Placental transfer of aspartate and its metabolites in the primate

Aspartame Intake Delayed Puberty Onset in Female Offspring Rats and Girls

SCOPE

The disturbance of the hypothalamic-pituitary-gonadal (HPG) axis, gut microbiota (GM) community, and short-chain fatty acids (SCFAs) is a triggering factor for pubertal onset. The study investigates the effects of the long-term intake of aspartame on puberty and GM in animals and humans.

METHODS AND RESULTS

Aspartame-fed female offspring rats result in vaginal opening time prolongation, serum estrogen reduction, and serum luteinizing hormone elevation. , 60 mg kg-1 aspartame treatment decreases the mRNA levels of gonadotropin-releasing hormone (GnRH), Kiss1, and G protein-coupled receptor 54 (GPR54), increases the mRNA level of RFamide-related peptide-3 (RFRP-3), and decreases the expression of GnRH neurons in the hypothalamus. Significant differences in relative bacterial abundance at the genus levels and decreased fecal SCFA levels are noted by 60 mg kg-1 aspartame treatment. Among which, Escherichia-Shigella is negatively correlated with several SCFAs. In girls, high-dose aspartame consumption decreases the risk of precocious puberty.

CONCLUSIONS

Aspartame reduces the chance of puberty occurring earlier than usual in female offspring and girls. Particularly, 60 mg kg-1 aspartame-fed female offspring delays pubertal onset through the dysregulation of HPG axis and GM composition by inhibiting the Kiss1/GPR54 system and inducing the RFRP-3. An acceptable dose of aspartame should be recommended during childhood.

Interactive effects of neonatal exposure to monosodium glutamate and aspartame on glucose homeostasis

BACKGROUND

Recent evidence suggests that the effects of certain food additives may be synergistic or additive. Aspartame (ASP) and Monosodium Glutamate (MSG) are ubiquitous food additives with a common moiety: both contain acidic amino acids which can act as neurotransmitters, interacting with NMDA receptors concentrated in areas of the Central Nervous System regulating energy expenditure and conservation. MSG has been shown to promote a neuroendocrine dysfunction when large quantities are administered to mammals during the neonatal period. ASP is a low-calorie dipeptide sweetener found in a wide variety of diet beverages and foods. However, recent reports suggest that ASP may promote weight gain and hyperglycemia in a zebrafish nutritional model.

METHODS

We investigated the effects of ASP, MSG or a combination of both on glucose and insulin homeostasis, weight change and adiposity, in C57BL/6 J mice chronically exposed to these food additives commencing in-utero, compared to an additive-free diet. Pearson correlation analysis was used to investigate the associations between body characteristics and variables in glucose and insulin homeostasis.

RESULTS

ASP alone (50 mg/Kgbw/day) caused an increase in fasting blood glucose of 1.6-fold, together with reduced insulin sensitivity during an Insulin Tolerance Test (ITT) P < 0.05. Conversely MSG alone decreased blood triglyceride and total cholesterol (T-CHOL) levels. The combination of MSG (120 mg/Kgbw/day) and ASP elevated body weight, and caused a further increase in fasting blood glucose of 2.3-fold compared to Controls (prediabetic levels); together with evidence of insulin resistance during the ITT (P < 0.05). T-CHOL levels were reduced in both ASP-containing diets in both genders. Further analysis showed a strong correlation between body weight at 6 weeks, and body weight and fasting blood glucose levels at 17 weeks, suggesting that early body weight may be a predictor of glucose homeostasis in later life.

CONCLUSIONS

Aspartame exposure may promote hyperglycemia and insulin intolerance. MSG may interact with aspartame to further impair glucose homeostasis. This is the first study to ascertain the hyperglycemic effects of chronic exposure to a combination of these commonly consumed food additives; however these observations are limited to a C57BL/6 J mouse model. Caution should be applied in extrapolating these findings to other species.

Gender dimorphism in aspartame-induced impairment of spatial cognition and insulin sensitivity

Previous studies have linked aspartame consumption to impaired retention of learned behavior in rodents. Prenatal exposure to aspartame has also been shown to impair odor-associative learning in guinea pigs; and recently, aspartame-fed hyperlipidemic zebrafish exhibited weight gain, hyperglycemia and acute swimming defects. We therefore investigated the effects of chronic lifetime exposure to aspartame, commencing in utero, on changes in blood glucose parameters, spatial learning and memory in C57BL/6J mice. Morris Water Maze (MWM) testing was used to assess learning and memory, and a random-fed insulin tolerance test was performed to assess glucose homeostasis. Pearson correlation analysis was used to investigate the associations between body characteristics and MWM performance outcome variables. At 17 weeks of age, male aspartame-fed mice exhibited weight gain, elevated fasting glucose levels and decreased insulin sensitivity compared to controls (P<0.05). Females were less affected, but had significantly raised fasting glucose levels. During spatial learning trials in the MWM (acquisition training), the escape latencies of male aspartame-fed mice were consistently higher than controls, indicative of learning impairment. Thigmotactic behavior and time spent floating directionless was increased in aspartame mice, who also spent less time searching in the target quadrant of the maze (P<0.05). Spatial learning of female aspartame-fed mice was not significantly different from controls. Reference memory during a probe test was affected in both genders, with the aspartame-fed mice spending significantly less time searching for the former location of the platform. Interestingly, the extent of visceral fat deposition correlated positively with non-spatial search strategies such as floating and thigmotaxis, and negatively with time spent in the target quadrant and swimming across the location of the escape platform. These data suggest that lifetime exposure to aspartame, commencing in utero, may affect spatial cognition and glucose homeostasis in C57BL/6J mice, particularly in males.

