Indian Journal of Experimental Biology Vol. 45, September 2007, pp. 802-806 Effects of monosodium glutamate on food acceptance and toxicity of selenium in rats R K Parshad & J K Natt Department of Zoology, Punjab Agricultural University, Ludhiana 141 004, India Received 15 January 2007; revised 12 April 2007 Food acceptance and toxic effects of feeding sodium selenite (Se) alone and in combination with monosodium glutamate (MSG), a taste enhancer were studied in the laboratory rat. Dose-dependent stimulation of daily food intake was observed with MSG offered in no-choice or bi-choice with the plain food. Consumption of pellets containing 0.05, 0.5 and 1.0% Se was significantly low than the plain or MSG containing pellets but their active ingredient was sufficient to cause mortality of rats. Food pellets containing both MSG and Se in no-choice feeding trial were not preferred by the rats, as their consumption remained low as compared to pellets containing only MSG. However, prior feeding on MSG containing pellets for two days increased the amount of intake of Se-containing pellets. No mortality of rats feeding on pellets containing different concentrations of MSG was recorded. Feeding on Se-containing pellets caused dose-dependent mortality on the third day of the trial. As compared to rats feeding on Se-containing pellets, the mortality rate was reduced in those provided Se in combination with MSG but the intake of active ingredient of Se in both these trials did not differ significantly. Decrease in death rate of rats feeding on Se in combination with MSG containing pellets suggested that addition of MSG to seleniferous food probably provide protection to some extent from the toxic effects of selenium. However, combination of excess doses of MSG and Se in food pellets caused mortality of all experimental animals. Keywords: Food consumption, Monosodium glutamate, Rat, Sodium selenite, Toxicity. Selenium is a crucial factor in maintaining appropriate antioxidant defense. However, under some conditions it can also act as a pro-oxidant 1.The mechanism by which selenium exerts its beneficial effects on physiological functions may be through Se-containing proteins. Because of it being an integral part of the active site of a widely dispersed enzyme glutathione peroxidase, it helps to keep low levels of cellular peroxides that damage cell membranes 2-4. On the other hand, excess dietary selenium, which may be ingested in its organic or inorganic form accumulates in different tissues and causes toxic effects 5-9. Selenium is present everywhere in the world but is rather unevenly distributed. In some areas, its amount is so high that the crops grown in these areas are toxic to both animals and human. The toxicity of selenium varies with the species, the chemical form of selenium, and duration of intake and nature of diet. It can be markedly affected by arsenic, organic sulphates and high protein intake of the animal 3, 10. Therefore, the dietary modifications capable of Phone: 91-161-2401960 ext. 382 Fax: 91-161-2400945 E-mail: rkparshad@yahoo.co.in alleviating the toxicity of selenium provide a successful means to prevent and control its harmful effects. Monosodium glutamate (MSG) is a non-essential amino acid, which is deliberately added to foods due to its flavor enhancing properties and has recently been considered to be a main energy source for the intestine 11,12. The present study is an attempt to determine the effects of MSG on the level of selenium toxicity induced by sodium selenite in rats. Materials and Methods Albino rats bred under standard laboratory conditions were caged individually and acclimatized for 2-3 days. They were provided with commercial pellet feed before using them for a specific trial. Pellets containing different concentration of sodium selenite (Se) and/or monosodium glutamate (MSG) were prepared and used for different groups of experimental animals. Generally male rats were used except for one trial where a group of females were also tested. Animal care and handling were in accordance with the guidelines by the Institutional Animal Ethics Committee of the university.
