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From the 29 May 1995 issue of Smart Drug News [v4n2]. Copyright (c) 1997. All rights reserved.
When aspartame in liquid is subjected to high temperatures, the breakdown of aspartame and the formation of large amounts of DKP (diketopiperazine) happens very quickly [Prudel, 1986]. In addition, Boehm  and Bada  showed that high temperatures can cause racemization of the free amino acids leading to significant amounts of unnatural D-type amino acids much more than is produced through cooking normal, healthy foods. The health affects of large amounts of these D-type amino acids are not well known. In a statement provide to U.S. Senate hearings, Jeffrey Bada, Ph.D. , had this to say about aspartame decomposition:
Aspartame, a dipeptide containing the amino acids phenylalanine and aspartic acid, is prone to a number of decomposition/alteration reactions. Dominant are cyclization to the cyclic dipeptide or diketopiperazine [DKP] and stereochemical inversion (racemization) producing the unnatural D-stereoisomers of the amino acids. [...] In some instances, however, these reactions are very significant, and the reaction products which are produced are not well-studied as far as their nutritional/toxicological properties are concerned. Some examples where these reactions could be significant are in soft drinks exposed to warm temperatures for prolonged periods and in consumer misuse of aspartame such as in cooking or baking.
In an article for the Wednesday Journal, Dr. Bada discusses some of his concerns relating to the chemical rearrangement of aspartame [Mullarkey, 1992]:
The chemistry of aspartame is changed when boiled... There is internal rearrangement of its structure. The L-isomers of phenylalanine and aspartic acid change to unnatural D-isomers which are metabolized differently. How it is metabolized is anybodys guess... Searle people tend to dismiss stereochemical inversion as unimportant. Chris Tschanz, director of aspartame clinical research, and Louis D. Stegnik, M.D., of the University of Iowa College of Medicine, visited me and admitted that nobody thought of looking at aspartame the way we did.
In 1993, the FDA approved aspartame for use in tea beverages, baked goods and mixes, frostings and toppings [Mullarkey, 1994]. There are many aspartame-containing products on the market which are intended to be heated to high temperatures. Therefore, Dr. Badas comment of aspartames misuse in cooking or baking no longer appliesit is now a condoned use of aspartame.
It has been shown that aspartame can react with other food additives to form chemicals of unknown health consequence. Hussein showed that aspartame reacts with aldehydes which are commonly used flavoring compounds in sodas and chewing gum [Hussein, 1984]. Cha  has shown that aspartame can react with vanillin in foods. These reactions are very important considerations. As an example of how additive reactions can cause the formation of toxic substances, in 1973 an experiment reported in the Journal of Food Science tested three different food additives on mice. None of the mice reacted negatively to the individual food additives. But when the additives were tested in pairs, the mice became ill. When all three food additives were tested at once, the mice died.
Aspartame-containing products which are ingested in the real-world are chemically very different than the 98-100% pure aspartame that is given in laboratory experiments. The large amount of breakdown products may play the most important role in aspartames negative health affects. Aspartames strong tendency to react with other food ingredients to form unique chemical compounds and the tendency of the free amino acids to racemize at high temperatures may also be important factors. The aspartame found in the real world is no longer the pure aspartame that was originally put into the food. It is a chemical witchs brew of aspartame breakdown and reaction products. SWF