Also known as NutraSweet®, Equal®, Canderel®, L-aspartyl-L-phenylalanine methyl ester
Aspartame is composed of two naturally occuring amino acids (aspartic acid and phenylalanine), esterified with methanol.
Aspartame has a sweet taste with minimal bitterness. The sweetness potency relative to sucrose is about 180, but depends upon the concentration of sucrose which is being matched.
Aspartame is composed of two amino acids which are building blocks of proteins; like proteins, it provides about 4 calories per gram. The high potency, relative to sucrose, means that you don't need to use very much, so the the calorie contribution in food systems is typically 0-2 calories.
The stability of aspartame in the dry state is very good. In food systems its stability is pH-dependent. It is most stable at pH 4.3, in the pH range of many soft drinks. Its stability is also temperature-dependent, breaking down faster at higher temperatures. This limits aspartame's use in baked goods.
Aspartame is fully digested in the body. The safety of aspartame has been extensively reviewed by the FDA, the European Food Safety Authority, and several other regulatory agencies around the world. On the basis of hundreds of scientific studies, these agencies all conclude that aspartame is safe. The exception is for people with the rare genetic disorder phenylketonuria (PKU); these people must carefully monitor their consumption of the essential amino acid phenylalanine, which is a component of aspartame.
While aspartame itself does not occur in nature, it is digested to three substances that are ubiquitous in the diet. I consider aspartame to be the safest of the high potency sweeteners.