- Email: [email protected]
Sham-feeding sucrose or corn oil stimulates food intake in rats
Appetite, 1991, 17, 97-103
Sham-feeding in Rats MICHAEL
Sucrose or Corn Oil Stimulates
Food Intake
G. TORDOFF and DANIELLE R. REED
Monell Chemical Senses Center, Philadelphia
In separate experiments, rats with open gastric cannulas were sham-fed either 32% sucrose solution or 15% corn oil emulsion. The rats’ cannulas were then closed, and food intake was measured for 2 h. Food intake was greater after sham-feeding either fluid than after tests when no fluid was available. These results suggest that the oral stimulation produced by ingestion of sweet or oily fluids can stimulate appetite in the rat.
In a recent series of studies, we found that rats increased food intake after drinking saccharin solution (Tordoff & Friedman, 1989a,b,c,d). The experiments suggested that the taste of saccharin elicited a cephalic-phase neural reflex-mediated alteration in liver metabolism, which increased hunger (Tordoff, 1988; Tordoff & Friedman, 1989d). The feeding response could be augmented by a favorable osmotic milieu, and by appropriate past experience (Tordoff & Friedman, 1989b,c). Inferences from these results with rats provide a theoretical framework that could help elucidate the conditions that allow sweeteners to influence, or fail to influence, human appetite and food intake (e.g. Birch et al., 1989; Blundell & Hill, 1986; Brala & Hagen, 1983; Mattes, 1990; Rogers dz Blundell, 1989; Rogers et al., 1988; Rolls et al., 1990; Tordoff & Alleva, 1990). There are several problems associated with using saccharin as a test solution for studying the influence of sweeteners on food intake. First, there is evidence that saccharin has pharmacological effects (Jorgensen, 1950; Purdom et al., 1973; Thompson & Mayer, 1959; see Rogers & Blundell, 1989, for additional references), and so the increased food intake could potentially be due to saccharin’s postingestive action rather than its taste. Second, even if saccharin itself is inert, the vehicle for saccharin may not be. Overhydration resulting from ingestion of large volumes of hyperosmotic saccharin solution can stimulate food intake (Tordoff & Friedman, 1989b). Third, it is questionable whether rats find saccharin particular sweet. For example, they would rather drink 4% sucrose solution (which is not very sweet to humans) than the most preferred saccharin concentrations (O.l-0.3%; Collier & Novell, 1967). One way to circumvent these problems is to use another sweetener. This is not as easy as it first appears because sugars have profound postingestive effects, and the rat This work was supported by National Institutes of Health Grant, DK-36339, and by the Howard Heinz Endowment. We thank Fred Sandler and Patricia Ulrich for their expert technical assistance. Address correspondence and reprint requests to: Dr M. G. Tordoff, Monell Chemical Senses Center, 3500 Market St., Philadelphia, PA 19104, U.S.A. 019%6663/91/050097+07 $03.00/O
0 1991 Academic Press Limited
98
M. G. TORDOFF
AND D. R. REED
is indifferent to, or avoids, most high-intensity sweeteners (Murray et al., 1953; Naim et al., 1982; Sclafani & Abrams, 1986; Wagner, 1971). In the present work, we solved this problem by measuring food intake after rats sham-fed a 32% sucrose solution. This provided an intensely sweet taste with minimal postingestive effects. The easy availability and high potency of artificial sweeteners has focused attention on the relationship between sweetness and appetite. This emphasis has left open the question of whether the increased appetite is a specific effect of sweetnessper se or a general effect of all “good-tasting” foodstuffs. A second goal of the present work was to determine if sweetness was a necessary stimulus to increase food intake. To do this, we measured short-term food intake after rats sham-fed corn oil, a food that is preferred but not sweet.
