Monosodium glutamate kinetic studies in human volunteers

Glutamic acid, twenty years later

This review examines progress in understanding the physiologic functions of glutamic acid in the body since the first symposium on glutamic acid physiology and biochemistry was held at the Mario Negri Institute in Milan in 1978. The topics reviewed, although not exhaustive, include the metabolism of glutamic acid, umami taste, the role of glutamic acid as a neurotransmitter, glutamate safety and the development of new drugs resulting from the knowledge of the neurodegeneration induced by high doses of glutamic acid.

Effect of branched-chain amino acids on glutamate metabolism in amyotrophic lateral sclerosis

Data from the literature about glutamate metabolism remain controversial. To refine such analysis we have studied plasma glutamate and aspartate levels after glutamate loading (60 mg/kg) in 6 fasting controls and ALS patients, before and after at least 2 weeks treatment with branched-chain amino acids. ALS patients showed no difference from age-matched controls in basal plasma glutamate or aspartate levels, but significantly elevated levels of glutamate and aspartate at 30 and 45 min after loading, and an increased area under the curve in plasma for glutamate following oral glutamate loading. Two weeks BCAAs treatment did not affect plasma glutamate metabolism in ALS patients.

Monosodium L-glutamate: a double-blind study and review

71 healthy subjects were treated with placebos and monosodium L-glutamate (MSG) doses of 1.5, 3.0 and 3.15 g/person, which represented a body mass-adjusted dose range of 0.015-0.07 g/kg body weight before a standardized breakfast over 5 days. The study used a rigorous randomized double-blind crossover design that controlled for subjects who had MSG after-tastes. Capsules and specially formulated drinks were used as vehicles for placebo and MSG treatments. Subjects mostly had no responses to placebo (86%) and MSG (85%) treatments. Sensations, previously attributed to MSG, did not occur at a significantly higher rate than did those elicited by placebo treatment. A significant (P < 0.05) negative correlation between MSG dose and after-effects was found. The profound effect of food in negating the effects of large MSG doses was demonstrated. The common practice of extrapolating food-free experimental results to 'in use' situations was called into question. An exhaustive review of previous methodologies identified the strong taste of MSG as the factor invalidating most 'blind' and 'double-blind' claims by previous researchers. The present study led to the conclusion that 'Chinese Restaurant Syndrome' is an anecdote applied to a variety of postprandial illnesses; rigorous and realistic scientific evidence linking the syndrome to MSG could not be found.

Hypersensitivity reactions to food additives

Only seldom have food additives been shown to cause true allergic (immunological) reactions. Adverse effects due to various pharmacological or other mechanisms are much more common. The individual tolerance may be decreased for one reason or another, and may fluctuate from time to time. Many patients suffering from food additive reactions have atopic constitutions and such clinical symptoms as flexural dermatitis, rhinitis and asthma. The most important skin symptoms caused by food additives are urticaria, angioneurotic edema, and contact urticaria. Azo dyes, benzoic acid and several other common food additives may aggravate or, more rarely, even cause urticaria. Spices are one of the most common causes of immunological contact urticaria. Non-immunological contact urticaria is produced by numerous spices, benzoic acid, sorbic acid, cinnamic acid, and many essential oils. Asthma and rhinitis are the main hypersensitivity symptoms in the respiratory tract, and azo dyes, benzoic acid, and sulfitic food additives are the most common causative agents. Systemic and respiratory reactions to food colorants and benzoates have been claimed to occur more frequently in acetylsalicylic acid- (ASA-)sensitive patients than in non-reactors. Hypersensitivity reactions in organs other than the skin and respiratory tract are rare or poorly documented. Psychological factors play an essential role in both food and food additive reactions.

Does monosodium glutamate cause flushing (or merely "glutamania")?

