Aspartame administration to the infant monkey: hypothalamic morphology and plasma amino acid levels

Alterations in behaviour, cerebral cortical morphology and cerebral oxidative stress markers following aspartame ingestion

OBJECTIVE

The study evaluated changes in open field behaviours, cerebral cortical histomorphology and biochemical markers of oxidative stress following repeated administration of aspartame in mice.

METHODOLOGY

Adult mice were assigned into five groups of twelve each. Vehicle (distilled water), or aspartame (20, 40, 80 and 160mg/kg body weight) were administered orally for 28days. Horizontal locomotion, rearing and grooming were assessed after the first and last dose of aspartame. Sections of the cerebral cortex were processed and stained for general histology, and also examined for neuritic plaques using the Bielschwosky's protocol. Glial fibrillary acidic protein (GFAP) and neuron specific enolase (NSE) immunoreactivity were assessed using appropriate antibodies. Aspartate and antioxidant levels were also assayed from cerebral cortex homogenates. Data obtained were analysed using descriptive and inferential statistics.

RESULTS

Body weight and food consumption decreased significantly with aspartame consumption. Locomotion, rearing and grooming increased significantly after first dose, and with repeated administration of aspartame. Histological changes consistent with neuronal damage were seen at 40, 80 and 160mg/kg. Neuritic plaque formation was not evident; while GFAP-reactive astrocytes and NSE-reactive neurons increased at 40 and 80mg/kg but decreased at 160mg/kg. Superoxide dismutase and nitric oxide increased with increasing doses of aspartame, while aspartate levels showed no significant difference.

CONCLUSION

The study showed morphological alterations consistent with neuronal injury and biochemical changes of oxidative stress. These data therefore supports the need for caution in the indiscriminate use of aspartame as a non-nutritive sweetener.

Toxic effects of l-aspartic acid at high dose levels on kidneys and salivary glands in Fischer 344 rats detected in a 90-day feeding study

A subchronic oral toxicity study of l-aspartic acid (l-Asp) was conducted with groups of 10 male and 10 female Fischer 344 rats fed a powder diet containing 0%, 0.05%, 1.25%, 2.5% and 5.0% concentrations for 90 days. Serum biochemistry showed treatment-related decreases of blood urea nitrogen, creatinine and uric acid levels in both sexes. In addition, incidences of urinary ketone and protein were significantly increased in treated both sexes, while relative kidney weight was significantly increased in the 5.0% male rat, and regenerative renal tubules with tubular dilation were histopathologically observed in male rats of the 2.5% or greater groups. The observed renal injury was confirmed not to be due to accumulation of alpha2u-globulin. Acinar cell hypertrophy of salivary glands was histopathologically evident in male and female rats of the 2.5% or greater groups. The present results indicate that l-Asp causes toxic effects on kidneys and possibly salivary glands at high dose levels in male and female Fischer 344 rats. Such toxic effects were observed only in animals given 2.5% and/or higher doses of l-Asp. In conclusion, the no-observed-adverse-effect-level (NOAEL) for l-Asp is 1.25% (696.6 mg/kg body weight/day for males and 715.2 mg/kg body weight/day for females) under the present experimental conditions.

Aspartame: a safety evaluation based on current use levels, regulations, and toxicological and epidemiological studies

Aspartame is a methyl ester of a dipeptide used as a synthetic nonnutritive sweetener in over 90 countries worldwide in over 6000 products. The purpose of this investigation was to review the scientific literature on the absorption and metabolism, the current consumption levels worldwide, the toxicology, and recent epidemiological studies on aspartame. Current use levels of aspartame, even by high users in special subgroups, remains well below the U.S. Food and Drug Administration and European Food Safety Authority established acceptable daily intake levels of 50 and 40 mg/kg bw/day, respectively. Consumption of large doses of aspartame in a single bolus dose will have an effect on some biochemical parameters, including plasma amino acid levels and brain neurotransmitter levels. The rise in plasma levels of phenylalanine and aspartic acid following administration of aspartame at doses less than or equal to 50 mg/kg bw do not exceed those observed postprandially. Acute, subacute and chronic toxicity studies with aspartame, and its decomposition products, conducted in mice, rats, hamsters and dogs have consistently found no adverse effect of aspartame with doses up to at least 4000 mg/kg bw/day. Critical review of all carcinogenicity studies conducted on aspartame found no credible evidence that aspartame is carcinogenic. The data from the extensive investigations into the possibility of neurotoxic effects of aspartame, in general, do not support the hypothesis that aspartame in the human diet will affect nervous system function, learning or behavior. Epidemiological studies on aspartame include several case-control studies and one well-conducted prospective epidemiological study with a large cohort, in which the consumption of aspartame was measured. The studies provide no evidence to support an association between aspartame and cancer in any tissue. The weight of existing evidence is that aspartame is safe at current levels of consumption as a nonnutritive sweetener.