METHOD
Two experiments were conducted, each employing 20 male Sprague-Dawley CD rats purchased from Charles River (Wilmington, MA). The rats were maintained in a vivarium at approximately 23°C with a 12:12 h light/dark cycle (lights on at 0700 hrs). Throughout the experiments, each rat was given powdered Purina Laboratory Chow (no. 5001) in a 120-ml glass jar, and tap water in a 300-ml inverted bottle with a stainless steel drinking spout. All rats were implanted with a stainless steel gastric cannula according to established procedures (Young et al., 1974). The rats were housed in hanging stainless steel mesh cages, except during the first week after surgery when they were housed in cages with a solid floor covered with wood shavings. This prevented the rat’s newly implanted cannula from becoming entangled in wire mesh. Food and water were available at all times except during sham-feeding tests. The rats weighed 304-408 g at the start of testing. After being given at least 14 days to recover from surgery, the rats began a series of daily tests. Because we could only test 10 rats simultaneously, half the rats in each experiment were tested in the morning (starting N 1000 hrs). and the other half in the afternoon (starting N 1400 hrs). Each rat was removed from its cage and lavaged to empty the stomach of food. To do this, the cap of the rat’s cannula was unscrewed, the rat was held over a sink, and N lOm1 boluses of water, warmed to body temperature, were injected through the cannula into the stomach. The injections were repeated until the effluent draining from the stomach was clear, indicating that all gastric contents had been removed. A drainage tube was screwed to the cannula, and the rat was placed in a Plexiglas box. The box was 16 cm long x 5 cm wide x 16 cm high and had a 1 cm-wide slot running along the longest axis of the floor to accept the drainage tube. Each rat was allowed to adapt to the box for 5- 15 min. The time was the same from day-to-day for each rat but it differed from rat-to-rat so that we could test several rats at once. The rats were then given 15-min access to either a 32% sucrose solution (Experiment 1) or a 15% corn oil emulsion (Experiment 2), or continued to wait for the same period (control test; both experiments). Measurements were made of fluid intake (+0*5 ml) and drainage from the cannula (kO.1 g), which was collected in preweighed plastic cups. The fluids were freshly prepared every day. The sucrose solution was made by adding tap water to 320g sucrose (Sigma Chemical Co.) to bring the total volume to 1000 ml. The corn oil emulsion was made by homogenizing
SHAM-FEEDING
AND FOOD INTAKE
99
with an ultrasonic mixer (Heat Systems Ultrasonics Inc., Plainview, NY) 150g Mazola corn oil with 850ml water and 0.6g xanthan gum. Immediately after the 15-min sham-feeding or control period, each rat was removed from its Plexiglas box, its drainage tube unscrewed, and its cannula cap replaced. The rat was returned to its home cage, and food intake was measured ( f 0.1 g) after 60 and 120 min. Whether or not the rat received a fluid to sham-feed or a control test was determined by a counterbalanced, repeating ABBAABBA . . . design. Because rats differ widely in their proclivity to sham-feed when not food-deprived, a criterion was used to exclude results from initial tests, when some rats did not sham-feed any fluid. Data were collected from the 14 tests (7 with sham feeding and 7 control tests) starting when the rat first sham-fed > 3 ml fluid on two successive occasions. According to this criterion, it took an average of 3.1 f 05 tests (range O-8) before rats started to shamdrink sucrose (Experiment l), and 6.8 kO.9 tests (range 1-12) before they started to sham-drink corn oil (Experiment 2). All the rats sham-fed more than 3 ml on their five remaining tests. The data from three rats from each experiment were excluded from analysis. In the sucrose experiment, one rat’s cannula cap could not be removed and so it could not be tested. Two rats sham-fed more sucrose than was collected in drainage, indicating that a significant fraction of the ingested sucrose was absorbed. In the corn oil experiment, two rats developed wound infections and were killed. The other rat never sham-fed > 3 ml corn oil (after 15 pairs of tests). Food intake data from the remaining 17 rats in each experiment were analyzed by within-subject ANOVAs, with factors of prior test condition (sham-feed or control), test pair (l-7), and time (1st or 2nd h). Intakes of sham-fed fluid during the tests were analyzed by one-way ANOVAs, with test pair (l-7) as the factor. Post hoc t-tests were used to compare the two means of each test pair, and individual test pairs to each other (Bruning & Kintz, 1977). The critical level of significance for all tests was p
Sucrose Rats sham-fed less sucrose during the first two 15-min tests than during the final five tests [F(6,96) = 18.6, p
100
M. G. TORDOFF AND D. R. REED
TABLE 1 Intake of sucrose and corn oil during 15-min sham-feeding tests
Test 1
2 3 4 5 6 7 Average
Experiment 1 32% sucrose intake (ml) (n= 17)
Experiment 2 15% corn oil intake (ml) (Fl= 17)
5.4kO.7 12.8f 1.8 16.7f2.1 18.0+ 1.6 18.0f 1.6 16.8+ 1.0 19.lfl.6 15.3+ 1.3
3.2f0.5 4.2kO.8 2.9 + 0.8 54f 1.1 5.0+ 1.0 5.0f 1.0 5.5f0.9 4.5 +0.7
Values are means + SE.
