Sweet deal? The safety of sugar replacements has been challenged over the years.© Punchstock

Italian researchers are challenging the safety of aspartame, the low-calorie sweetener found in NutraSweet, diet sodas and thousands of other food products.

Rats that have been fed the sweetener can develop cancer, the researchers report, even at doses below the recommended limits for people. The findings contradict most other studies, which have suggested that aspartame is safe.

However, experts note that this recent study was conducted in an unorthodox way, and critics are finding it tough to understand how the sweetener could cause cancer. This is because aspartame breaks down into compounds that are normally found in the body and in food before they enter the bloodstream.

"It is intriguing data," says James Popp, vice president of the Society of Toxicology, based in Reston, Virginia. "The scientific community will have to look at this and ask what the two types of study are telling us."

Both the US Food and Drug Administration (FDA) and the European Food Safety Authority have promised to review the group's findings. Neither has recommended any changes to the use of aspartame. "We have seen a lot of very solid data and are very comfortable with it," says George Pauli, at the FDA office of food additive safety.

Rats on a diet

The FDA's approval of aspartame, in 1981, for use in food was based in part on several cancer-safety studies. The European Food Safety Authority re-evaluated aspartame in 2002 and did not change its previous assessment.

Morando Soffritti and colleagues at the Cesare Maltoni Cancer Research Center in Bologna, Italy, decided to re-test aspartame. They are also testing several other common substances thought to be harmless, from vitamin C to cola. They have just published their conclusions on aspartame in the journal Environmental Health Perspectives¹.

To evaluate the sweetener, the researchers fed it to a test group of 1,800 rats, from the early age of eight weeks. They then allowed the rats to live out a full lifetime, up to about three years. After death they probed the rat tissues for signs of cancer.

Some types of cancer were increased in rats fed relatively low doses. For example, at doses from 20 to 500 milligrams per kilogram of body weight, about 20% of female rats fed aspartame had lymphoma and leukaemia, compared with 9% of the aspartame-free females. Male rats had to consume much more than the females to increase their risk of these blood cancers. Both the control animals and those fed aspartame lived to about the same age.

For people, the safe daily limit is 40 milligrams per kilogram of body weight, according to the World Health Organization. A person would have to drink more than 28 cans of diet cola a day to exceed that.

Not so sweet

The research was done in a nontraditional way, says John Bucher, deputy director of the environmental toxicology programme at the US National Institute of Environmental Health Sciences: rats are generally killed at two years of age in such studies.

The two types of approach have their own advantages, he says. Letting the animals live for longer enables researchers to get a fuller sense of whether cancer will develop, since cancer risk increases with age. But the statistical analysis is "problematic" since it's tough to compare animals that have died at different ages. Nonetheless, the study has a lot of strengths, says Bucher, including the large group of test animals.

Other researchers say that exactly how aspartame could cause cancer is unclear. In the body, aspartame breaks down into two amino acids, which are components of normal food proteins, and methanol. Methanol, although it can be toxic, is not generally regarded as a carcinogen and is present in common products including juice.

Michele Medinsky, a private toxicology consultant in Durham, North Carolina, says that some of the previous carcinogenicity studies done by the Italian group have been inconsistent with those of other researchers.

More than 8,000 tons of aspartame, found in about 6,000 products, are consumed each year in the United States. 

• References

  1. Soffritti M., et al. Environ. Health Perspect, doi: 10.1289/eh.8711 (2005).