Some animal studies have shown that aspartame poses antipyretic, analgesic and anti-inflammatory action.[21,22] Interference of aspartame with rheumatoid factor activity has been proposed to alleviate the pain and immobility resulting from chronic inflammation of joints.[23]

It has also been suggested that the components of aspartame can lead to a number of health problems.[3] Double-blind trials have been carried out with aspartame at Duke University and in one of the best-designed of these studies, the effects of a single large dose of aspartame in people who had claimed to be sensitive to the substance was investigated. The results showed no difference in headache frequency, blood pressure, or blood histamine concentrations (a measure of the allergenic potential) between the experimental and control groups.[24] In another study, at the University of Illinois, which involved diabetics, subjects in the placebo group actually had more reactions than those in the aspartame group. Reported anecdotal experiences are not confirmed by carefully controlled scientific studies. This, of course, does not mean that the problems are not real, but it does imply that in many cases the symptoms may not be caused by aspartame itself. One study has, however, confirmed allergic symptoms such as hives and swelling in sensitive individuals.[19] It is unclear how the allergy comes about, since none of the components of aspartame are believed to be capable of producing allergic reactions. It has been suggested that diketopiperazine, a compound which forms when aspartame decomposes, may be responsible.[16] When the temperature of Aspartame exceeds 86 degrees F, the wood alcohol in aspartame coverts to formaldehyde and then to formic acid, which in turn causes metabolic acidosis. The methanol toxicity mimics multiple sclerosis; thus people may be misdiagnosed with having multiple sclerosis. Multiple sclerosis does not lead to death whereas methanol toxicity does.[25] In some case reports associations have been made between aspartame intake, in particular the subsequent exposure to the aspartame metabolite formaldehyde, and Type IV Delayed Type Hypersensitivity (DTH) reactions in patients with proven contact sensitization to formaldehyde.[26,27] However, to confirm the associations observed in these two case studies with only a limited number of patients (seven in total), larger studies would be needed involving double-blind placebo-controlled challenges with aspartame and placebo exposures and the inclusion of well-defined control-patient groups. Although in in vivo studies by Parthasarathy et al., effects of the aspartame metabolite methanol on organs/tissues/cells and function of the immune system were described,[28,29] these observations are considered more likely to result from an indirect stress-effect due to the high methanol levels used, in addition these high dose levels are not considered relevant for aspartame exposure. In addition, the quality of the studies was poor.

Further experiments show that aspartame is no more likely to cause an allergic reaction than a placebo.[30] The three breakdown products of aspartame are all toxic in high doses. Phenylalanine is an essential amino acid which must be included in the diet for normal growth and maintenance, but sustained high blood levels can lead to brain damage. This is of major concern to the one out of roughly 20,000 children who are born with an inherited condition called “phenylketonuria” (PKU). These children cannot metabolize phenylalanine, which then builds up to dangerous levels in their brains. The condition, therefore, necessitates a severe curtailment of phenylalanine intake for at least the first six years of life. This means that aspartame, due to its phenylalanine content, is not suitable for PKU sufferers and consequently requires a warning to that effect on products in which it is an ingredient. It was suggested that some of the untoward effects of aspartame may be caused by a sudden increase in brain phenylalanine levels, especially when the sweetener is consumed along with foods high in carbohydrates. Carbohydrates trigger insulin release into the bloodstream which, in turn, makes it easier for phenylalanine to cross the blood-brain barrier.[31]

Aspartame has seizure-promoting activity in animal models that are widely used to identify compounds affecting (i.e., usually protecting against) seizure incidence. In a similar manner, it is possible that doses of the sweetener that cause a sufficient increase in brain phenylalanine might increase seizure frequency among susceptible humans, or might allow seizures to occur in people who are vulnerable but without prior episodes.[31] On the other hand, human studies disprove it. It was shown that there was no difference between the results for aspartame and those for the placebo.[32,33] In a crossover design by Ralph et al., they concluded that individuals with mood disorders are particularly sensitive to this artificial sweetener and its use in this population should be discouraged.[34]

