Does the thrifty phenotype result from chronic glutamate intoxication? A hypothesis

Alanine and aspartate aminotransferase and glutamine-cycling pathway: Their roles in pathogenesis of metabolic syndrome

Although new research technologies are constantly used to look either for genes or biomarkers in the prediction of metabolic syndrome (MS), the pathogenesis and pathophysiology of this complex disease remains a major challenge. Interestingly, Cheng et al recently investigated possible pathways underlying MS by high-throughput metabolite profiling in two large and well characterized community-based cohorts. The authors explored by liquid chromatography and mass spectrometry the plasma concentrations of 45 distinct metabolites and examined their relation to cardiometabolic risk, and observed that metabolic risk factors such as obesity, insulin resistance (IR), high blood pressure, and dyslipidemia were associated with several metabolites, including branched-chain amino acids, other hydrophobic amino acids, tryptophan breakdown products, and nucleotide metabolites. In addition, the authors found a significant association of IR traits with glutamine, glutamate and the glutamine-to-glutamate ratio. These data provide new insight into the pathogenesis of MS-associated phenotypes and introduce a crucial role of glutamine-cycling pathway as prominently involved in the development of metabolic risk. We consider that the hypothesis about the role of abnormal glutamate metabolism in the pathogenesis of the MS is certainly challenging and suggests the critical role of the liver in the global metabolic modulation as glutamate metabolism is linked with aminotransferase reactions. We discuss here the critical role of the “liver metabolism” in the pathogenesis of the MS and IR, and postulate that before fatty liver develops, abnormal levels of liver enzymes, such as alanine and aspartate aminotransferases might reflect high levels of hepatic transamination of amino acids in the liver.

The potential role of glutamate in the current diabetes epidemic

In the present article, we propose the perspective that abnormal glutamate homeostasis might contribute to diabetes pathogenesis. Previous reports and our recent data indicate that chronically high extracellular glutamate levels exert direct and indirect effects that might participate in the progressive loss of β-cells occurring in both T1D and T2D. In addition, abnormal glutamate homeostasis may impact all the three accelerators of the "accelerator hypothesis" and could partially explain the rising frequency of T1D and T2D.

Clinical psychopharmacology of eating disorders: a research update

The paper presents a critical review (with search date 2010) of the major psychotropic medications assessed in eating disorders, namely antipsychotics, antidepressants, mood-stabilizing medications, anxiolytic and other agents. The evidence of efficacy of drug treatments is mostly weak or moderate. In addition, attrition rates are usually higher than for psychotherapies. However, there is support for use of antidepressants, particularly high-dose fluoxetine in bulimia nervosa, and anticonvulsants (topiramate) for binge-eating disorder. Low-dose antipsychotic medication may be clinically useful as adjunct treatment in acute anorexia, particularly where there is high anxiety and obsessive eating-related ruminations and failure to engage, but more trials are needed. Drug therapies such as topiramate and anti-obesity medication may aid weight loss in obese or overweight patients with binge-eating disorder; however, common or potentially serious adverse effects limit their use.

Consumption of monosodium glutamate in relation to incidence of overweight in Chinese adults: China Health and Nutrition Survey (CHNS)

BACKGROUND

It has been hypothesized that monosodium glutamate (MSG), a flavor enhancer, is positively associated with weight gain, which influences energy balance through the disruption of the hypothalamic signaling cascade of leptin action.

OBJECTIVE

The objective was to examine the longitudinal association between MSG consumption and incidence of overweight.

DESIGN

Data were collected from the China Health and Nutrition Survey (CHNS), a prospective open-cohort, ongoing nationwide health and nutrition survey, consisting of 10,095 apparently healthy Chinese adults aged 18-65 y at entry from 1991 to 2006. Diet, including MSG and other condiments, was assessed with a weighed food inventory in combination with three 24-h recalls. Incident overweight was defined as a body mass index (BMI; in kg/m(2)) ≥ 25 or ≥23 based on World Health Organization recommendations for Asian populations. Multilevel mixed-effects models were constructed to estimate change in BMI, and Cox regression models with gamma shared frailty were used to determine the incidence of overweight.

RESULTS

The mean follow-up was 5.5 y. The cumulative mean (±SD) MSG intake of 2.2 ± 1.6 g/d was positively associated with BMI after adjustment for potential confounders and cluster effects at different levels (individual, household, and community). The adjusted hazard ratio of overweight was 1.33 (95% CI: 1.01, 1.75; P for trend < 0.01) for participants in the highest quintile of MSG intake compared with those in the lowest quintile after adjustment for age, physical activity, total energy intake, and other major lifestyle factors.

CONCLUSIONS

MSG consumption was positively, longitudinally associated with overweight development among apparently healthy Chinese adults. Additional studies are needed to elucidate mechanisms of action and to establish causal inference.