PARSHAD & NATT: MONOSODIUM GLUTAMATE AND SELENIUM TOXICITY IN RATS 803 No-choice feeding trials Three groups of rats having 6 rats each were fed on pellets containing 0.5, 5.0 and 10% MSG. Similarly in separate trials three groups of rats were provided 0.05, 0.5 and 1% Secontaining pellets. Bi-choice feeding trial with MSG Three groups of rats having 6 rats each were offered 4, 10 and 20% MSG-containing pellets in choice with the plain pellets for 5 days. Water was provided ad libitum to all rats during the experimental period. A record of daily consumption of MSG-containing and plain pellets was maintained for each animal. No-choice feeding trial with combined MSG and Se Seven groups having 6 rats each previously feeding on plain food pellets were offered pellets containing different concentrations of both MSG and Se for three days. During the experimental period, a record of daily consumption of food for each rat and mortality rate was maintained. Care was taken to avoid contamination of other laboratory material. Left over treated and plain feed were collected daily and buried in soil. No-choice pretreatment feeding trials One group of rats, previously on plain food, was offered pellets containing 5% MSG for two days and then shifted to pellets containing 0.5% Se for another two days. Another group of rats having 6 rats that were feeding on plain pellets were offered 0.5% Se for two days and then were shifted to pellets containing 5% MSG. Daily food consumption for each rat and mortality were recorded for all the groups. Statistical analysis Significance of differences between the treated and untreated groups was arrived at by using Student s t-test. Results Daily consumption of food pellets containing 5% MSG offered in no-choice was significantly higher than the plain pellets, whereas, pellets containing different concentrations of Se were least preferred by the animals (Table 1). Maximum mortality occurred in rats feeding on pellets containing 1.0% Se. Food consumption data revealed dose-dependent acceptance of MSG containing pellets offered with plain alternative in bi-choice (Table 2). The consumption of pellets containing 4% MSG was higher than the plain pellets from day 2 to day 5 but the differences were significant (P <0.05) only on day 4. Food consumption was significantly higher (P<0.05) at 10% MSG on day 1 to day 3 and reduced thereafter. The consumption of pellets containing 20% MSG remained significantly low throughout the testing period (Table 2). The effects of combinations of varied concentrations of both MSG and Se in the same food pellets offered in no-choice on food consumption and percentage mortality are given in Table 3. Day 1 consumption of pellets containing both MSG and Se declined with their increased concentration. All the combinations except for 0.5% MSG and 0.05% Se caused mortality within 96 hrs. Sex differences were observed with respect to day 1 feeding (Table 3). Females totally avoided feeding on these pellets containing high amounts of MSG and Se, however feeding on these pellets on day 2 and 3 caused death of all rats of the group. In another set of experiment, the rats were fed on pellets containing 5% MSG in no-choice on day 1 and day 2 and then shifted to food containing 0.5% Se on day 3 and day 4 and vice versa. The consumption of Table 1 Consumption of plain and treated food pellets containing different concentrations of monosodium glutamate (MSG) and sodium selenite (Se) [Values are mean ± SE. Figures in parentheses are mortality] Type of food Concentration of MSG and Se (%) Food consumption (g/100g body weight.) Day 1 Day 2 Day 3 Plain food -- 4.50±0.35 4.86±0.31 4.83±0.29 MSG 0.50 5.32±0.30 5.36±0.52 5.69±0.28 5.00 7.19±0.72* 7.37±0.39* 7.59±0.35* Treated food Se 10.00 5.05±0.69 5.74±0.26 4.39±0.93 0.05 1.93±0.68* 1.55±0.73* 0.25±0.1* (2/6) 0.50 1.33±0.52* 1.27±0.34* 0.46±0.26*(5/6) 1.00 0.75±0.45* 0.46±0.26* (6/6) *P<0.01, significant differences by t-test between treated and plain food consumption of the same day.
