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The effects of pregnancy and lactation on dietary self-selection in the rat
Physiology & Behavior, Vol. 49, pp. 811-813. ©Pergamon Press plc, 1991. Printed in the U.S.A.
0031-9384/91 $3.00 + .00
BRIEF COMMUNICATION
The Effects of Pregnancy and Lactation on Dietary Self-Selection in the Rat J A C K I E D I A L A N D D A V I D D. A V E R Y 1
Department o f Psychology, Colorado State University, Fort Collins, CO 80523 R e c e i v e d 4 June 1990
DIAL, J. AND D. D. AVERY. The effects of pregnancy and lactation on dietary self-selection in the rat. PHYSIOL BEHAV 49(4) 811-813, 1991. --Total food intake and intake of discrete dietary components, carbohydrate, fats, and protein, throughout pregnancy and lactation were measured in the albino rat, with virginal rats acting as controls. The dietary components were isocaloric and presented in a self-selection situation. Although the reproductive subjects ate more carbohydrate, protein, and total food than controls during the lactation phase, their intake did not significantly exceed that of controls during pregnancy. In addition, reproductive but not virginal animals changed patterns of selection over the reproductive periods. They consumed more total food during gestation and lactation, and more carbohydrate and protein during lactation. Pregnancy lactation
Dietary self-selection
Macronutrients
the breeding female rat requires a minimum of 12% net protein and 5% net fat. If the dam is to gain weight during lactation, she must consume at least 24% protein. Thus the female rats in this study were expected to adjust their intakes of specific macronuIrients in the dietary self-selection situation during gestation and lactation in comparison with virgin animals in a control group.
ANIMALS vary their selection of nutrients, when possible, to accommodate changing physiological needs. These changes in food preference, called "body wisdom," are caused by such external factors as cold ambient temperatures and increases in energy demand. One internal factor associated with rather dramatic changes in food preferences is reproductive state in female rats. Although research has been done on dietary self-selection during reproductive phases (2, 4, 6), previous researchers did not provide free access to diets that were both isocaloric and composed of the three macronutfients, carbohydrate, fat, and protein. Thus our objective in this experiment was to measure changes in patterns of macronutrient intake in a dietary self-selection paradigm during pregnancy and lactation in the rat. During the three weeks of lactation, female rats have been shown to increase food intake by as much as 300% (4). Tribe (6) investigated the ability of rats to self-select nutrients during pregnancy. Although the nutritional values of the components were equivalent to recommendations for stock diets, the nutritional requirements of the animals were not met. Dams failed to gain weight normally during pregnancy, and the pups that were born were not viable. Increases in food intake during pregnancy are not equally apportioned among the macronutrients. Although in one previous study (2) only carbohydrate and protein components were freely available, an increased intake of protein and no change in carbohydrate intake during pregnancy and lactation were observed. According to the Subcommittee on Laboratory Animal Nutrition (5),
METHOD This experiment had a mixed design. The subjects were randomly assigned to one of two groups. The experiment consisted of four three-week intervals: Baseline--weeks 1 to 3, Gestation-weeks 4 to 6, Lactation--weeks 7 to 9, and Postlactation--weeks 10 to 12. Total food intake and separate intakes in grams of carbohydrate, fat, and protein, were measured daily. Any differences were determined by analysis of variance and were considered significant when they occurred at the 0.05 level or less.
Subjects The animals used in the study were 15 female and five male Sprague-Dawley albino rats from Charles River Laboratories, Wilmington, MA (the males were used for breeding). Sixteen female subjects were randomly assigned to one of the two betweensubjects groups, but one female was dropped from the experiment because she refused to accept the food. Thus eight subjects served in the reproductive experimental group (REG) and seven in the
1Requests for reprints should be addressed to David D. Avery.