Aspartame: review of safety

Over 20 years have elapsed since aspartame was approved by regulatory agencies as a sweetener and flavor enhancer. The safety of aspartame and its metabolic constituents was established through extensive toxicology studies in laboratory animals, using much greater doses than people could possibly consume. Its safety was further confirmed through studies in several human subpopulations, including healthy infants, children, adolescents, and adults; obese individuals; diabetics; lactating women; and individuals heterozygous (PKUH) for the genetic disease phenylketonuria (PKU) who have a decreased ability to metabolize the essential amino acid, phenylalanine. Several scientific issues continued to be raised after approval, largely as a concern for theoretical toxicity from its metabolic components--the amino acids, aspartate and phenylalanine, and methanol--even though dietary exposure to these components is much greater than from aspartame. Nonetheless, additional research, including evaluations of possible associations between aspartame and headaches, seizures, behavior, cognition, and mood as well as allergic-type reactions and use by potentially sensitive subpopulations, has continued after approval. These findings are reviewed here. The safety testing of aspartame has gone well beyond that required to evaluate the safety of a food additive. When all the research on aspartame, including evaluations in both the premarketing and postmarketing periods, is examined as a whole, it is clear that aspartame is safe, and there are no unresolved questions regarding its safety under conditions of intended use.

Dietary amino acids and brain function

Two groups of amino acids--the aromatic and the acidic amino acids--are reputed to influence brain function when their ingestion in food changes the levels of these amino acids in the brain. The aromatic amino acids (tryptophan, tyrosine, phenylalanine) are the biosynthetic precursors for the neurotransmitters serotonin, dopamine, and norepinephrine. Single meals, depending on their protein content, can rapidly influence uptake of aromatic amino acid into the brain and, as a result, directly modify their conversion to neurotransmitters. Such alterations in the production of transmitters can directly modify their release from neurons and, thus, influence brain function. The acidic amino acids glutamate and aspartate are themselves brain neurotransmitters. However, they do not have ready access to the brain from the circulation or the diet. As a result, the ingestion of proteins, which are naturally rich in aspartate and glutamate, has no effect on the level of acidic amino acid in the brain (or, thus, on brain function by this mechanism). Nevertheless, the food additives monosodium glutamate and aspartame (which contains aspartate) have been reputed to raise the level of acidic amino acid in the brain (when ingested in enormous amounts), to modify brain function, and even to cause neuronal damage. Despite such claims, a substantial body of published evidence clearly indicates that the brain is not affected by ingestion of aspartame and is affected by glutamate only when the amino acid is administered alone in extremely large doses. Therefore, when consumed in the diet neither compound presents a risk to normal brain function.

A report of the proceedings of an MSG workshop held August 1991

This report of the proceedings of a workshop on monosodium glutamate (MSG) represents the output of an exchange of scientific information, discussed and debated, by a group of experts representing a variety of disciplines. Experts in the areas of food science, potential adverse reactions to foods, pharmacology, neuroscience, biochemistry, nutrition, pediatrics, and anatomy reviewed the current scientific literature relative to the safe use of MSG in foods. These proceedings supplement the extensive literature compiled by various prestigious international expert committees since the flavor-enhancing properties of MSG were identified around the turn of the century,

Behavioral assessment of the toxicity of aspartame

Six experiments with rats assessed the toxicity of aspartame with behavioral measures. The first three experiments used a conditioned taste aversion procedure since taste aversions are typically observed after a taste is followed by a toxin. Thirty min after thirsty rats drank a sweet solution they were intraperitoneally injected (Experiment 1) or intragastrically intubated (Experiment 2) with saline or 176, 352, or 704 mg/kg of aspartame. Relative to rats given saline, rats injected with 704 and 352 mg/kg aspartame showed strong and mild aversions, respectively. Rats injected with 176 mg/kg of aspartame or intubated with any dose of aspartame did not show taste aversions. In Experiment 3, rats voluntarily consumed an aspartame solution sweetened with saccharin for 7 hr each day. Consumption of the taste paired with aspartame was not reduced. When 352 mg/kg aspartame was injected (Experiment 4), but not when intubated (Experiment 5), 5 min prior to access to a running wheel, running was reduced. Wheel running was not affected by the voluntary consumption of aspartame (Experiment 6). The route of administration effect (intraperitoneal vs. intragastric) on behavior corresponded with the amino acid levels in blood plasma (Experiment 7). Aspartate, phenylalanine, tyrosine and glutamate levels increased more after the injection, than the intubation, of aspartame (176 mg/kg). Overall, the results suggest that aspartame may have adverse effects when intraperitoneally injected but not when the route of administration is oral.