Relative to food intake after tests without corn oil to drink, food intake during the 2 h after sham corn oil ingestion was significantly increased [F(l, 16) = 6.35, p < 0.051. Intake after the 6th and 7th sham-ingestion tests was reliably higher than after their respective control tests (test condition x trial pair interaction [F(6,96) = 2.56, p < 0.051 (Figure 1). Most eating occurred during the 1st hour of the 2-h tests [F(l, 16)= 92.8, p
DISCUSSION Rats ate more food during the 2 h after they sham-fed 32% sucrose solution or 15% corn oil emulsion than after similar tests without a fluid to sham-feed. The finding that sham-feeding sucrose increased subsequent food intake corroborates previous reports that ingestion of non-nutritive sweeteners increases appetite (Blundell dz Hill, 1986; Brala & Hagen, 1983; Rogers & Blundell, 1989; Rogers, et al., 1988; Tordoff & Alleva, 1990; Tordoff & Friedman, 1989a,b,c,d). It lends support to the argument that the increased appetite is due to the sweeteners’ common taste rather than to their pharmacological or osmotic effects (see discussions in Rogers & Blundell, 1989; Rogers et al., 1990; Tordoff & Alleva, 1990). In particular, although overhydration is a cause of the feeding response produced by drinking saccharin solution (Tordoff SC Friedman, 1989b), it is improbable that this is involved afer sham-feeding because any absorbed 32% sucrose solution would be strongly hyperosmotic (see also Tordoff & Friedman, 1989b). Sham-ingestion of sucrose, sham-ingestion of corn oil, and “normal” ingestion of saccharin all produced very similar effects on food intake (compare the results here with Tordoff & Friedman, 1989~). In each case, it took several pairs of trials before increases in food intake were evident. With saccharin, we found that the rats required two or three tests to learn that eating food was rewarding (i.e. a response that counteracted the aversive, appetite-inducing effects of drinking saccharin; Tordoff & Friedman, 1989~). This probably explains the absence of a response on the first few tests after sham-feeding as well. Similarly, we found with saccharin that the increase
SHAM-FEEDING
4
B 2
32%
AND
101
FOOD INTAKE
Sucrose
4
3
2
I
0
2
3
4
5 Pairs
6
7
Meon
of tests
FIGURE 1. Food intake during the 2 h after rats sham-fed 32% sucrose solution (top panel) or 15% corn oil emulsion (bottom panel) for lfi-min. (n = 17 in each experiment; *p
in food
intake was due to a cephalic-phase metabolic reflex initiated by oral stimulation (Tordoff & Friedman, 1989d). We suspect this is also true for sham-fed sucrose and corn oil. Because a small proportion of ingested sugar or corn oil does not drain through the type of gastric cannula used here (Reed et al., 1990; Sclafani & Nissenbaum, 1985) it is conceivable that postgastric controls of intake contributed to the response. However, it is difficult to see how these could be significant because any sham-fed nutrients that entered the duodenum would be expected to decrease food intake (Booth & Jarman, 1976; Greenberg et al., 1990; Snowdon, 1975) which is the opposite of the results found here. Judging by the relative intake of sucrose and corn oil in Experiments 1 and 2, rats found sucrose to be the more palatable. Despite this, the increase in food intake
102
M. G. TORDOFF AND D. R. REED
produced by sham-feeding sucrose was no greater than that produced by shamfeeding corn oil. This suggests that the volume and palatability of the sham-fed fluid did not influence the feeding response. Perhaps both sucrose and corn oil were so palatable that they produced maximal stimulation of food intake, or the stimulation of feeding resulted from an all-or-none response. This is interesting with respect to the involvement of a cephalic-phase reflex because there is little or no evidence to suggest that the magnitude of such reflexes are modulated by palatability, except when grossly unpalatable and palatable stimuli are compared. On the other hand, the effects of sham-feeding on subsequent food intake were fairly small, and it would be difficult to discriminate more than one level of increased intake under the test conditions used here. This is the first report that sham-feeding corn oil can increase food intake. It raises the possibility that non-nutritive fat substitutes may have the potential to increase appetite, and demonstrates that sweetness is not a necessary sensory component of the response. The “taste” of fat is ill-defined, but clearly pleasant to rats: they consume it avidly and will ingest non-nutritive fats (e.g. Corbit & Stellar, 1964; Mindell et al., 1990; Reed & Friedman, 1990; Reed et al., 1990). There is also evidence that corn oil can elicit a cephalic-phase reflex in rats (Ramirez, 1985). We conclude that the effective oral stimulus to increase appetite is a common attribute of sucrose and corn oil, such as a pleasant orosensation.
REFERENCES
Birch, L. L., McPhee, L. & Sullivan, S. (1989) Children’s food intake following drinks sweetened with sucrose and aspartame: Time course effects. Physiology and Behaoior, 45, 387-395. Blundell, J. E. & Hill, A. J. (1986) Paradoxical effects of an intense sweetener (aspartame) on appetite. Lancet, i, 1092-1093. Booth, D. A. & Jarman, S. P. (1976) Inhibition of food intake in the rat following complete absorption of glucose delivered to the stomach, intestine, or liver. Journal of Physiology (London), 259, 501-522.
Brala, P. M. & Hagen, R. L. (1983) Effects of sweetness Perception and caloric value of a preload on short-term intake. Physiology and Behavior, 30, l-9. Bruning, J. L. & Kintz, B. L. (1977) Computational Handbook of Statistics. Dallas: Scott, Foresman and Co. Collier, G. & Novell, K. (1967) Saccharin as a sugar surrogate. Journal of Comparative and Physiological Psychology, 64, 404-408.