811
812
DIAL AND AVERY
TABLE 1 ISOCALORICDIETARYCOMPONENTS* Protein
Carbohydrate
960 g Vitamin-free Casein (ICN Pharmaceuticals, Cleveland, OH)
Fat
580 g Corn Starch (Teklad Test Diets, Madison, WI)
40 g U.S,P. XIV Salt Mixture (ICN Pharmaceuticals)
280 g Dextrin (Teklad Test Diets)
22 g Vitamin Diet Fortification Mixture (ICN Pharmaceuticals)
100 g Commercial-Grade Sucrose
404 g Hydrogenated Vegetable Fat (Crisco) 22 g Safflower Oil (Hollywood Health Foods) 267 g Alphacei Nonnutritive Bulk (ICN Pharmaceuticals)
40 g U.S.P. XIV Salt Mixture (ICN Pharmaceuticals)
267 g Petroleum Jelly
22 g Vitamin Diet Fortification Mixture (ICN Pharmaceuticals)
40 g U.S.P. XIV Salt Mixture (ICN Pharmaceuticals) 22 g Vitamin Diet Fortification Mixture (ICN Pharmaceuticals
*Each Dietary component has a caloric density of 3.8 kcal/g. Data taken from [(3), Table 1]. virginal control group (VCG). The age of each rat at the beginning of the experiment was approximately 60 days, and the females averaged about 100 grams in body weight. The animals were housed in plastic cages equipped with three metal food dishes, one for each separate food component. The experimental room was maintained on a 12-hour light/dark cycle. The temperature of the room was 23 - I°C. Access to the dietary components was ad libitum for 24 hours each day. Diets
Each rat's diet consisted of isocaloric carbohydrate, fat, and protein, presented in aluminum cups. To avoid position preferences, locations of the feeding cups were varied randomly. A block of wood was available for each subject to prevent excessive tooth growth. The compositions of the three diets are listed in Table 1. Procedure. After acceptance of the diet had been established, the rats were placed on a free-feeding schedule with the three macronutrients always available. Water was also constantly available. Rats were weighed daily, and the amount of each separate macronutrient consumed was measured daily. Spilled dry food was collected and included in daily measures. After three weeks, the REG rats were mated. Litters born to the REG dams were kept intact until the pups reached two weeks of age and were then culled to three pups per litter to minimize interference with feeding and spillage of dry food while maintaining demands for production of milk. The remaining pups were removed from the mothers' cages when they reached 20 days of age. Food intake and weight continued to be monitored for both groups of adults until each REG rat had gone through all four planned reproductive periods. RESULTS Litter Characteristics
All eight females in the REG group bore litters, which ranged in size from four to 14 pups. All pups were born alive, although
a total of four offspring from three different litters died within several days after birth. The mean number of pups-standard error for REG animals was 1 1 . 0 _ 3 . 3 8 and the mean w e i g h t+- standard error at birth was 06.2 +-0.53 grams. Food intake measures. Food intake measures were compared between and within each group for total intake and for intake of individual dietary components. Mean values of these measures are shown in Fig. 1. Analysis of variance measured differences in food intake for each group, and post hoc tests (Tukey's HSD) determined specifically where significant differences occurred, both between and within the two groups. During lactation REG animals consumed more carbohydrate, fat, and protein than did VCG animals during various reproductive periods: prepregnancy and gestation for
C
F Bile
P
C
F Preg
p
C
F Llct
P
C
F
P
Post
FIG. l. Mean (+ SEM) intake in grams of isocaloric carbohydrate (C), fat (F), and protein (P) during four three-week periods associated with reproduction--baseline (Base), pregnancy (Preg), lactation (Lact), and postweaning (Post), for two groups of rats, a reproductive experimental group (REG) and a virginal control group (VCG). Significant differences (p<0.05) are as follows: 10>1, 2, 5, 6, and 13; 11>14; 12>3, 4, 9, 15 and 16; 8>7.