Effects of perinatal exposure to aspartame on rat pups

Possible effects of perinatal exposure to L-aspartyl-L-phenylalanine methyl ester (aspartame) on rat pups were investigated. Adult female rats, and later their pups, were exposed, via their drinking water, to aspartame (0.007%, 0.036%, 0.18% or 0.9% w/v) or phenylalanine (0.45% w/v) for 12 days prior to conception until the pups were 38 days old. Control rats were given plain water. The adults exposed to aspartame consumed an average of 14, 68, 347 and 1614 mg/kg/day of aspartame and those exposed to phenylalanine consumed an average of 835 mg/kg/day of phenylalanine. After weaning the pups given aspartame consumed an average of 32, 154, 836, and 3566 mg/kg/day of aspartame and those given phenylalanine consumed an average of 1795 mg/kg/day of phenylalanine. No effect of aspartame or phenylalanine was detected on either two measures of morphological development (i.e., latencies to pinnae detachment and eye opening) or two tests of reflex development (i.e., latencies for surface righting at 7 days of age and negative geotaxis at 8 days of age). All groups were similar in spatial memory as assessed with two different mazes with pups 30-36 days old. The number of arms before reentry in an 8-arm radial-arm maze and the acquisition curves from a milk maze did not differ between groups. Furthermore, the latencies of mothers to retrieve their litters was also unaffected by the aspartame and phenylalanine. These results indicate that perinatal exposure to aspartame, when voluntarily consumed by mothers (14-1614 mg/kg/day) and later directly by the rat pups (32 to 3566 mg/kg/day) does not affect reflex development, morphological development or spatial memory.

Plasma amino acid concentrations in normal adults ingesting aspartame and monosodium L-glutamate as part of a soup/beverage meal

We tested the hypothesis that ingestion of monosodium L-glutamate with aspartame produces a marked increase in plasma glutamate and aspartate concentrations. Twelve normal adults (6 males, 6 females) ingested three different soup/beverage meals in a balanced Latin square design. One meal (A) provided no aspartame (APM) or monosodium L-glutamate (MSG); a second (B) provided 50 mg MSG/kg body weight; while the third (C) provided 50 mg MSG and 34 mg APM per kg body weight. Plasma glutamate (Glu) concentrations were not significantly affected by meal A but increased significantly after meals B and C (no significant difference between B and C). Plasma aspartate (Asp) concentrations were not significantly affected by meal A but increased significantly after meals B and C (values significantly higher after meal C than meal B). Plasma Glu + Asp concentrations were not significantly affected by meal A but increased significantly from a mean (+/- SD) baseline value of 5.64 +/- 2.62 mumol/dL to high mean values of 23.1 +/- 7.29 and 26.8 +/- 9.74 mumol/dL after ingestion of meals B and C, respectively (no significant difference between meals B and C). Similarly, the area under the plasma Glu + Asp concentration-time curve did not differ significantly between meals B and C (624 +/- 197 v 763 +/- 277 mumol/dL x min, respectively). Peak plasma Glu + Asp concentrations for each subject (ignoring time) were also examined. The mean peak plasma Glu + Asp concentrations were 7.39 +/- 2.77, 23.0 +/- 6.61, and 27.3 +/- 9.07 mumol/dL, respectively after meals A, B, and C.

Correlation of glutamate plus aspartate dose, plasma amino acid concentration and neuronal necrosis in infant mice

Eight-day-old mice were given by gavage glutamate and aspartate mixtures providing each amino acid at 125, 250 or 500 mg/kg body weight (250, 500 and 1000 mg total dicarboxylic amino acids/kg) and the degree and extent of neuronal necrosis were determined. Similar studies were carried out in mice given monosodium L-glutamate at 250 or 500 mg/kg body weight. Plasma aspartate and glutamate concentrations were determined at each dose level. No animal given either glutamate or the glutamate plus aspartate mixture at 250 mg/kg developed neuronal necrosis. However, neuronal necrosis developed in 30% of animals given glutamate at 500 mg/kg (12+/-2 necrotic neurons/section in the region of maximal damage) and in 17% of animals given 250 mg glutamate/kg plus 250 mg aspartate/kg (11-13 necrotic neurons/section in the region of maximal damage). The threshold mean peak plasma glutamate plus aspartate concentration associated with neuronal necrosis was 128+/-24 mumol/dl. Using these data, and previously published data for aspartate-induced neurotoxicity (Finkelstein et al. Toxicology 1983, 29, 109), the individual threshold plasma glutamate and aspartate concentrations associated with neuronal necrosis were calculated to be 110 mumol/dl for aspartate and 75 mumol/dl for glutamate.

