Answering this question is no easy task since it may be a lot like asking "Which is worse: heart disease or nervous system disease?" Each product is associated with different possible connections with disease. Aspartame is thought to possibly cause or promote nervous system disorders such as Parkinson's disease, at least when consumed in fairly high amounts. The steady consumption of high fructose corn syrup, on the other hand, is said to possibly cause or promote diabetes, heart disease, and damage to other organs such as the liver, pancreas, and kidneys.
Studies suggest that the amount of both aspartame and high fructose corn syrup consumed seem to be a factor in whether or not ill effects may occur. Therefore, if people avoid the high fructose corn syrup but consume over the recommended limit for aspartame per day — often said to be three cans of diet soda — then they would perhaps be at a high risk for nervous system damage.
Neither form of sweetener is a natural product. High fructose corn syrup is derived from corn but is completely synthetic. Corn starch is boiled and distilled to create syrup with a very high fructose, or fruit sugar, content. The percentage of fructose varies between 45% and 85%. The problem is that the human body is not made to digest this very high sugar concentration, just as it are not made to digest the high concentration of ingredients in aspartame.
Aspartame is made up of 50% phenylalanine, 40% aspartic acid, and 10% methyl alcohol. Phenylalanine and aspartic acid are actually important amino acids that are found naturally in many foods, but again the percentages used in the man-made sweetener is much too high for our bodies to consume naturally. Methyl alcohol is sometimes called wood alcohol, and it can cause blindness as well as kidney and liver failure.
Methyl alcohol is thought to turn into formaldehyde in the body. Formaldehyde is a chemical used in embalming and is considered poisonous to the central nervous system. High levels of aspartic acid are said to damage the basal ganglia part of the brain and damage to the basal ganglia has been shown in many studies to be connected to the development of Parkinson's disease. Phenylalanine is high amounts is said to cause a reduction in dopamine production in the brain and dopamine levels have also been found to play a role in diseases such as Parkinson's.
Aspartame is usually found only in sugar-free versions of foods such as desserts and drinks, while high fructose corn syrup is found in a large range of packaged foods. High fructose corn syrup is commonly used in foods such as tomato sauce, cake mixes, cereals, bread,s and juice. Manufacturers use it rather than natural sugar as it is cheaper and has a much longer shelf life. Several studies have linked high fructose corn syrup to obesity, heart disease, and cirrhosis of the liver. Since the choice between the two sweeteners seems to be like choosing between a rock and a hard place, maybe the answer is to read labels and consume only a small amount, if any, of both ingredients.