804 INDIAN J EXP BIOL, SEPTEMBER 2007 MSG (%) Body weight Table2 Daily food consumption of plain and treated food (g/100g b.wt) offered in bi-choice. [Values are mean ± SE.] Day 1 Day 2 Day 3 Day 4 Day 5 (g) P T P T P T P T P T 4 191±8.17 4.32±0.30 3.78±0.41 3.36±10.52 3.85±0.14 2.79±0.66 4.03±0.31 2.95±0.24 4.0±0.35** 3.69±0.29 4.06±0.22 10 177.5±8.00 2.14±0.84 4.35±0.18** 2.36±0.72 4.42±0.22** 5.55±0.31 4.08±0.64** 4.52±0.55 3.51±0.49 4.17±0.67 2.44±0.64 20 170.0±7.9 5.42±0.27 1.71±0.51 5.68±0.17 1.33±0.27* 5.10±0.34 1.66±0.29* 5.01±0.49 1.34±0.42* 4.75±0.15 1.23±0.29 P, plain food; T, monosodium glutamate treated food P values: **<0.05; *<0.01 Table 3 Efficacy of monosodium glutamate (A) and sodium selenite (B) - containing food against laboratory rat Concentration of A and B ingredients (%) in food Food consumption (g) (mean ±SE) Day 1 Day 2 Day 3 Mortality within 96 hr 0.5A+0.05B 1.15 ± 0.26 1.87 ± 0.19 0.75±0.20 0/6 a 5.75 ± 1.31 9.35 ± 0.95 3.75±1.00 b 0.57 ± 0.14 0.93 ± 0.09 0.37±0.01 5A+ 0.5B 0.60 ± 1.15 1.90 ± 0.82 * 2/6 a 30.0 ± 7.27 75.0 ± 31.0 b 3.0 ± 0.72 7.5 ± 3.1 5A+1B 0.71 ± 0.06 0.13 ± 0.09 * 2/6 a 35.68±3.42 6.33 ± 4.57 b 7.13 ± 0.68 1.26 ± 0.91 10A+1B 0.52 ± 0.07 0.25 ± 0.09 * 1/6 a 51.83 ±7.41 25.75 ±9.7 b 5.18 ± 0.74 2.57 ± 0.97 20A+2B 0.43 ± 0.10 * 6/6 a 85.0 ± 20.5 b 8.5 ± 2.05 20A+3B(M) 0.25 ± 0.13 0.2 ± 0.07 0.38 ± 0.12 6/6 a 50 ± 25.98 40.0 ± 4.14 77.0 ± 4.06 b 7.5 ± 3.89 6.0 ± 0.31 11.55±3.61 20A+3B(F) 0 0.88 ± 0.31 0.16 ± 0.12 6/6 a 0 177.0± 62.93 49.5 ±35.00 b 0 26.55 +9.43 4.95 ± 3.50 a,b, active ingredients (mg) of monosodium glutamate and selenite respectively; F, female; M, male rats; *, mortality pellets containing 0.5% Se after pretreatment with MSG was significantly higher (P< 0.01) and caused mortality of 5 out of 6 rats (Table 4). Prior feeding on 0.5% Se on day 1 and day 2 significantly reduced the consumption of MSG-containing pellets on day 3 and day 4 and also the number of deaths. Discussion The results of the present study have shown that the presence of MSG in the rat feed pellets offered in either no-choice or bi-choice caused dose-related stimulation of food intake. At its lower concentration (4%), the amount of food intake increased gradually
PARSHAD & NATT: MONOSODIUM GLUTAMATE AND SELENIUM TOXICITY IN RATS 805 Table 4 Effect of pre-baiting with monosodium glutamate on the consumption of pellets containing sodium selenite and vice-versa. [Values are mean ± SE.] Days Consumption of treated food pellets (g/100g b.wt.) 5% MSG 0.5% Se 0.5% Se 5% MSG 1 7.49±0.52 -- 1.22±0.46** -- 2 7.17±0.24 -- 1.0±0.35* -- 3 -- 3.35±0.74 -- 5.24±0.13* 4 -- 2.65±0.35 -- 4.21±0.59* -- (5/6) (1/6) P values: **<0.01; *<0.05 significant differences by Student s t-test between food consumption with and without pre-baiting. Figures in parentheses indicate the mortality. but at 10% MSG concentration, marked increase occurred in the first three meals, and declined thereafter indicating that the higher intake on first three days probably had the detrimental effects as a result of which consumption of MSG pellets decreased. It was also evident from the trial in which there was very low consumption of 20% MSG containing pellets but intake of this lesser amount was sufficient to deliver the higher amount of active ingredient and the animal learned to avoid the food. It has also been observed earlier that MSG added in excess, diminish the palatability of foods 13. In contrast to MSG, presence of sodium selenite in the food has negative effect on its flavour and taste as the intake of Se-containing pellets was less than the plain alternative. The rats consumed lesser amount of wheat containing 12.5 ppm selenium grown in the seleniferous area of Punjab as compared to the normal wheat 6. Hence, the rats possess the ability to detect the occurrence of Se