Correlation of aspartate dose, plasma dicarboxylic amino acid concentration, and neuronal necrosis in infant mice

Eight-day-old mice were administered aspartate at 0, 1.88, 3.76, 4.89, 5.64 and 7.52 mmol/kg body wt and the degree and extent of neuronal necrosis determined. In addition, plasma aspartate and glutamate concentrations were determined at each aspartate dose. Animals administered aspartate at 0, 1.88 and 3.76 mmol/kg body wt did not develop neuronal necrosis. Hypothalamic neuronal necrosis (7.33 +/- 1.52 necrotic neurons/section of maximal damage) was found in 3 of 10 animals administered aspartate at 4.89 mmol/kg body wt. The extent of neuronal necrosis was proportional to dose once a neurotoxic dose of aspartate was reached. All 12 animals administered aspartate at 5.64 mmol/kg body wt developed lesions (49.5 +/- 7.2 necrotic neurons/section of maximal damage). Similarly, all 18 mice administered aspartate at 7.52 mmol/kg developed hypothalamic lesions (80.8 +/- 17.8 necrotic neurons/section of maximal damage). Infant mice administered the highest dose of aspartate not producing neuronal necrosis (3.76 mmol/kg) had a mean (+/- S.D.) peak plasma aspartate concentration of 87 +/- 23 mumol/dl and a mean peak plasma glutamate concentration of 64 +/- 22 mumol/dl. Thus, the toxic threshold for these amino acids must be greater than those values.

Plasma and erythrocyte concentrations of free amino acids in adult humans administered abuse doses of aspartame

Plasma and erythrocyte concentrations of amino acids were measured in 18 fasting adult subjects (9 male, 9 female) administered abuse doses of aspartame (100, 150, and 200 mg/kg body weight) dissolved in 500 ml orange juice. Six subjects were studied at each dose. Plasma aspartate concentrations increased significantly (p less than or equal to 0.05) over baseline values after ingestion of each dose. However, the increase was small in each case, and maximal levels observed were below those noted postprandially in formula-fed infants. No significant changes (p greater than 0.05) were noted in erythrocyte glutamate, or erythrocyte aspartate concentrations after any dose. Plasma phenylalanine concentrations increased significantly over fasting concentrations (p less than 0.01) from 15 min to 6 h after each dose, and the increase was proportional to dose. Mean (+/- SD) peak plasma phenylalanine concentrations were 20.3 +/- 2.03, 35.1 +/- 11.3, and 48.7 +/- 15.5 mumol/dl, respectively, after aspartame doses of 100, 150, and 200 mg/kg. Erythrocyte phenylalanine concentrations showed similar changes. Although these phenylalanine concentrations are considerably above the normal postprandial range (12 +/- 3 mumol/dl), they are below values associated with toxic findings. These data indicate little risk to normal subjects from excessive aspartate or phenylalanine levels after ingestion of single abuse loads of aspartame.

Plasma phenylalanine levels in phenylketonuric heterozygous and normal adults administered aspartame at 34 mg/kg body weight

Following administration of aspartame (34 mg/kg body wt) in orange juice, plasma concentrations of free amino acids were measured in 12 female subjects known to be heterozygous for phenylketonuria and 22 normal subjects (12 male, 10 female). No change in fasting plasma aspartate concentrations were noted after aspartame loading in either group. In normal male subjects, the mean (+/-S.D.) plasma phenylalanine concentration increased from a fasting value of 5.86 +/- 1.25 mumol/dl. Plasma phenylalanine levels in normal female subjects increased from a mean fasting concentration of 4.83 +/- 0.84 mumol/dl to a men peak value of 8.95 +/- 1.49 mumol/dl suggesting a more rapid absorption, metabolism, and/or clearance of phenylalanine by females. In female heterozygous subjects, the mean peak plasma phenylalanine concentration was significantly higher than in normal females. Plasma phenylalanine values increased from a mean fasting value of 5.92 +/- 1.51 mumol/dl to a mean peak value of 15.1 +/- 4.76 mumol/dl. Similarly, the area under the plasma phenylalanine concentration-time curve was significantly greater in heterozygous female subjects (21.36 +/- 5.10 IU) than in normal female subjects (10.84 +/- 2.32 IU). However, peak plasma phenylalanine levels were well below those associated with toxic effects in all cases.