Portal Hyperglutamatemia after Dietary Supplementation with Monosodium Glutamate in Pigs

Abstract

The aim of the present work was to examine in pigs the effect of a dietary supplementation with the flavor enhancer monosodium glutamate (MSG) on intestinal amino acid metabolism. For this purpose, pigs weighing 60 ± 2 kg received a standard meal twice a day for 1 week, supplemented with either 10 g MSG per meal or, as control experiments, an isonitrogenous amount of glycine together with an equal amount of sodium in the form of NaCl, the animals being their own control in all experiments. At the end of this period, pigs received a MSG or glycine-NaCl-supplemented meal and samples of portal and arterial blood were collected for amino acid analysis in plasma. The results demonstrate after MSG supplementation rapid significant increases in glutamate concentration in the portal and arterial blood plasma after a test meal which resulted in a positive portoarterial difference. In comparison, after glycine-NaCl supplementation, glutamate concentrations were almost identical in portal and arterial plasma. Furthermore, significant increased aspartate concentration in the portal blood plasma was observed after MSG supplementation when compared with control experiments. When enterocytes were isolated at the end of the supplementation period from the jejunum and examined for their metabolic capacities towards L-glutamate and L-glutamine, it was found that metabolism did not differ according to the supplement used, with glutamate and glutamine being oxidized and transaminated at a similar level. It is concluded that the portal hyperglutamatemia observed shortly after the ingestion of a MSG- supplemented meal is likely due to the saturation of the intestinal capacity to metabolize glutamate with no measurable adaptation of the metabolic pathways controlling glutamate metabolism in enterocytes.

Metabolism of Dietary Glutamate in Adults

“…In these and other studies, there was some change in the concentrations of other AAs: ornithine [25] and aspartate [26].…”

“…In pigs, after ingestion of a meal containing 10 g MSG (i.e., ~167 mg/kg bw), plasma glutamate peaked at 60 min and then slowly decreased without returning to the basal value at 360 min. This was accompanied by an increase in plasma aspartate [26].…”

“…Interestingly this process appears to be saturable because if the level of perfused glutamate increases, it appears in the portal vein [26]: after a protein-rich meal, small amounts of glutamate therefore appear in portal blood and are taken up by the liver. In rats, on administering 13 N-glutamate in the portal vein [41] or 15 N-glutamate intravenously [42], the label appears within seconds in liver alanine and aspartate, underlining the importance of transamination reactions.…”

The Roles of Dietary Glutamate in the Intestine

“…When MSG was added at 15, 40, and 45 mg/kg (total, 100 mg/kg/day) respectively to the breakfast, lunch, and dinner meals, plasma Glu concentrations slightly but significantly increased after lunch and dinner, but the circadian variations of plasma glutamate were small (between 32 and 53 µM) and varied significantly as a function of the time of day, indicating that Glu is actively metabolized in the gut [46]. In adult pigs, transient portal and arterial increase in Glu concentration was observed when the diet was supplemented with 10 g MSG, but the most part of MSG was metabolized in the gut [51]. In post-weaning pigs, dietary supplementation with 4% MSG (2 g MSG/kg BW/day) results in a transient, approximately 75% increase in circulating Glu levels at 1 h after feeding but the resulting concentrations in plasma remain low [52].…”

The potential role of glutamate in the current diabetes epidemic

“…In piglet, increased glutamate concentrations were observed in the portal and arterial blood when the basal milk formula was supplemented with MSG [119]. Also in larger pigs, transient portal and arterial hyperglutamatemia was observed when the diet was supplemented with 10 g of MSG [120], indicating that large doses of glutamate may exceed the intestinal capacity to catabolize this amino acid. In men, the ingestion of 150 mg/kg of MSG determines a rise in plasma glutamate concentration of the 700-800% and increases intracellular free glutamate pool in the skeletal muscle for more than 1 h [121].…”