really should analyze this study very carefully. If it is accepted as top quality, it could lead to a ban on aspartame," Jacobson tells WebMD. "I think a lot of companies are going to see the writing on the wall from this study and switch to newer artificial sweeteners. Meanwhile, I think consumers should switch to Splenda, the sweetener known as sucralose."
But Jacobson urges consumers not to panic.
"The risk to an individual is quite small," he says. "So people shouldn't fear that if they have one diet soda a day they are going to develop cancer. And I must say, the one qualm I have about the study is they found an increased risk of cancer at such a low level of exposure. If aspartame were that potent a carcinogen, I wonder if we wouldn't be seeing a real epidemic of cancer."
Soffritti has presented his findings to the European Food Safety Authority. In its 2002 review of aspartame safety, the EFSA found no cause for alarm. It promises that the new data will get a "high priority" evaluation.
"EFSA does not consider it appropriate to suggest any change in consumers' diets relative to aspartame on the basis of the information it currently has," the EFSA announced on July 14.
Low-Calorie Industry: No Cause for Alarm
The new findings fly in the face of all previous studies of aspartame safety, says the Calorie Control Council, an international association representing the low-calorie and reduced-fat food and beverage industry.
The Soffritti study findings "are not consistent with the extensive scientific research and regulatory reviews done on aspartame," the CCC says in a statement. "Aspartame has been used by hundreds of millions of consumers around the world for over 20 years. With billions of man-years of safe use, there is no indication of an association between aspartame and cancer in humans."
The CCC points to four long-term studies on aspartame that failed to find any relationship between aspartame and any form of cancer.
It's true that reports linking brain and breast cancer to aspartame had little merit, says blood-cancer specialist Martin R. Weihrauch, MD, of the University of Cologne, Germany. Last year, Weihrauch reported on his analysis of all published studies on artificial sweeteners in the Annals of Oncology.
So what does he think of the new study linking aspartame to leukemia and lymphoma?
"I think it is shocking news," he says. "However, the data have to be carefully reviewed and the study redone. Not because of their methods, probably they are fine. But for a study like this, which brings out data that would make a big change in what consumers do every day, it certainly has to be confirmed. It is worrisome."
What Happened to the Rats
Soffritti's study findings may be a first report, but the study was quite thorough. It looked at 1,800 rats fed various doses of aspartame -- or no aspartame at all -- from age 8 weeks until death. When the animals died, the researchers did a thorough autopsy.
They found that:
- A daily dose of 20 milligrams of aspartame per kilogram of body weight was linked to lymphomas and leukemias in female -- but not male -- rats.
- Rats that got daily doses of as little as 4 mg/kg aspartame got lymphomas and leukemias 62% more often than those that got no aspartame, but this finding could have been due to chance.
- A few brain tumors were seen in rats fed aspartame, while those who did not get the sweetener did not get brain tumors. But this finding, too, could have been due to chance.
The findings are scheduled to appear in the European Journal of Oncology.