Physiology & Behavior, Vol. 19, pp. 627--634. Pergamon Press and Brain Research Publ., 1977. Printed in the U.S.A.

Effects of Monosodium Glutamate on Eating and Drinking Behavior in R a t s I IKUO OHARA 2 AND MICHAEL NAIM 3

Monell Chemical Senses Center, University o f Pennsylvania Philadelphia PA 19104 ( R e c e i v e d 17 May, 1 9 7 7 ) OHARA, I. AND M. NAIM. Effects o f monosodium glutamate on eating and drinking behavior in rats. PHYSIOL. BEHAV. 19(5) 6 2 7 - 6 3 4 , 1977. - The effect of monosodium glutamate (MSG) on eating and drinking behavior of rats was studied by means of preference tests in free choice situations. Low or high protein casein diets containing up to 3% MSG were selected indifferently compared to plain casein diet in 7 day trials. The acceptability of diets containing 7% MSG, particularly those of low protein level, was significantly reduced. Except for Day 1, the proportional intake (MSG flavored diet intake/total diet intake) of 9% protein - 7% MSG diet was consistently lower than 18% protein - 7% MSG diet. In brief exposure tests (10 min) solutions of MSG (0.02-8%) were preferred over deionized water. The acceptance of solutions containing higher concentrations of MSG was significantly reduced. The total volume intake of both choices (MSG flavored water and water) was significantly increased in tests using solutions containing 3 - 8 % MSG. In long-term tests ( 1 - 1 4 days), solutions containing 0.05-1% MSG were preferred over water. The acceptance of a solution containing 5% MSG was significantly reduced. For maximum intake of moles of MSG, rats selected solutions which offered 3 - 5 x 10-I M MSG whether the experimental period was 10 min or up to 2 weeks. In a series of two choice preference tests using solutions, where Na÷ content and pH were kept equal, MSG was preferred over sodium acetate and over sodium glutarate but was less preferred than monosodium aspartate. The results of this study can be explained in terms of sensory quality. MSG

Taste

Preference tests

Eating behavior

Drinking behavior

C O N T R O L O F i n g e s t i o n of n u t r i e n t s is k n o w n t o b e s u b j e c t t o t h e i n f l u e n c e s of taste, smell or t e x t u r e as well as p o s t - i n g e s t i o n a l f a c t o r s [2, 3, 16, 23 ]. Brief e x p o s u r e t y p e s of p r e f e r e n c e tests are m o r e valid t h a n long e x p o s u r e tests as m e a s u r e s of sensory f a c t o r s a l o n e [ 6 ] . M o n o s o d i u m g l u t a m a t e ( M S G ) is widely used as a flavor in c o o k e r y , p r e s u m a b l y as a t a s t e s t i m u l a n t a n d a flavor e n h a n c e r [ 5 , 2 0 ] . Its free acid L-glutamic is a c o n s t i t u e n t of p r o t e i n a n d also occurs as a free a m i n o acid ( 0 . 0 1 - 0 . 2 % ) in a variety o f vegetables, m e a t a n d s e a f o o d [ 13,21 ]. Milk f r o m l a c t a t i n g h u m a n females c o n t a i n s 0 . 0 0 7 - 0 . 0 2 % free L-glut a m a t e [ 3 2 ] . It has b e e n suggested t h a t o n l y t h e f o r m o f g l u t a m a t e w i t h b o t h c a r b o x y l g r o u p s ionized is active as a taste s t i m u l a n t in h u m a n s [ 8 ] . Taste t h r e s h o l d s o f MSG f o r h u m a n s are in m i l l i m o l a r c o n c e n t r a t i o n s [ 2 0 ] . E l e c t r o p h y siological m e a s u r e m e n t s in rats s h o w e d t h a t M S G is effective f o r s t i m u l a t i n g t h e c h o r d a t y m p a n i nerve [ 2 9 ] . T h e taste o f s o m e o t h e r a m i n o acids has b e e n r e f e r r e d to as a n MSG-like taste [ 1 7 ] . M S G has b e e n s t u d i e d for its flavor