Corbit, J. D. & Stellar, E. (1964) Palatability, food intake, and obesity in normal and hyperphagic rats. Journal of Comparative and Physiological Psychology, 58, 63-67. Greenberg, D., Smith, G. P. & Gibbs, J. (1990) Intraduodenal infusions of fats elicit satiety in sham-feeding rats. American Journal of Physiology, 259, Rl lo-R1 18. Jorgensen,
H. (1950) The influence of saccharin
Scandanavia, 20, 33-37.
on the blood-sugar.
Acta Physiologica
Mattes, R. (1990) Effects of aspartame and sucrose on hunger and energy intake in humans. Physiology and Behavior, 47, 1037-1044.
Mindell, S., Smith, G. P. & Greenberg, D. (1990) Corn oil and mineral oil stimulate sham feeding in rats. Physiology and Behavior, 48, 283-287. Murray, E. J., Wells, H., Kohn, M. & Miller, N. E. (1953). Sodium sucaryl: A substance which tastes sweet to human subjects but is avoided by rats. Journal of Comparative and Physiological Psychology, 46, 134- 137.
Naim, M., Rogatka, H., Yamamoto, T. & Zehavi, U. (1982) Taste responses to neohesperidin dihydrochalcone in rats and baboon monkeys. Physiology and Behavior, 28, 979-986.
SHAM-FEEDING AND FOOD INTAKE
103
Purdom, M. E., Hyder, K. & Pybas, M. D. (1973) Effects of saccharin on rats fed chemically defined diets. Journal of the American Dietetic Association, 63, 635-638. Ramirez, I. (1985) Oral stimulation alters digestion of intragastric meals in rats. American Journal of Physiology, 248, R459-R463. Reed, D. R. & Friedman, M. I. (1990). Diet composition alters the preference for fat in rats. Appetite, 14, 219-230.
Reed, D. R., Tordoff, M. G. & Friedman, M. I. (1990) Sham feeding of corn oil by rats: sensory and postingestive factors. Physiology and Behavior, 47, 779-781. Rogers, P. J. & Blundell, J. E. (1989) Separating the actions of sweetness and calories: effects of saccharin and carbohydrates on hunger and food intake in human subjects. Physiology and Behavior, 45, 1093-1099. Rogers, P. J., Carlyle, J.-A., Hill, A. J. & Blundell, J. E. (1988) Uncoupling sweet taste and calories: Comparison of the effects of glucose and three intense sweeteners on hunger and food intake. Physiology and Behavior, 43, 547-552. Rogers, P. J., Pleming, H. C. & Blundell, J. E. (1990) Aspartame ingested without tasting inhibits hunger and food intake. Physiology and Behavior, 47, 1239-1243. Rolls, B. J., Kim, S. & Federoff, I. C. (1990) Effects of drinks sweetened with sucrose or aspartame on hunger, thirst and food intake in men. Physiology and Behavior, 48, 19-26. Sclafani, A. & Abrams, M. (1986) Rats show only a weak preference for the artificial sweetener aspartame. Physiology and Behavior, 37, 253-256. Sclafani, A. & Nissenbaum, J. W. (1985) Is gastric sham feeding really sham feeding? American Journal of Physiology, 248, R387-R390. Snowdon, C. T. (1975) Production of satiety with small intraduodenal infusions in the rat. Journal of Comparative and Physiological Psychology, 88, 23 l-238.
Thompson, M. M. & Mayer, J. (1959) Hypoglycemic effects of saccharin in experimental animals. American Journal of Clinical Nutrition, 7, 80-85. Tordoff, M. G. (1988) How do nonnutritive sweeteners increase food intake? Appetite, II (Suppl.), 5- 11. Tordoff, M. G & Alleva, A. M. (1990) Oral stimulation with aspartame increases hunger. Physiology and Behavior, 47, 555-559.
Tordoff, M. G. & Friedman, M. I. (1989a) Drinking saccharin increases food preference: I. Comparison with other drinks. Appetite, 12, l-10. Tordoff, M. G. & Friedman, M. I. (1989b) Drinking saccharin increases food preference: II. Hydrational factors. Appetite, 12, 1l-21. Tordoff, M. G. & Friedman, M. I. (1989~) Drinking saccharin increases food preference: III. Sensory and associative factors. Appetite, 12, 23-35. Tordoff, M. G. & Friedman, M. I. (1989d) Drinking saccharin increases food preference: IV. Cephalic phase and metabolic factors. Appetite, 12, 37-56. Wagner, M. W. (1971) Comparative rodent preferences for artificial sweeteners.
intake and intake and intake and intake and Journal of
Comparative and Physiological Psychology, 75,483-490.