Monosodium glutamate is widely regarded as the provocative agent in the "Chinese restaurant syndrome," of which flushing is regarded as part of the reaction. Six subjects were monitored by laser Doppler velocimetry for changes in facial cutaneous blood flow during challenge with monosodium glutamate and its cyclization product, pyroglutamate. Additionally, records of patients challenged with monosodium glutamate in the laboratory were reviewed. No flushing was provoked among the twenty-four people tested, eighteen of whom gave a positive history of Chinese restaurant syndrome flushing. These results indicate that monosodium glutamate-provoked flushing, if it exists at all, must be rare. Monosodium glutamate and its cyclization product, pyroglutamate, may provoke edema and associated symptoms.

Kinetics of monosodium glutamate in human volunteers under different experimental conditions

The kinetics of glutamic acid (GA) in plasma was studied in human volunteers after administration of monosodium glutamate (MSG) at different doses--43 mg/kg (3 g/70 kg) and 64 mg/kg (4.5 g/70 kg)--and in bouillon solutions of different concentrations (1.5-3.5%). MSG was administered to the subjects either during fasting or immediately after a standard meal. In the fasted subjects MSG administration caused a dose-dependent increase in plasma GA levels. In contrast, ingestion of MSG with a meal did not result in any significant increases in plasma GA levels in comparison with the wide variations observed in plasma GA after ingestion of a meal without added MSG.

Effect of water restriction on the development of hypothalamic lesions in weanling rodents given MSG. I. Drinking behaviour and physiological parameters in mice (Mus musculus)

Deprivation of water overnight (ON: 19.00-09.00 h) induced approx. 20% haemoconcentration in weanling ICR strain mice and resulted in avid consumption of fluid offered for 30 min subsequently, with delayed restoration of serum osmolality and sodium to normal levels in those offered 8% w/v monosodium L-glutamate monohydrate (MSG) aqueous solution. Neuronal necrosis of the arcuate nucleus of the hypothalamus was found in 12/180 dehydrated mice deprived ON and subsequently consuming 3.0 g or more MSG/kg body weight offered hypertonic MSG solution as the sole source of drinking water. The lesions were less severe than in mice given MSG s.c. or by forced intubation. Mice deprived ON and subsequently consuming MSG solution with water in a free choice experiment, and those deprived during daytime (DT: 07.00-21.00 h) and subsequently consuming MSG solution, with or without water, developed fewer and less severe lesions than did mice deprived ON and subsequently consuming MSG solution solely. The provision of other nutrients (e.g. glucose, arginine) reduced glutamate-induced hypothalamic lesions and elevation of plasma L-glutamate. The findings are in accord with earlier observations that hypothalamic lesions do not result even in this most susceptible species from the 'voluntary' consumption of MSG by physiologically normal animals. No lesions were produced even in mice deprived of food and water ON and subsequently consuming up to 7.3 g MSG/kg body weight in diet and in soup.

Plasma pyroglutamic acid levels after oral administration of monosodium glutamate to human volunteers

Plasma levels of pyroglutamic (PY) and glutamic (GA) acids have been measured after oral administration of monosodium glutamate (MSG) to 6 human volunteers. MSG (43 mg/kg) was administered either after an overnight fast or immediately after a normal meal. In fasting subjects plasma GA peak levels were about 2.3 times the basal levels whereas the increase in plasma PY levels were only 1.5-fold. When MSG was administered with a meal plasma GA levels were raised, to a lesser extent, while no increase could be observed in plasma PY.

Plasma and brain levels of glutamate and pyroglutamate after oral monosodium glutamate to rats

Plasma and brain levels of pyroglutamate (Py), a compound connected with the pathway of glutamate (GA) metabolism, were measured in rats after oral administration of monosodium glutamate (MSG) or Py. Oral MSG (1 g/kg) was followed by only a small rise in plasma Py levels. No increase of Py or GA brain levels was observed in these experimental conditions. Oral administration of Py (0.05, 0.5 and 1 g/kg) resulted in a marked dose-dependent increase of plasma Py, but no increase of plasma or brain GA levels. However, Py accumulated in brain in a dose-related manner. After 0.05 mg/kg the basal brain levels remained unchanged at all the times considered. Rat brain Py levels significantly increased when the oral dose of Py was raised to 0.5 g/kg. Peak brain levels were reached at 240 min, and were about 3 and 4 times the basal levels after 0.5 and 1 g/kg, respectively.