PREGNANCY AND DIET
813
carbohydrate, postweaning for fat, and all four periods for protein. In addition, VCG intake of fat exceeded that of REG animals during gestation. Within-group comparisons revealed significant effects for REG animals only. (Virgin animals did not alter patterns of self-selection across the four time periods.) Total food intake for REG subjects during gestation exceeded total intake during the prepregnancy period, and total intake during lactation exceeded total intake during the other three reproductive periods. In addition, carbohydrate intake during lactation exceeded carbohydrate intake during the other periods and protein intake during lactation surpassed protein intake during prepregnancy and postweaning. There were no REG differences in fat intake. DISCUSSION The primary purpose of this study was to measure the effects of gestation and lactation on intake of individual, isocaloric components of the diet. Because nutritional needs affect both amount of food consumed and selection patterns, REG rats were expected to consume more food, particularly protein, during gestation and lactation, than nortreproducing rats. This prediction was essentially confirmed. During gestation, REG animals ate more total
food than they had before pregnancy; during lactation, they increased their total intake and intake of protein and carbohydrate over their own baseline levels and over that of controls. The increase in protein intake by REG animals conformed with Leshner's (2) findings, although in this study only the increase during lactation was significant. Over the course of the present study, both reproductive and control rats ate more protein and fat than the net amounts (12% and 5%, respectively), considered minimal for breeding dams. In fact, both groups consistently consumed at least 25% of their food as protein and, during gestation and lactation, REG animals maintained 38% and 35% protein intakes, respectively. Litter-characteristics statistics gathered in the present study conform to the average litter size, birthweight, and pup fatality rate cited earlier. As the pups were born apparently healthy and the majority successfully reached maturity, it would appear that the protein percentages selected by the mothers were appropriate. ACKNOWLEDGEMENT This research was supported by a BRSG Grant No. 2 S07 RR05458-27 awarded by the Biomedical Research Support Grant program, Division of Research Resources, National Institutes of Health.
REFERENCES 1. Collins, G. R., ed. Syllabus for the laboratory animal technologist. Joliet, IL: American Association for Laboratory Animal Science; 1972. 2. Leshner, A. I.; Siegel, H.; Collier, G. Dietary self-selection by pregnant and lactating rats. Physiol. Behav. 8:151-154; 1972. 3. Marks-Kaufmann, R. Increased fat consumption induced by morphine administration in rats. Pharmacol. Biochem. Behav. 16:949-955; 1982. 4. Slonaker, J. B. The effect of copulation, pregnancy, pseudopregnancy and lactation on the voluntary activity and food consumption of the
albino rat. Am. J. Physiol. 7:362-394; 1925. 5. Subcommittee on Laboratory Animal Nutrition (1978). Nutrient requirements of laboratory animals, 3rd ed. Washington, DC: National Academy of Sciences; 1978. 6. Tribe, D. E. Choice of diets by rats, 4: The choice of purified food constituents during growth, pregnancy, and lactation. Br. J. Nutr. 9:103-109; 1955.
0031-9384/91 $3.00 + .00
BRIEF COMMUNICATION
The Effects of Pregnancy and Lactation on Dietary Self-Selection in the Rat J A C K I E D I A L A N D D A V I D D. A V E R Y 1
Department o f Psychology, Colorado State University, Fort Collins, CO 80523 R e c e i v e d 4 June 1990
DIAL, J. AND D. D. AVERY. The effects of pregnancy and lactation on dietary self-selection in the rat. PHYSIOL BEHAV 49(4) 811-813, 1991. --Total food intake and intake of discrete dietary components, carbohydrate, fats, and protein, throughout pregnancy and lactation were measured in the albino rat, with virginal rats acting as controls. The dietary components were isocaloric and presented in a self-selection situation. Although the reproductive subjects ate more carbohydrate, protein, and total food than controls during the lactation phase, their intake did not significantly exceed that of controls during pregnancy. In addition, reproductive but not virginal animals changed patterns of selection over the reproductive periods. They consumed more total food during gestation and lactation, and more carbohydrate and protein during lactation. Pregnancy lactation
Dietary self-selection
Macronutrients
the breeding female rat requires a minimum of 12% net protein and 5% net fat. If the dam is to gain weight during lactation, she must consume at least 24% protein. Thus the female rats in this study were expected to adjust their intakes of specific macronuIrients in the dietary self-selection situation during gestation and lactation in comparison with virgin animals in a control group.
