For decades, controversy has surrounded the question of whether the artificial sweetener, aspartame can cause cancer.
Aspartame was first manufactured in 1965. In 1981, following cursory assessment of its safety and toxicity , aspartame was approved by the U.S. Food and Drug Administration for use in foods . Today with an annual production of 3000–5000 metric tons, aspartame is one the world’s most widely used artificial sweeteners. It is an ingredient in more than 5000 food and beverage products including cereals, chewing gum, yogurt, pharmaceuticals, and instant coffee. A particularly important use in the United States is in the manufacture of low-calorie beverages that are extensively consumed by children and pregnant women .
The Ramazzini Institute studies of aspartame
In 1997, in response to rising concerns about the safety of aspartame, the Ramazzini Institute (RI), an independent, not-for-profit research laboratory in Bologna, Italy initiated a series of large-scale toxicological studies of the possible carcinogenicity of aspartame. In the first of these studies (BT 6008), aspartame was administered to Sprague-Dawley rats in their feed at seven dose levels ranging from 0 to 100,000 ppm (ppm) throughout their lives beginning at 8 weeks of age . The second study (BT 6009) used the three lowest doses of the first study, but began dosing prenatally, thus resulting in exposures to fetal rat pups in utero . The third study (BT 6010) was performed on Swiss mice, used five dose levels, and again began dosing prenatally . In total, 2270 Sprague-Dawley rats and 852 Swiss mice were included in these three studies.
The main finding in these RI studies was that aspartame caused increased incidence of malignant tumors in multiple organs in rodents. Increases were seen in hematopoietic and lymphoid tissue tumors (HLTs) in animals of both sexes, carcinomas of the renal pelvis and the ureter in females, mammary cancers in females, and malignant schwannomas of the peripheral nerves in males. Positive dose-response relationships were observed, in which the highest incidence of malignancies was seen in the animals exposed to the highest levels of aspartame [4,5,6,7].
Increased incidence of malignant tumors was seen even in animals exposed to relatively low doses of aspartame – exposures close to the current Acceptable Daily Intake (ADI) levels of 40 mg/Kg body weight in the European Union [8, 9] and 50 mg/Kg body weight in the United States .
Prenatal exposures of rat pups to aspartame in utero produced dose-related increases in malignancies at lower exposure levels and with shorter latency periods than body weight equivalent exposures in adults  (Fig. 1). This finding indicates that aspartame may initiate carcinogenesis in utero. It is consistent with a large body of literature indicating that young animals, especially in the fetal period, are more sensitive than older animals to a range of chemical and physical carcinogens .
Three unique features of the RI’s toxicological testing protocol distinguish it from most other carcinogenesis bioassays [7, 12]:
Large numbers of animals are used, thus increasing statistical power to detect increases in cancer incidence;
Animals are maintained and followed across their entire lifetimes to natural death. This design replicates the human experience, in which approximately 80% of all cancers are diagnosed beyond the age of 60 years . It enables the RI to detect malignancies that are missed by many other rodent bioassays that truncate follow-up and sacrifice their experimental animals at 104 weeks (or earlier) – often before the chemical under examination has had an opportunity to express its carcinogenic potential, and before many chemically induced malignancies have become evident [7, 14];
Systematic histopathological analyses are undertaken in all organs and tissues, and not merely in a subset.
Publication of the RI findings on the carcinogenicity of aspartame generated intense controversy . At the heart of this debate were doubts raised about the accuracy of the RI’s histopathological diagnoses - in particular the RI’s diagnoses of pulmonary lymphomas and leukemias – in animals exposed to aspartame [14, 16].
The European Food Safety Agency (EFSA) made the unsubstantiated claim that the RI’s animal colony was poorly managed and that the experimental animals were subject to uncontrolled infections [8, 9, 17]. Schoeb et al. speculated further that pulmonary lesions diagnosed as lymphomas and leukemias by RI might have been inflammatory lesions caused by Mycoplasma pulmonis infections .
Of note is that none of these explanations accounted for the strongly positive dose-response relationships between aspartame exposure level and cancer incidence observed in the RI studies or for the increased incidence of neoplasms in animals exposed in utero .