in foods. The results of the present study have shown that addition of MSG that is well known for its taste enhancing properties of food was ineffective in stimulating the amount of food intake, which also had Se in it. However, pretreatment with MSG containing pellets increased the amount of intake of Se-containing pellets. On the other hand, pretreatment with Se-containing food reduced the consumption of food containing MSG within palatable limits, which indicated that learned avoidable help them to protect from feeding on food containing toxic compounds. Though, the amount of intake of Se-containing pellets was very less, the amount of Se consumed was sufficient to cause mortality even at its lowest dose (0.05%). Simultaneous consumption of MSG (10%) and Se (1.0%) reduced the mortality rate. These results suggest that the intake of MSG may have the protective function against Se toxicity. This appears to be effective only when, both MSG and Se were consumed simultaneously, as pretreatment with MSG was ineffective in reducing the mortality rate; rather, it appeared to act as pre-bait to enhance the consumption of toxic bait for rodent pest species. Studies have shown that MSG provides a signal to the animal that a protein food is being ingested 14 through the stimulation of oral and intestinal sensors 15. Previous investigations of alkali disease revealed that high protein diets provide some protection against selenium toxicity 16 and added MSG that is a amino acid present in consumable proteins may be exerting a similar effects in alleviating the Se toxicity in rats. References 1 Surai P F, Selenium in poultry nutrition 1 Antioxidant properties, deficiency and toxicity, World s Poult Sci, 58 (2002) 333. 2 Rotruck J T, Pope P L, Ganther H E, Swanson A B, Hafeman D G & Hoekstra W G, Selenium : Biochemical role as component of glutathione peroxidase, Science, 179 (1973) 588. 3 Nève J, Physiological and nutritional importance of selenium, Experientia, 47 (1991) 187. 4 Kitahara J, Seko Y & Imura N, Possible involvement of active oxygen species in selenium toxicity in isolated rat hepatocytes, Arch Toxicol, 67 (1993) 497. 5 Gissel-Nielsen G, Gupta U C, Lamand M & Westermarck T, Selenium in soils and plants and its importance in livestock and human nutrition, Adv Agron, 37 (1984) 397. 6 Parshad R K & Sud M, Effects of selenium-rich wheat on rat spermatogenesis, Andrologia, 21 (1989) 486. 7 Kaur R & Parshad V R, Effects of dietary selenium on the differentiation, morphology and functions of spermatozoa of the house rat, Rattus rattus, Mut Res, 309 (1994) 29. 8 Parshad R K, Effects of selenium toxicity on oestrous cyclicity, ovarian follicles, ovulation and foetal survival in rats, Indian J Exp Biol, 37 (1999) 615. 9 Parshad R K & Kaur N, Antifertility effects of sodium selenite in the Indian mole rat Bandicota bengalensis, Indian J Anim Sci, 73 (2003) 903. 10 Schrauzer G N, Selenomethionine: A review of its nutritional significance, metabolism and toxicity, J Nutr, 130 (2000) 1653. 11 Hegenbart S, Flavour enhancers: Part of the building blocks, Food Product Design, 2 (1993) 38. 12 Fernstrom J D & Garattini S, International Symposium on Glutamate, J Nutr, 130 (2000) 891S.
806 INDIAN J EXP BIOL, SEPTEMBER 2007 13 Yamaguchi S & Takachachi C, Hedonic function of monosodium glutamate and four basic taste substances used at various concentration levels in simple and complex systems, Agric Biol Chem, 48 (1984) 1077. 14 Kondoh T, Mori M, Ono T & Torii K, Mechanism of umami taste preference and aversion in rats, J Nutr, 130 (2000) 966S. 15 Niijima A, Effects of oral and intestinal stimulation with umami substance on gastric vagus activity, Physiol Behav, 49 (1991) 1025. 16 Underwood E J, Selenium in Trace elements in human and animal nutrition (Academic Press, New York) 1977, 291.