This type of diketopiperazine (DKP) is created in products as aspartame breaks down over time. For example, researchers found that 6 months after aspartame was put into carbonated beverages, 25% of the aspartame had been converted to DKP.^[53] Concern among some scientists has been expressed that this form of DKP would undergo a nitrosation process in the stomach producing a type of chemical that could cause brain tumors.^[54][55] Other scientists think that the nitrosation of aspartame or the DKP in the stomach would not produce a chemical that would cause brain tumors. In addition, only a minuscule amount of the nitrosated chemical would be produced.^[56] There are very few human studies on the effects of this form of DKP. However, a (one-day) exposure study showed that the DKP was tolerated without adverse effects.^[57]

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Mario Negri research institute

^{A 2007 study, published in Annals of Oncology of the European Society for Medical Oncology, reviewed Italian studies of incidences of cancer from 1991 and 2004 and concluded a "lack of association between saccharin, aspartame and other sweeteners and the risk of several common neoplasms".[58]}