e n h a n c i n g ability in various f o o d s [ 12,31 ]. A synergistic effect b e t w e e n MSG a n d 5 ' - r i b o n u c l e o t i d e s in e n h a n c i n g f o o d flavor has b e e n r e p o r t e d [ 19]. Relatively little c o n s i d e r a t i o n has b e e n given t o MSG p r e f e r e n c e in animals a l t h o u g h a n i m a l m o d e l s can b e useful in t h e analysis o f f o o d i n t a k e m e c h a n i s m s . O n e p e r c e n t MSG in t h e diet has b e e n r e p o r t e d n o t t o i n f l u e n c e t h e diet p r e f e r e n c e o f rats [ 3 0 ] . F e m a l e rats selected 5 x 10 -2 M - 1 . 5 x 10 -1M MSG-flavored w a t e r over w a t e r in 2-choice, l o n g - t e r m p r e f e r e n c e test [ 1 4 ] . This p r e f e r e n c e for MSG s o l u t i o n s was o b s e r v e d o n l y in a s i t u a t i o n w h e r e s o l u t i o n s c o n t a i n i n g MSG were o f f e r e d in a s c e n d i n g c o n c e n t r a t i o n sequence. No s u c h p r e f e r e n c e was s h o w n in d e s c e n d i n g c o n c e n t r a t i o n s e q u e n c e tests [ 14]. Weanling calves selected m o r e o f a 0.2% MSG-flavored diet t h a n n o n f l a v o r e d diet b e g i n n i n g at t h r e e weeks o f age [ 3 3 ] . Taste r e s p o n s e s b y p y g m y goats to sucrose and q u i n i n e h y d r o c h l o r i d e solut i o n s were m o d e r a t e d b y a d d i n g 5, 50, 5 0 0 PPM MSG [ 2 2 ] . W i t h regard t o physiological effects, w h e n circulating

1Portions of this paper were presented at the 61st Annual Meeting of the Federation of American Societies for Experimental Biology, Chicago, April 1977, Abstract published in Fedn Proc. 3 6 : 1 1 3 2 , 1977. 2 Present address: Central Research Laboratories, Ajinomoto Company, Inc., Yokohama, Japan. aReprint requests should be sent to Michael Naim, Monell Chemical Senses Center, University of Pennsylvania, 3500 Market Street, Philadelphia, PA 19104. 627

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levels of glutamic acid are markedly raised by such methods as injection or the feeding of massive doses of MSG to immature animals, CNS pathology resulting in undesirable physiological responses including obesity without hyperphagia may occur [1, 25, 26]. However, it appears that pathology does not occur when large amounts of MSG are incorporated in diets fed to mature animals [4,34]. Neither does this pathology occur when normal amounts of MSG (2%) are incorporated in diet of immature rats [28]. This is probably due to the fact that the intestine transaminates much of the dietary glutamic acid and slowly absorbs the rest. Thus little glutamic acid is transferred to portal blood [7, 10, 15]. Recently [ 11 ], intracisternal injection of a-amino butyric acid (GABA) in rats greatly decreased behavioral activity whereas injections of glutamic acid, its precursor, showed no effect. In light of the paucity of data on effects of MSG flavoring on animals, this study was designed to examine in some detail the responses of rats to MSG solutions and MSG in food using both long-term and short-term 2-choice preference tests. METHOD

A nimals Naive male rats (CD Charles River, Wilmington, MA) were used. The rats were acclimated to our laboratory for at least 5 days before testing. The animals were individually housed in cages 30 x 25 × 25 cm at a temperature of 25°C -+ 2°C with a cycle of 12 hr light and 12 hr dark. Each cage contained two holders for food cups and two holders for drinking tubes. During the acclimation period, the animals were kept on a maintenance diet described below and tap water ad lib.

Evaluation o f the Data Paired t-tests or analyses of variance (ANOVA) were performed on the intake data. Neuman-Keuls' test was used for post hoc comparisons of the difference among the means. In all cases the criteria for accepting a difference was set at least at 0.05 level.

period of 10 min. Animals were tested 3 times during the week on alternate days. Tests were conducted between 6 - 9 p.m. in the dark cycle using a red lamp for illumination. Position effects were avoided by randomizing the location, as well as alternating the position of graduated burrettes. After testing, animals were returned to their home cages. Each group of animals was subjected sequentially from lower to higher concentrations of MSG (Ajinomoto Company, Japan) in the following percentages: Group 1: 0.0005, 0.05, 0.5, 5.0, 50.0 (2.7 × 10- ' - 2.7 M). Group 2: 0.001, 0.02, 0.1, 1.0, 8.0, 16.0 (5.3 × 10-2 - 8.6 x 10-1 M). and Group 3: 0.005, 0.25, 3.0, 12.0 (2.7 ~. 10-4 6.4 × 10-' M). - -

Results Overall analysis of variance (ANOVA) of the proportional volume intake of MSG (ml of MSG solution/total ml intake x 100) showed that rats selected solutions containing 0 . 0 2 - 8 % (1.1 × 10-3 - 4.3 × 10-' M) MSG over water (Fig. I). Group !: F(4,60) = 29.38, p