While it is important to consider the breakdown products separately, it is equally important to consider combinations of breakdown products that could damage a person's health when ingested over a long period of time. One combination which may prove to be one of the significant problems with aspartame is methanol and aspartic acid. Methanol breaks down into formaldehyde.
Aspartic acid is an excitatory amino acid.
Therefore, it is possible that damage caused by formaldehyde or formic acid may cause neurons with NMDA receptors to fire for a prolonged period of time. This negative effect is enhanced by the administration of an excitatory amino acid. Also, neurons damaged by formaldehyde or formic acid may be much more susceptible to excitotory amino acid damage. Certain breakdown products may have a cumulative effect such that slight or moderate damage or biochemical change caused by each breakdown product may add up to major damage over time. For example, methanol, aspartic acid, and phenylalanine have all been shown to cause vision damage in animals at some level.