ANIMALS vary their selection of nutrients, when possible, to accommodate changing physiological needs. These changes in food preference, called "body wisdom," are caused by such external factors as cold ambient temperatures and increases in energy demand. One internal factor associated with rather dramatic changes in food preferences is reproductive state in female rats. Although research has been done on dietary self-selection during reproductive phases (2, 4, 6), previous researchers did not provide free access to diets that were both isocaloric and composed of the three macronutfients, carbohydrate, fat, and protein. Thus our objective in this experiment was to measure changes in patterns of macronutrient intake in a dietary self-selection paradigm during pregnancy and lactation in the rat. During the three weeks of lactation, female rats have been shown to increase food intake by as much as 300% (4). Tribe (6) investigated the ability of rats to self-select nutrients during pregnancy. Although the nutritional values of the components were equivalent to recommendations for stock diets, the nutritional requirements of the animals were not met. Dams failed to gain weight normally during pregnancy, and the pups that were born were not viable. Increases in food intake during pregnancy are not equally apportioned among the macronutrients. Although in one previous study (2) only carbohydrate and protein components were freely available, an increased intake of protein and no change in carbohydrate intake during pregnancy and lactation were observed. According to the Subcommittee on Laboratory Animal Nutrition (5),
METHOD This experiment had a mixed design. The subjects were randomly assigned to one of two groups. The experiment consisted of four three-week intervals: Baseline--weeks 1 to 3, Gestation-weeks 4 to 6, Lactation--weeks 7 to 9, and Postlactation--weeks 10 to 12. Total food intake and separate intakes in grams of carbohydrate, fat, and protein, were measured daily. Any differences were determined by analysis of variance and were considered significant when they occurred at the 0.05 level or less.
Subjects The animals used in the study were 15 female and five male Sprague-Dawley albino rats from Charles River Laboratories, Wilmington, MA (the males were used for breeding). Sixteen female subjects were randomly assigned to one of the two betweensubjects groups, but one female was dropped from the experiment because she refused to accept the food. Thus eight subjects served in the reproductive experimental group (REG) and seven in the
1Requests for reprints should be addressed to David D. Avery.
811
812
DIAL AND AVERY
TABLE 1 ISOCALORICDIETARYCOMPONENTS* Protein
Carbohydrate
960 g Vitamin-free Casein (ICN Pharmaceuticals, Cleveland, OH)
Fat
580 g Corn Starch (Teklad Test Diets, Madison, WI)
40 g U.S,P. XIV Salt Mixture (ICN Pharmaceuticals)
280 g Dextrin (Teklad Test Diets)
22 g Vitamin Diet Fortification Mixture (ICN Pharmaceuticals)
100 g Commercial-Grade Sucrose
404 g Hydrogenated Vegetable Fat (Crisco) 22 g Safflower Oil (Hollywood Health Foods) 267 g Alphacei Nonnutritive Bulk (ICN Pharmaceuticals)
40 g U.S.P. XIV Salt Mixture (ICN Pharmaceuticals)
267 g Petroleum Jelly
22 g Vitamin Diet Fortification Mixture (ICN Pharmaceuticals)
40 g U.S.P. XIV Salt Mixture (ICN Pharmaceuticals) 22 g Vitamin Diet Fortification Mixture (ICN Pharmaceuticals
*Each Dietary component has a caloric density of 3.8 kcal/g. Data taken from [(3), Table 1]. virginal control group (VCG). The age of each rat at the beginning of the experiment was approximately 60 days, and the females averaged about 100 grams in body weight. The animals were housed in plastic cages equipped with three metal food dishes, one for each separate food component. The experimental room was maintained on a 12-hour light/dark cycle. The temperature of the room was 23 - I°C. Access to the dietary components was ad libitum for 24 hours each day. Diets
Each rat's diet consisted of isocaloric carbohydrate, fat, and protein, presented in aluminum cups. To avoid position preferences, locations of the feeding cups were varied randomly. A block of wood was available for each subject to prevent excessive tooth growth. The compositions of the three diets are listed in Table 1. Procedure. After acceptance of the diet had been established, the rats were placed on a free-feeding schedule with the three macronutrients always available. Water was also constantly available. Rats were weighed daily, and the amount of each separate macronutrient consumed was measured daily. Spilled dry food was collected and included in daily measures. After three weeks, the REG rats were mated. Litters born to the REG dams were kept intact until the pups reached two weeks of age and were then culled to three pups per litter to minimize interference with feeding and spillage of dry food while maintaining demands for production of milk. The remaining pups were removed from the mothers' cages when they reached 20 days of age. Food intake and weight continued to be monitored for both groups of adults until each REG rat had gone through all four planned reproductive periods. RESULTS Litter Characteristics
All eight females in the REG group bore litters, which ranged in size from four to 14 pups. All pups were born alive, although
a total of four offspring from three different litters died within several days after birth. The mean number of pups-standard error for REG animals was 1 1 . 0 _ 3 . 3 8 and the mean w e i g h t+- standard error at birth was 06.2 +-0.53 grams. Food intake measures. Food intake measures were compared between and within each group for total intake and for intake of individual dietary components. Mean values of these measures are shown in Fig. 1. Analysis of variance measured differences in food intake for each group, and post hoc tests (Tukey's HSD) determined specifically where significant differences occurred, both between and within the two groups. During lactation REG animals consumed more carbohydrate, fat, and protein than did VCG animals during various reproductive periods: prepregnancy and gestation for
C
F Bile
P
C
F Preg
p
C
F Llct
P
C
F
P
Post
FIG. l. Mean (+ SEM) intake in grams of isocaloric carbohydrate (C), fat (F), and protein (P) during four three-week periods associated with reproduction--baseline (Base), pregnancy (Preg), lactation (Lact), and postweaning (Post), for two groups of rats, a reproductive experimental group (REG) and a virginal control group (VCG). Significant differences (p<0.05) are as follows: 10>1, 2, 5, 6, and 13; 11>14; 12>3, 4, 9, 15 and 16; 8>7.