Young, R. C., Gibbs, J., Antin, J., Holt, J. & Smith, G. P. (1974) Absence of satiety during sham-feeding in the rat. Journal of Comparative and Physiological Psychology, 87, 795-800. Received
20 September
1990, revision 27 February
1991
Sham-feeding in Rats MICHAEL
Sucrose or Corn Oil Stimulates
Food Intake
G. TORDOFF and DANIELLE R. REED
Monell Chemical Senses Center, Philadelphia
In separate experiments, rats with open gastric cannulas were sham-fed either 32% sucrose solution or 15% corn oil emulsion. The rats’ cannulas were then closed, and food intake was measured for 2 h. Food intake was greater after sham-feeding either fluid than after tests when no fluid was available. These results suggest that the oral stimulation produced by ingestion of sweet or oily fluids can stimulate appetite in the rat.
In a recent series of studies, we found that rats increased food intake after drinking saccharin solution (Tordoff & Friedman, 1989a,b,c,d). The experiments suggested that the taste of saccharin elicited a cephalic-phase neural reflex-mediated alteration in liver metabolism, which increased hunger (Tordoff, 1988; Tordoff & Friedman, 1989d). The feeding response could be augmented by a favorable osmotic milieu, and by appropriate past experience (Tordoff & Friedman, 1989b,c). Inferences from these results with rats provide a theoretical framework that could help elucidate the conditions that allow sweeteners to influence, or fail to influence, human appetite and food intake (e.g. Birch et al., 1989; Blundell & Hill, 1986; Brala & Hagen, 1983; Mattes, 1990; Rogers dz Blundell, 1989; Rogers et al., 1988; Rolls et al., 1990; Tordoff & Alleva, 1990). There are several problems associated with using saccharin as a test solution for studying the influence of sweeteners on food intake. First, there is evidence that saccharin has pharmacological effects (Jorgensen, 1950; Purdom et al., 1973; Thompson & Mayer, 1959; see Rogers & Blundell, 1989, for additional references), and so the increased food intake could potentially be due to saccharin’s postingestive action rather than its taste. Second, even if saccharin itself is inert, the vehicle for saccharin may not be. Overhydration resulting from ingestion of large volumes of hyperosmotic saccharin solution can stimulate food intake (Tordoff & Friedman, 1989b). Third, it is questionable whether rats find saccharin particular sweet. For example, they would rather drink 4% sucrose solution (which is not very sweet to humans) than the most preferred saccharin concentrations (O.l-0.3%; Collier & Novell, 1967). One way to circumvent these problems is to use another sweetener. This is not as easy as it first appears because sugars have profound postingestive effects, and the rat This work was supported by National Institutes of Health Grant, DK-36339, and by the Howard Heinz Endowment. We thank Fred Sandler and Patricia Ulrich for their expert technical assistance. Address correspondence and reprint requests to: Dr M. G. Tordoff, Monell Chemical Senses Center, 3500 Market St., Philadelphia, PA 19104, U.S.A. 019%6663/91/050097+07 $03.00/O
0 1991 Academic Press Limited
98
M. G. TORDOFF
AND D. R. REED
is indifferent to, or avoids, most high-intensity sweeteners (Murray et al., 1953; Naim et al., 1982; Sclafani & Abrams, 1986; Wagner, 1971). In the present work, we solved this problem by measuring food intake after rats sham-fed a 32% sucrose solution. This provided an intensely sweet taste with minimal postingestive effects. The easy availability and high potency of artificial sweeteners has focused attention on the relationship between sweetness and appetite. This emphasis has left open the question of whether the increased appetite is a specific effect of sweetnessper se or a general effect of all “good-tasting” foodstuffs. A second goal of the present work was to determine if sweetness was a necessary stimulus to increase food intake. To do this, we measured short-term food intake after rats sham-fed corn oil, a food that is preferred but not sweet.