Position of the American Dietetic Association: use of nutritive and nonnutritive sweeteners

Sweeteners elicit pleasurable sensations with (nutritive) or without (nonnutritive) energy. Nutritive sweeteners (eg, sucrose, fructose) are generally recognized as safe (GRAS) by the Food and Drug Administration (FDA), yet concern exists about increasing sweetener intakes relative to optimal nutrition and health. Dietary quality suffers at intakes above 25% of total energy (the Institutes of Medicine's suggested maximal intake level). In the United States, estimated intakes of nutritive sweeteners fall below this, although one in four children (ages 9 to 18 years) can surpass this level. Polyols (sugar alcohols), GRAS-affirmed or petitions filed for GRAS, add sweetness with reduced energy and functional properties to foods/beverages and promote dental health. Five nonnutritive sweeteners with intense sweetening power have FDA approval (acesulfame-K, aspartame, neotame, saccharin, sucralose) and estimated intakes below the Acceptable Daily Intake (level that a person can safely consume everyday over a lifetime without risk). By increasing palatability of nutrient-dense foods/beverages, sweeteners can promote diet healthfulness. Scientific evidence supports neither that intakes of nutritive sweeteners by themselves increase the risk of obesity nor that nutritive or nonnutritive sweeteners cause behavioral disorders. However, nutritive sweeteners increase risk of dental caries. High fructose intakes may cause hypertriglyceridemia and gastrointestinal symptoms in susceptible individuals. Thus, it is the position of The American Dietetic Association that consumers can safely enjoy a range of nutritive and nonnutritive sweeteners when consumed in a diet that is guided by current federal nutrition recommendations, such as the Dietary Guidelines for Americans and the Dietary References Intakes, as well as individual health goals. Dietetics professionals should provide consumers with science-based information about sweeteners and support research on the use of sweeteners to promote eating enjoyment, optimal nutrition, and health.

Acceptable daily intake vs actual intake: the aspartame example

This article discusses the acceptable daily intake (ADI) and the postmarketing surveillance of consumption levels for a food additive, using the widely used food additive aspartame (APM, L-aspartyl-L-phenylalanine methyl ester) as an example. The safety implications of the ADI and consumption levels are also discussed. Aspartame has been assigned an ADI of 40 mg/kg/day by the World Health Organization and regulatory authorities in Europe and Canada, and of 50 mg/kg/day by the US Food and Drug Administration. A number of different methods have been used to measure consumption levels of food additives. Consumption estimations for aspartame from one such method, the food intake survey, have been done in the United States, Canada, Germany, and Finland. APM consumption in all age groups and selected subpopulations, even at the 90th percentile, is approximately 2-10 mg/kg/day and is thus well below the ADI.

The relationship between placental protein synthesis and transfer of amino acids

The relationship between placental protein synthesis and transfer of amino acids from mother to foetus was studied in the guinea pig, by using [U-14C]-lysine, -leucine, -glycine, -aspartate and -alpha-aminoisobutyrate. The uptake of label by protein was 12-16% of total label transferred. Cycloheximide inhibited incorporation of all naturally occurring amino acids into protein by 81-96% and transfer by 62-75%; the concentration of label in the free pool was increased for each. These findings were confirmed when specific-radioactivity measurements were made with L-[U-14C]lysine. The transfer of the non-protein amino acid alpha-aminoisobutyrate was not significantly decreased by cycloheximide. A model, linking protein synthesis to the generation of a transfer pool of amino acids, is proposed whereby inhibition of protein synthesis decreases the amount of amino acid available for transfer.

The uptake of naturally occurring amino acids by the plasma membrane of the human placenta

The characteristics of the uptake of four naturally occurring amino acids, alanine, leucine, lysine and aspartate, by microvesicles prepared from the plasma membrane on the maternal surface of the human placenta were studied. Electron microscopy showed that a variety of fragments was interspersed between the vesicles and this heterogeneity probably accounted for the considerable variation in the results. Nevertheless, the properties of the uptake of amino acids were similar to those of vesicle preparations from other tissues. Insulin did not enhance the uptake of any of the amino acids.

Maternal and fetal plasma levels of 3-methylhistidine in pregnant nonhuman primates

Available data indicate little reutilization of 3-methylhistidine (3-MH) in the rat and man. These data led to the use of urinary 3-MH excretion as a measure of muscle protein catabolism in those animal species. However, 3-MH excretion does not accurately measure protein catabolism in the sheep, pig, and rabbit. This is due, at least in part, to the fact that renal amino acid (AA) transport systems reabsorb 3-MH from the glomerular filtrate. The monkey differs from man in that its plasms contains significant quantities of 3-MH, suggesting an active renal transport system for this AA. The present study measured maternal and fetal plasma 3-MH levels in 33 pregnant rhesus monkeys to determine whether the non-human primate placenta contained transport sites concentrating this AA to the fetal plasma. Mean fetal plasma 3-MH concentrations were 16.4 +/- 6.71 micrometers/100 ml, while maternal levels were 9.45 +/- 3.69 micrometers/100 ml. The fetal to maternal gradient was maintained between 1.6 to 1.7 during the course of maternal infusions of various AA. Since placental AA transport systems are similar to those in the kidney and intestine, the data also suggest the presence of AA transport systems for 3-MH in the monkey, indicating that urinary 3-MH excretion would be a poor method for measuring muscle protein catabolism in this species.