The effect of aspartame during reproduction, development and lactation has been evaluated in rats, mice, hamsters, and rabbits. No-effect levels of exposure during reproduction and gestation have been reported to range from 1,600 mg/kg bw/day in rabbits to 4,000 mg aspartame/kg bw/day in rodents. Human clinical studies with daily doses of 75 mg/kg bw/day (more than 15 times the estimated daily average intake and 1.5 times the established Acceptable Daily Intake ( ADI) by the FDA) of aspartame for 24 weeks were not associated with any significant changes in clinical measures or adverse effects. The effect of aspartame on behavior, cognitive function, and seizures has been studied extensively in animals, and in healthy children, hyperactive children, sugar-sensitive children, healthy adults, individuals with Parkinson's disease, and individuals suffering from depression.[35]

The effects of aspartic acid, another aspartame breakdown product, have also been rigorously examined. Administration of extremely large amounts to non-human primates produced no damage even though blood levels were greatly elevated. It is a fact that in large doses, methanol can lead to blindness and even to death. Methanol occurs naturally in foods. In fact, the “natural” methanol content of fruit juice is about 2.5 times higher than from aspartame-sweetened drinks. Even at the 99^th percentile level of 34 mg per kg of body weight consumed per day, blood levels of methanol are undetectable. A study published in 1996, claimed that a 10% increase in brain tumors noted in the 1980s was associated with the introduction of aspartame.[36] The suggestion was that aspartame or its diketopiperazine breakdown product may combine with nitrites in the diet to form nitrosated compounds. Nitrosoureas are indeed known to produce brain tumors in animals.[18] In 2005, researchers at the Ramazzini Foundation in Bologna, Italy, conducted a study which showed a significant dose dependent increase in incidence of lymphomas/leukemias in both male and female rats. They have stated that aspartame is a multi-potential carcinogenic compound whose carcinogenic effects are evident even at a daily dose of 20 mg/kg body weight, much less than the current ADI for humans in Europe (40 mg/kg bw) and in the United States (50 mg/kg bw). However, in 2009 the European Food Safety Authority reviewed the study and concluded that the tumors probably occurred just by chance.[37] In a study conducted in 2006, the U.S. National Cancer Institute researchers studied a large number of adults 50 to 69 years of age over a five-year period. There was no evidence that aspartame posed any risk. However, the study was limited in three major regards: It did not involve truly elderly people (the rat studies monitored the rats until they died a natural death), the subjects had not consumed aspartame as children, and it was not a controlled study (the subjects provided only a rough estimate of their aspartame consumption, and people who consumed aspartame might have had other dietary or lifestyle differences that obscured the chemical's effects).[38] In 2007, the same Italian researchers published a follow-up study that began exposing rats to aspartame in utero. This study found that aspartame caused leukemias/lymphomas and mammary (breast) cancer.[39] It is likely that the new studies found problems and the earlier company-sponsored studies did not because the Italian researchers monitored the rats for three years instead of two. The Italian tests remain controversial, with the industry contending that they were flawed in several ways and with the FDA stating that its scientists could not evaluate the studies because the researchers refused to provide their original data.

The Advisory Forum of EFSA (European Food Safety Authority) has reviewed the information on aspartame with national experts. The objectives of the Organizing Team were to identify, collect and review all published papers since the review carried out by the SCF (Scientific Committee on Food) in 2002. In addition the Organizing Team considered available non-peer-reviewed information and anecdotal evidence. They analyzed 26 reviews. The areas which were considered include exposure data, brain function, satiation and appetite, allergenicity and immunotoxicity, metabolic aspects and diabetes, carcinogenicity (including cancer epidemiology) and genotoxicity and reported on.[40] The review relating to brain function includes reports on the direct and indirect cellular effects of aspartame or its metabolites on the nervous system including neurotoxicity and functional aspects published or accessible after 2002. Several studies, in vitro or in vivo, indicate that aspartame or its metabolites may affect certain enzyme activities in the brain, for example, acetylcholinesterase,[41] Na+/K+-ATPase,[42] or cytochrome P450 (CYP) enzymes.[43] The National Experts consider that the biological relevance of such findings is not clear, particularly the relevance of findings in in vitro studies in which the toxicokinetic and toxicodynamic behavior of aspartame in vivo is not fully reflected. The National Experts consider, however, that the scientific literature needs to be monitored for further research and mechanistic explanations related to this area. The National Experts note that no new publications were identified reporting a link between aspartame intake and enhanced susceptibility to seizures, behavior, mood and cognitive function, and conclude that there is still no substantive evidence that aspartame can induce such effects, as earlier concluded by the SCF.