METHOD
Two experiments were conducted, each employing 20 male Sprague-Dawley CD rats purchased from Charles River (Wilmington, MA). The rats were maintained in a vivarium at approximately 23°C with a 12:12 h light/dark cycle (lights on at 0700 hrs). Throughout the experiments, each rat was given powdered Purina Laboratory Chow (no. 5001) in a 120-ml glass jar, and tap water in a 300-ml inverted bottle with a stainless steel drinking spout. All rats were implanted with a stainless steel gastric cannula according to established procedures (Young et al., 1974). The rats were housed in hanging stainless steel mesh cages, except during the first week after surgery when they were housed in cages with a solid floor covered with wood shavings. This prevented the rat’s newly implanted cannula from becoming entangled in wire mesh. Food and water were available at all times except during sham-feeding tests. The rats weighed 304-408 g at the start of testing. After being given at least 14 days to recover from surgery, the rats began a series of daily tests. Because we could only test 10 rats simultaneously, half the rats in each experiment were tested in the morning (starting N 1000 hrs). and the other half in the afternoon (starting N 1400 hrs). Each rat was removed from its cage and lavaged to empty the stomach of food. To do this, the cap of the rat’s cannula was unscrewed, the rat was held over a sink, and N lOm1 boluses of water, warmed to body temperature, were injected through the cannula into the stomach. The injections were repeated until the effluent draining from the stomach was clear, indicating that all gastric contents had been removed. A drainage tube was screwed to the cannula, and the rat was placed in a Plexiglas box. The box was 16 cm long x 5 cm wide x 16 cm high and had a 1 cm-wide slot running along the longest axis of the floor to accept the drainage tube. Each rat was allowed to adapt to the box for 5- 15 min. The time was the same from day-to-day for each rat but it differed from rat-to-rat so that we could test several rats at once. The rats were then given 15-min access to either a 32% sucrose solution (Experiment 1) or a 15% corn oil emulsion (Experiment 2), or continued to wait for the same period (control test; both experiments). Measurements were made of fluid intake (+0*5 ml) and drainage from the cannula (kO.1 g), which was collected in preweighed plastic cups. The fluids were freshly prepared every day. The sucrose solution was made by adding tap water to 320g sucrose (Sigma Chemical Co.) to bring the total volume to 1000 ml. The corn oil emulsion was made by homogenizing
SHAM-FEEDING
AND FOOD INTAKE
99
with an ultrasonic mixer (Heat Systems Ultrasonics Inc., Plainview, NY) 150g Mazola corn oil with 850ml water and 0.6g xanthan gum. Immediately after the 15-min sham-feeding or control period, each rat was removed from its Plexiglas box, its drainage tube unscrewed, and its cannula cap replaced. The rat was returned to its home cage, and food intake was measured ( f 0.1 g) after 60 and 120 min. Whether or not the rat received a fluid to sham-feed or a control test was determined by a counterbalanced, repeating ABBAABBA . . . design. Because rats differ widely in their proclivity to sham-feed when not food-deprived, a criterion was used to exclude results from initial tests, when some rats did not sham-feed any fluid. Data were collected from the 14 tests (7 with sham feeding and 7 control tests) starting when the rat first sham-fed > 3 ml fluid on two successive occasions. According to this criterion, it took an average of 3.1 f 05 tests (range O-8) before rats started to shamdrink sucrose (Experiment l), and 6.8 kO.9 tests (range 1-12) before they started to sham-drink corn oil (Experiment 2). All the rats sham-fed more than 3 ml on their five remaining tests. The data from three rats from each experiment were excluded from analysis. In the sucrose experiment, one rat’s cannula cap could not be removed and so it could not be tested. Two rats sham-fed more sucrose than was collected in drainage, indicating that a significant fraction of the ingested sucrose was absorbed. In the corn oil experiment, two rats developed wound infections and were killed. The other rat never sham-fed > 3 ml corn oil (after 15 pairs of tests). Food intake data from the remaining 17 rats in each experiment were analyzed by within-subject ANOVAs, with factors of prior test condition (sham-feed or control), test pair (l-7), and time (1st or 2nd h). Intakes of sham-fed fluid during the tests were analyzed by one-way ANOVAs, with test pair (l-7) as the factor. Post hoc t-tests were used to compare the two means of each test pair, and individual test pairs to each other (Bruning & Kintz, 1977). The critical level of significance for all tests was p
Sucrose Rats sham-fed less sucrose during the first two 15-min tests than during the final five tests [F(6,96) = 18.6, p
100
M. G. TORDOFF AND D. R. REED
TABLE 1 Intake of sucrose and corn oil during 15-min sham-feeding tests
Test 1
2 3 4 5 6 7 Average
Experiment 1 32% sucrose intake (ml) (n= 17)
Experiment 2 15% corn oil intake (ml) (Fl= 17)
5.4kO.7 12.8f 1.8 16.7f2.1 18.0+ 1.6 18.0f 1.6 16.8+ 1.0 19.lfl.6 15.3+ 1.3
3.2f0.5 4.2kO.8 2.9 + 0.8 54f 1.1 5.0+ 1.0 5.0f 1.0 5.5f0.9 4.5 +0.7
Values are means + SE.
