The majority of NutraSweet is composed of phenylalanine. Phenylalanine is an essential amino acid (i.e., our body cannot manufacture it independently and it must be obtained). Phenylalanine is one of the most abundant amino acids and its primary food sources are meat and cheese. Phenylalanine is involved with the central nervous system -- it can also elevate mood, decrease pain, aid in memory and learning.
Fifty percent of aspartame is phenylalanine. That means that one 12 ounce can of diet soda may contain 90 mg of phenylalanine.
Phenylalanine is normally found in the brain but excessive levels can be dangerous and can lead to schizophrenia, depression, and higher risk of seizure. In large quantities it may interfere with the production of serotonin and cause amino acid imbalances.
The best information I could find on the adult daily requirement of phenylalanine is 14 mg per kilogram (or 950 mg for 150 lb person). Simple math implies that a 150 lb individual could have 10 cans of diet soda a day without consuming "excessive" amounts. That being said, most individuals consume multiple sources of the amino acid (for example: meat, cheese, other sugar-free products).
I scanned nearly six websites for the most recent stats on how much soda U.S. consumers are drinking. The middle of the range is 4-6 cans of soda a day. It is highly likely that many people are hitting the maximum daily intake of phenylalanine.
Typically, amino acids are consumed in combination and not in isolation. When phenylalanine is consumed in isolation there is a much higher likelihood that it will cross the blood brain barrier.