Feto-placental adaptations to maternal obesity in the baboon

Maternal obesity is present in 20-34% of pregnant women and has been associated with both intrauterine growth restriction and large-for-gestational age fetuses. While fetal and placental functions have been extensively studied in the baboon, no data are available on the effect of maternal obesity on placental structure and function in this species. We hypothesize that maternal obesity in the baboon is associated with a maternal inflammatory state and induces structural and functional changes in the placenta. The major findings of this study were: 1) decreased placental syncytiotrophoblast amplification factor, intact syncytiotrophoblast endoplasmic reticulum structure and decreased system A placental amino acid transport in obese animals; 2) fetal serum amino acid composition and mononuclear cells (PBMC) transcriptome were different in fetuses from obese compared with non-obese animals; and 3) maternal obesity in humans and baboons is similar in regard to increased placental and adipose tissue macrophage infiltration, increased CD14 expression in maternal PBMC and maternal hyperleptinemia. In summary, these data demonstrate that in obese baboons in the absence of increased fetal weight, placental and fetal phenotype are consistent with those described for large-for-gestational age human fetuses.

Metabolic fate and function of dietary glutamate in the gut

Glutamate is a main constituent of dietary protein and is also consumed in many prepared foods as an additive in the form of monosodium glutamate. Evidence from human and animal studies indicates that glutamate is a major oxidative fuel for the gut and that dietary glutamate is extensively metabolized in first pass by the intestine. Glutamate also is an important precursor for bioactive molecules, including glutathione, and functions as a key neurotransmitter. The dominant role of glutamate as an oxidative fuel may have therapeutic potential for improving function of the infant gut, which exhibits a high rate of epithelial cell turnover. Our recent studies in infant pigs show that when glutamate is fed at higher (4-fold) than normal dietary quantities, most glutamate molecules are either oxidized or metabolized by the mucosa into other nonessential amino acids. Glutamate is not considered to be a dietary essential, but recent studies suggest that the level of glutamate in the diet can affect the oxidation of some essential amino acids, namely leucine. Given that substantial oxidation of leucine occurs in the gut, ongoing studies are investigating whether dietary glutamate affects the oxidation of leucine in the intestinal epithelial cells. Our studies also suggest that at high dietary intakes, free glutamate may be absorbed by the stomach as well as the small intestine, thus implicating the gastric mucosa in the metabolism of dietary glutamate. Glutamate is a key excitatory amino acid, and metabolism and neural sensing of dietary glutamate in the developing gastric mucosa, which is poorly developed in premature infants, may play a functional role in gastric emptying. These and other recent reports raise the question as to the metabolic role of glutamate in gastric function. The physiologic significance of glutamate as an oxidative fuel and its potential role in gastric function during infancy are discussed.

Association of monosodium glutamate intake with overweight in Chinese adults: the INTERMAP Study

Animal studies indicate that monosodium glutamate (MSG) can induce hypothalamic lesions and leptin resistance, possibly influencing energy balance, leading to overweight. This study examines the association between MSG intake and overweight in humans. We conducted a cross-sectional study involving 752 healthy Chinese (48.7% women), aged 40-59 years, randomly sampled from three rural villages in north and south China. The great majority of participants prepared their foods at home, without use of commercially processed foods. Diet was assessed with four in-depth multipass 24-h recalls. Participants were asked to demonstrate MSG amounts added in food preparation. Amounts shaken out were weighed by trained interviewers. Overweight was defined as BMI > or =25.0 or > or =23.0 kg/m(2)(based on World Health Organization recommendations for Asian populations). Eighty-two percent of participants were MSG users. Average intake was 0.33 g/day (s.d. = 0.40). With adjustment for potential confounders including physical activity and total energy intake, MSG intake was positively related to BMI. Prevalence of overweight was significantly higher in MSG users than nonusers. For users in the highest tertile of MSG intake compared to nonusers, the multivariable-adjusted odds ratios of overweight (BMI > or =23.0 and > or =25.0) were 2.10 (95% confidence interval, 1.13-3.90, P for trend across four MSG categories = 0.03) and 2.75 (95% confidence interval, 1.28-5.95, P = 0.04). This research provides data that MSG intake may be associated with increased risk of overweight independent of physical activity and total energy intake in humans.

Obesity, voracity, and short stature: the impact of glutamate on the regulation of appetite

BACKGROUND

World-wide obesity has risen to alarming levels. We present experimental support for a new and very challenging hypothesis linking obesity, voracity, and growth hormone (GH) deficiency, to the consumption of elevated amounts of the amino-acid glutamate (GLU). Supraphysiological doses of GLU are toxic for neuronal cells.

METHODS

Human data were obtained from 807,592 German conscripts born between 1974 and 1978, and from 1,432,368 women of the German birth statistics (deutsche Perinatalerhebung) 1995-1997. The effects of orally administered monosodium glutamate (MSG) were investigated in 30 pregnant Wistar rats and their offspring. Pregnant animals either received no extra MSG, or 2.5 g MSG, or 5 g MSG per day, up to the end of the weaning period. In all, 2.5 g, respectively 5 g, MSG accounted for some 10%, respectively 20%, of dry weight of the average daily food ration. After weaning, MSG feeding was continued in the offspring.