Relative to food intake after tests without corn oil to drink, food intake during the 2 h after sham corn oil ingestion was significantly increased [F(l, 16) = 6.35, p < 0.051. Intake after the 6th and 7th sham-ingestion tests was reliably higher than after their respective control tests (test condition x trial pair interaction [F(6,96) = 2.56, p < 0.051 (Figure 1). Most eating occurred during the 1st hour of the 2-h tests [F(l, 16)= 92.8, p
DISCUSSION Rats ate more food during the 2 h after they sham-fed 32% sucrose solution or 15% corn oil emulsion than after similar tests without a fluid to sham-feed. The finding that sham-feeding sucrose increased subsequent food intake corroborates previous reports that ingestion of non-nutritive sweeteners increases appetite (Blundell dz Hill, 1986; Brala & Hagen, 1983; Rogers & Blundell, 1989; Rogers, et al., 1988; Tordoff & Alleva, 1990; Tordoff & Friedman, 1989a,b,c,d). It lends support to the argument that the increased appetite is due to the sweeteners’ common taste rather than to their pharmacological or osmotic effects (see discussions in Rogers & Blundell, 1989; Rogers et al., 1990; Tordoff & Alleva, 1990). In particular, although overhydration is a cause of the feeding response produced by drinking saccharin solution (Tordoff SC Friedman, 1989b), it is improbable that this is involved afer sham-feeding because any absorbed 32% sucrose solution would be strongly hyperosmotic (see also Tordoff & Friedman, 1989b). Sham-ingestion of sucrose, sham-ingestion of corn oil, and “normal” ingestion of saccharin all produced very similar effects on food intake (compare the results here with Tordoff & Friedman, 1989~). In each case, it took several pairs of trials before increases in food intake were evident. With saccharin, we found that the rats required two or three tests to learn that eating food was rewarding (i.e. a response that counteracted the aversive, appetite-inducing effects of drinking saccharin; Tordoff & Friedman, 1989~). This probably explains the absence of a response on the first few tests after sham-feeding as well. Similarly, we found with saccharin that the increase
SHAM-FEEDING
4
B 2
32%
AND
101
FOOD INTAKE
Sucrose
4
3
2
I
0
2
3
4
5 Pairs
6
7
Meon
of tests
FIGURE 1. Food intake during the 2 h after rats sham-fed 32% sucrose solution (top panel) or 15% corn oil emulsion (bottom panel) for lfi-min. (n = 17 in each experiment; *p
in food
intake was due to a cephalic-phase metabolic reflex initiated by oral stimulation (Tordoff & Friedman, 1989d). We suspect this is also true for sham-fed sucrose and corn oil. Because a small proportion of ingested sugar or corn oil does not drain through the type of gastric cannula used here (Reed et al., 1990; Sclafani & Nissenbaum, 1985) it is conceivable that postgastric controls of intake contributed to the response. However, it is difficult to see how these could be significant because any sham-fed nutrients that entered the duodenum would be expected to decrease food intake (Booth & Jarman, 1976; Greenberg et al., 1990; Snowdon, 1975) which is the opposite of the results found here. Judging by the relative intake of sucrose and corn oil in Experiments 1 and 2, rats found sucrose to be the more palatable. Despite this, the increase in food intake
102
M. G. TORDOFF AND D. R. REED
produced by sham-feeding sucrose was no greater than that produced by shamfeeding corn oil. This suggests that the volume and palatability of the sham-fed fluid did not influence the feeding response. Perhaps both sucrose and corn oil were so palatable that they produced maximal stimulation of food intake, or the stimulation of feeding resulted from an all-or-none response. This is interesting with respect to the involvement of a cephalic-phase reflex because there is little or no evidence to suggest that the magnitude of such reflexes are modulated by palatability, except when grossly unpalatable and palatable stimuli are compared. On the other hand, the effects of sham-feeding on subsequent food intake were fairly small, and it would be difficult to discriminate more than one level of increased intake under the test conditions used here. This is the first report that sham-feeding corn oil can increase food intake. It raises the possibility that non-nutritive fat substitutes may have the potential to increase appetite, and demonstrates that sweetness is not a necessary sensory component of the response. The “taste” of fat is ill-defined, but clearly pleasant to rats: they consume it avidly and will ingest non-nutritive fats (e.g. Corbit & Stellar, 1964; Mindell et al., 1990; Reed & Friedman, 1990; Reed et al., 1990). There is also evidence that corn oil can elicit a cephalic-phase reflex in rats (Ramirez, 1985). We conclude that the effective oral stimulus to increase appetite is a common attribute of sucrose and corn oil, such as a pleasant orosensation.
REFERENCES
Birch, L. L., McPhee, L. & Sullivan, S. (1989) Children’s food intake following drinks sweetened with sucrose and aspartame: Time course effects. Physiology and Behaoior, 45, 387-395. Blundell, J. E. & Hill, A. J. (1986) Paradoxical effects of an intense sweetener (aspartame) on appetite. Lancet, i, 1092-1093. Booth, D. A. & Jarman, S. P. (1976) Inhibition of food intake in the rat following complete absorption of glucose delivered to the stomach, intestine, or liver. Journal of Physiology (London), 259, 501-522.
Brala, P. M. & Hagen, R. L. (1983) Effects of sweetness Perception and caloric value of a preload on short-term intake. Physiology and Behavior, 30, l-9. Bruning, J. L. & Kintz, B. L. (1977) Computational Handbook of Statistics. Dallas: Scott, Foresman and Co. Collier, G. & Novell, K. (1967) Saccharin as a sugar surrogate. Journal of Comparative and Physiological Psychology, 64, 404-408.