A number of studies focused on the effects of aspartame on appetite/hunger and food intake,[44–46] as it has been suggested that aspartame may have modulating effects on these body responses, even resulting in the converse effect than that intended, namely obesity rather than body weight maintenance or loss. The National Experts have noted that there is little or no substantive data suggesting that aspartame affects appetite/hunger, food intake. A study focusing on aspartame, such as that performed by Just et al., which looked at cephalic insulin response in healthy fasting volunteers after taste stimulation, comparing sucrose, starch and saccharin, warrants further consideration.[47]

In the literature reviewed by the Organizing Team it has been observed that the metabolites of aspartame (aspartic acid, phenylalanine and methanol) could affect the metabolism of endogenous and exogenous compounds. Amino acids per se have an influence on metabolic pathways and it is known that high doses of aspartame may increase plasma levels of the metabolites of aspartame. High levels of specific amino acids can also affect transporters and protein synthesis. The National Experts note that there is very little new information about the effects of aspartame and its metabolites on the metabolism of endogenous and exogenous compounds. The available reports mainly used high doses, and focused on plasma changes in aspartic acid, phenylalanine and methanol.[48] One reference showed, in patients with Type II diabetes, that the reduction of plasma glucose and insulin levels during exercise was similar after a sucrose meal compared to an aspartame-sweetened meal.[49] These results were obtained even though the aspartame meal contained 22% less calories and 10% less carbohydrates. The National Experts considered that research investigating whether aspartame and its metabolites affect gene expression, protein synthesis and enzyme activities of Cytochrome P450 enzymes in the brain could be useful to extend knowledge in this area. While the use of novel techniques such as metabolomics has not been considered in previous evaluations of aspartame, as they were not available at the time, it is recognized that such research is at the forefront of toxicological science and the results of such work may usefully increase the evidence base. There is no evidence to suggest that aspartame is carcinogenic (as discussed earlier).

One animal study has shown that consumption of products containing saccharin may lead to increased body weight and obesity by interfering with fundamental homeostatic and physiological processes[15] Fear about saccharin increased when a study in 1960 showed that high levels of saccharin may cause bladder cancer in laboratory rats. In 1977, Canada banned saccharin due to the adverse effects reported in animal studies. In the United States, the FDA considered banning it in 1977, but Congress stepped in and placed a moratorium on such a ban which required a warning label and also mandated further study of saccharin safety. Subsequently, it was discovered that saccharin causes cancer in male rats by a mechanism not found in humans. At high doses, it forms a precipitate in rat urine. This precipitate damages the cells lining the bladder and a tumor forms when the cells regenerate. According to the International Agency for Research on Cancer, part of the World Health Organization, “Saccharin and its salts were downgraded from Group 2B, possibly carcinogenic to humans, to Group 3, not classifiable as carcinogenic to humans, despite sufficient evidence of carcinogenicity to animals, because it is carcinogenic by a non-DNA-reactive mechanism that is not relevant to humans because of critical interspecies differences in urine composition.”[11]

Safety concerns pertaining to sucralose revolve around the fact that it belongs to a class of chemicals called organic chlorides, some types of which are toxic or carcinogenic; however, the presence of chlorine in an organic compound does not in any way ensure toxicity. The way sucralose is metabolized may suggest a reduced risk of toxicity. For example, sucralose is extremely insoluble in fat and thus does not accumulate in fat as do some other organic chlorides; sucralose also does not break down and dechlorinates only under conditions that are not found during regular digestion.[50]