FINDINGS

Morbid obesity associates with short stature. Average stature of conscripts progressively declines when body mass index increases above 38 kg/m2. Also morbidly obese young women are shorter than average though to a lesser extent than conscripts. Oral administration of MSG to pregnant rats affects birth weight of the offspring. Maternal feeding with 5 g MSG per day results in severe birth weight reduction (P<0.01). Weight increments remain subnormal when MSG feeding to the mothers is maintained during weaning (P < 0.01). GH serum levels are affected in animals that received MSG during prenatal life via maternal feeding. Animals that are kept on high MSG diet (5 g MSG per day) continue to show serum GH levels that are as low or even lower than those of MSG injected animals (P < 0.05), both at day 30 and at day 90 of life. Animals that were kept on medium MSG diet (2.5 g MSG per day) showed low serum GH levels at day 30 of life (P < 0.01), but seemed to partially recover before day 90. Almost identical results were observed in IGF-1 serum levels. Oral MSG resulted in dose dependent voracity. The animals fed 5 g MSG per day increased water uptake by threefold (P < 0.01), and food uptake by almost two-fold (P < 0.01). The influence of MSG is in general more marked in males than in females.

INTERPRETATION

GLU is a widely used nutritional substance that potentially exhibits significant neuronal toxicity. Voracity, and impaired GH secretion are the two major characteristics of parenterally administered GLU-induced neuronal damage. GLU maintains its toxicity in animals even when administered orally. Males appear to be more sensitive than females. The present study for the first time demonstrates, that a widely used nutritional monosubstance--the flavouring agent MSG--at concentrations that only slightly surpass those found in everyday human food, exhibits significant potential for damaging the hypothalamic regulation of appetite, and thereby determines the propensity of world-wide obesity. We suggest to reconsider the recommended daily allowances of amino acids and nutritional protein, and to abstain from the popular protein-rich diets, and particularly from adding the flavouring agents MSG.

A new anti-obesity drug treatment: first clinical evidence that, antagonising glutamate-gated Ca2+ ion channels with memantine normalises binge-eating disorders

The regulation of appetite relies on complex hypothalamic neurocircuitry of which the arcuate nucleus, and the hormone leptin play important roles. Arcuate nucleus neurones are essential for the regulation of eating behaviour, but they can be intoxicated by elevated serum levels of the amino acid glutamate (GLU). Neurotoxic effects of GLU are mediated by the N-methyl-D-aspartate receptor (NMDA-R). But the neurotoxic effects of GLU can be prevented. Concurrent administration of dizocilpine maleate (MK-801), a selective and highly potent non-competitive NMDA-R antagonist, antagonises GLU-gated Ca2+ ion channels and completely prevents the adverse effects of GLU. Also the non-competitive NMDA-R antagonist memantine displays neuroprotective properties. In view of a previously published hypothesis that human obesity results from chronic over-consumption of GLU, we performed a therapeutic trial in five obese, but otherwise healthy women. Memantine treatment markedly decreased appetite within few hours and complete suppressed the binge-eating disorder within 24 h. Body weight decreased markedly within a few days. The findings strongly support the hypothesis that elevated levels of nutritional GLU play an important role in the pathomechanism of human obesity. We suggest to treat human obesity by protecting the hypothalamic signalling cascade of leptin action with low to moderate affinity, non-competitive NMDA-R antagonists that selectively block the GLU-gated Ca2+ ion channels.

Morbid obesity is associated with short stature

Body height and body mass index (BMI) were obtained from 807,592 German conscripts born between 1974 and 1978, aged 19-20 years. The conscripts had either completed Gymnasium (secondary school, more than 10 years, A-level), Realschule (secondary school, 10 years, O-level), or Hauptschule (elementary school, 9 years). Maternal data on body height and weight at the beginning of pregnancy from 1,432,368 women were obtained from the German birth statistics (deutsche Perinatalerhebung) 1995-1997. Morbid obesity is associated with short stature. Regardless of school education, average stature of conscripts progressively declines by up to 10 cm when the BMI increases above 38 kg/m2. In addition, morbidly obese young women are shorter than average, though the impact of overweight on adult stature appears to be less pronounced than in males.

How much glutamate is toxic in paediatric parenteral nutrition?

AIM

To discuss a hypothesis regarding the impact and possible side effects of glutamate in paediatric parenteral nutrition.

RESULTS

Published evidence suggests that the arcuate nucleus, which is a potent site of leptin action, is severely damaged by elevated glutamate levels. Early administration of glutamate (GLU) to the neonatal rat disrupts the hypothalamic signalling cascade of leptin action.

CONCLUSION

We are concerned that GLU-containing parenteral nutrition may not only increase the risk of hypothalamic damage in neurosurgical patients with an impaired blood-brain barrier, and in patients with periventricular leukomalacia, but may also permanently damage the arcuate nucleus neurones in the very immature infant. This may result in later impairment of feeding regulation, obesity, hyperleptinaemia, and other symptoms that characterize the "thrifty phenotype" and the dysmetabolic syndrome. We strongly suggest reconsidering the recommended daily allowances of amino acids, particularly the use of GLU, in current paediatric parenteral nutrition.