Corbit, J. D. & Stellar, E. (1964) Palatability, food intake, and obesity in normal and hyperphagic rats. Journal of Comparative and Physiological Psychology, 58, 63-67. Greenberg, D., Smith, G. P. & Gibbs, J. (1990) Intraduodenal infusions of fats elicit satiety in sham-feeding rats. American Journal of Physiology, 259, Rl lo-R1 18. Jorgensen,
H. (1950) The influence of saccharin
Scandanavia, 20, 33-37.
on the blood-sugar.
Acta Physiologica
Mattes, R. (1990) Effects of aspartame and sucrose on hunger and energy intake in humans. Physiology and Behavior, 47, 1037-1044.
Mindell, S., Smith, G. P. & Greenberg, D. (1990) Corn oil and mineral oil stimulate sham feeding in rats. Physiology and Behavior, 48, 283-287. Murray, E. J., Wells, H., Kohn, M. & Miller, N. E. (1953). Sodium sucaryl: A substance which tastes sweet to human subjects but is avoided by rats. Journal of Comparative and Physiological Psychology, 46, 134- 137.
Naim, M., Rogatka, H., Yamamoto, T. & Zehavi, U. (1982) Taste responses to neohesperidin dihydrochalcone in rats and baboon monkeys. Physiology and Behavior, 28, 979-986.
SHAM-FEEDING AND FOOD INTAKE
103
Purdom, M. E., Hyder, K. & Pybas, M. D. (1973) Effects of saccharin on rats fed chemically defined diets. Journal of the American Dietetic Association, 63, 635-638. Ramirez, I. (1985) Oral stimulation alters digestion of intragastric meals in rats. American Journal of Physiology, 248, R459-R463. Reed, D. R. & Friedman, M. I. (1990). Diet composition alters the preference for fat in rats. Appetite, 14, 219-230.
Reed, D. R., Tordoff, M. G. & Friedman, M. I. (1990) Sham feeding of corn oil by rats: sensory and postingestive factors. Physiology and Behavior, 47, 779-781. Rogers, P. J. & Blundell, J. E. (1989) Separating the actions of sweetness and calories: effects of saccharin and carbohydrates on hunger and food intake in human subjects. Physiology and Behavior, 45, 1093-1099. Rogers, P. J., Carlyle, J.-A., Hill, A. J. & Blundell, J. E. (1988) Uncoupling sweet taste and calories: Comparison of the effects of glucose and three intense sweeteners on hunger and food intake. Physiology and Behavior, 43, 547-552. Rogers, P. J., Pleming, H. C. & Blundell, J. E. (1990) Aspartame ingested without tasting inhibits hunger and food intake. Physiology and Behavior, 47, 1239-1243. Rolls, B. J., Kim, S. & Federoff, I. C. (1990) Effects of drinks sweetened with sucrose or aspartame on hunger, thirst and food intake in men. Physiology and Behavior, 48, 19-26. Sclafani, A. & Abrams, M. (1986) Rats show only a weak preference for the artificial sweetener aspartame. Physiology and Behavior, 37, 253-256. Sclafani, A. & Nissenbaum, J. W. (1985) Is gastric sham feeding really sham feeding? American Journal of Physiology, 248, R387-R390. Snowdon, C. T. (1975) Production of satiety with small intraduodenal infusions in the rat. Journal of Comparative and Physiological Psychology, 88, 23 l-238.
Thompson, M. M. & Mayer, J. (1959) Hypoglycemic effects of saccharin in experimental animals. American Journal of Clinical Nutrition, 7, 80-85. Tordoff, M. G. (1988) How do nonnutritive sweeteners increase food intake? Appetite, II (Suppl.), 5- 11. Tordoff, M. G & Alleva, A. M. (1990) Oral stimulation with aspartame increases hunger. Physiology and Behavior, 47, 555-559.
Tordoff, M. G. & Friedman, M. I. (1989a) Drinking saccharin increases food preference: I. Comparison with other drinks. Appetite, 12, l-10. Tordoff, M. G. & Friedman, M. I. (1989b) Drinking saccharin increases food preference: II. Hydrational factors. Appetite, 12, 1l-21. Tordoff, M. G. & Friedman, M. I. (1989~) Drinking saccharin increases food preference: III. Sensory and associative factors. Appetite, 12, 23-35. Tordoff, M. G. & Friedman, M. I. (1989d) Drinking saccharin increases food preference: IV. Cephalic phase and metabolic factors. Appetite, 12, 37-56. Wagner, M. W. (1971) Comparative rodent preferences for artificial sweeteners.
intake and intake and intake and intake and Journal of
Comparative and Physiological Psychology, 75,483-490.
Young, R. C., Gibbs, J., Antin, J., Holt, J. & Smith, G. P. (1974) Absence of satiety during sham-feeding in the rat. Journal of Comparative and Physiological Psychology, 87, 795-800. Received
20 September
1990, revision 27 February
1991