PREGNANCY AND DIET
813
carbohydrate, postweaning for fat, and all four periods for protein. In addition, VCG intake of fat exceeded that of REG animals during gestation. Within-group comparisons revealed significant effects for REG animals only. (Virgin animals did not alter patterns of self-selection across the four time periods.) Total food intake for REG subjects during gestation exceeded total intake during the prepregnancy period, and total intake during lactation exceeded total intake during the other three reproductive periods. In addition, carbohydrate intake during lactation exceeded carbohydrate intake during the other periods and protein intake during lactation surpassed protein intake during prepregnancy and postweaning. There were no REG differences in fat intake. DISCUSSION The primary purpose of this study was to measure the effects of gestation and lactation on intake of individual, isocaloric components of the diet. Because nutritional needs affect both amount of food consumed and selection patterns, REG rats were expected to consume more food, particularly protein, during gestation and lactation, than nortreproducing rats. This prediction was essentially confirmed. During gestation, REG animals ate more total
food than they had before pregnancy; during lactation, they increased their total intake and intake of protein and carbohydrate over their own baseline levels and over that of controls. The increase in protein intake by REG animals conformed with Leshner's (2) findings, although in this study only the increase during lactation was significant. Over the course of the present study, both reproductive and control rats ate more protein and fat than the net amounts (12% and 5%, respectively), considered minimal for breeding dams. In fact, both groups consistently consumed at least 25% of their food as protein and, during gestation and lactation, REG animals maintained 38% and 35% protein intakes, respectively. Litter-characteristics statistics gathered in the present study conform to the average litter size, birthweight, and pup fatality rate cited earlier. As the pups were born apparently healthy and the majority successfully reached maturity, it would appear that the protein percentages selected by the mothers were appropriate. ACKNOWLEDGEMENT This research was supported by a BRSG Grant No. 2 S07 RR05458-27 awarded by the Biomedical Research Support Grant program, Division of Research Resources, National Institutes of Health.
REFERENCES 1. Collins, G. R., ed. Syllabus for the laboratory animal technologist. Joliet, IL: American Association for Laboratory Animal Science; 1972. 2. Leshner, A. I.; Siegel, H.; Collier, G. Dietary self-selection by pregnant and lactating rats. Physiol. Behav. 8:151-154; 1972. 3. Marks-Kaufmann, R. Increased fat consumption induced by morphine administration in rats. Pharmacol. Biochem. Behav. 16:949-955; 1982. 4. Slonaker, J. B. The effect of copulation, pregnancy, pseudopregnancy and lactation on the voluntary activity and food consumption of the
albino rat. Am. J. Physiol. 7:362-394; 1925. 5. Subcommittee on Laboratory Animal Nutrition (1978). Nutrient requirements of laboratory animals, 3rd ed. Washington, DC: National Academy of Sciences; 1978. 6. Tribe, D. E. Choice of diets by rats, 4: The choice of purified food constituents during growth, pregnancy, and lactation. Br. J. Nutr. 9:103-109; 1955.