Abstract
Background: Dietary free glutamate promotes gastric emptying of protein-rich diet in healthy humans (Biol Pharm Bull.10:1841-3, 2008). However it is not known whether it could affect the process of digestion and/or absorption of dietary protein as a result of faster gastric emptying. Aim: In this study we examined the effects of glutamate on amino acids (AAs) increase both in portal and peripheral blood and on gastric emptying rate after intragastric protein-rich meal load in rats. Methods: Male SD rats (20 weeks) were fitted with portal vein cannula under nembutal anesthesia. Ten days later, blood samples were taken from a lateral tail vein and the portal vein of awake rats at 0, 5, 15, 30, 60, 90 and 120 min after protein-rich liquid diet load (10 ml/kg p.o., energy density: 1.0 kcal/ml) containing 12.5% casein-calcium and 12.5% dextrin. Plasma AAs were analyzed by AA analyzer (L-8800, Hitachi-high tech, Japan). Monosodium L-glutamate (0.5% w/v, normal habitual concentration for human) was used as an active ingredient and equimolar NaCl (0.18% w/v) was used as a control. To measure gastric emptying rate, 13CO2 excretion was measured by an open-circuit metabolic gas analysis system equipped with a mass spectrometer (Model RL600, ArcoSystem, Japan) after p.o. load of the same meal labeled by 13C-sodium acetate (100 mg/kg). Results: Intragastric load of the diet rapidly increased plasma glucose and all 20 AAs both in portal as well as peripheral vein with their peaks at 15 or 30 min. Free glutamate enrichment augmented the increase in portal plasma AUC (during 2hr) of AAs, but not in portal glucose. Significant augmentation was observed in both essential and nonessential AAs (mg/dl.hr, ** P<0.01, * P<0.05); alanine** (8.0±1.8 vs 4.3±1.2), leucine** (4.1±1.0 vs 2.4±0.5), glutamate**(1.9±0.7 vs 0.7±0.5), lysine*(7.5±2.4 vs 4.7±1.7), serine*(2.8±0.8 vs 1.8±0.7), arginine*(2.5±0.6 vs 1.8±0.4), lsoleucine*(2.2±0.2 vs 1.5±0.2), phenylalanine*(1.7±0.5 vs 1.0±0.3), histidine* (1.5±0.4 vs 1.0±0.4), methionine*(1.3±0.4 vs 0.8±0.3) . On the other hand, free glutamate enrichment did not change significantly either peripheral plasma AUC of glucose and AAs or gastric emptying rate of the meal (T 1/2, 1.07±0.03 vs 1.03±0.06 hr, P=0.10). Conclusions: Free glutamate significantly enhanced the portal increase in 10 out of 20 natural AAs composing proteins without affecting portal glucose increase or gastric emptying rate after the meal in rats, indicating that it facilitates the process of protein digestion and/or absorption rather than gastric motility. Differences of its effect on gastric emptying rate between species and its sites of action remain to be elucidated.
Published Version
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