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Social facilitation of eating: Effects of social instruction on food intake
Physiology&Behavior,Vol.52, pp. 749-754, 1992
0031-9384/92$5.00 + .00 Copyright © 1992PergamonPressLtd.
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Social Facilitation of Eating: Effects of Social Instruction on Food Intake M A R I E R E D D A N D J O H N M. DE C A S T R O 1
Department of Psychology and Psychological Sciences Program, Georgia State University, Atlanta, GA 30303 Received 7 J u n e 1991 REDD, M. AND J. M. DE CASTRO. Socialfacilitation of eating: Effects of social instruction onfood intake. PHYSIOL BEHAV 52(4) 749-754, 1992.--To investigatewhether social influencescause increasesin eating behavior, thirty undergraduatepsychology students completed a diet diary for three 5-day periods. Subjects were instructed to either eat alone or eat with other people, actively eating with them for two of these periods. For the third period, subjects were instructed to eat as they normally would (with or without other people present). When instructed to eat with others present, subjects overall consumed more food, water, sodium, and alcohol than when they were instructed to eat alone. In the normal condition, food intake was 60% higher when the subjects ate with others present than when they ate alone. These results suggest that social facilitation has a causal influence on eating which increases food intake. Meals
Eating
Carbohydrate
Socialfacilitation
Sodium
SOCIAL conditions associated with food intake have a powerful effect on the amount of food ingested both in animals (2,15,16,20,21,23) and in humans (9). The number of people eating with the subject influences the amount of food eaten in a meal. This social influence is the best predictor of how much food an individual will consume, independent of when the individual eats, where the individual eats, whether the eating episode is a meal or a snack, and whether the eating episode occurs on a weekday or a weekend (6,8). As the number of people eating with an individual increases, the amount of food consumed increases incrementally as a power function. Subjects eat an average of 28% more when one other person is present at a meal than when they eat alone; with two people present, 41% more; with three people present, 53% more; and 53%, 71%, and 76% more when eating with four, five, or six people present, respectively (7). This results in a strong positive correlation between the number of people eating with an individual and the size of the meal ingested (9). People appear to eat more when other people are present because, in social meals, people tend to eat for a longer period of time (5). Previous correlational studies have shown that when other people are present at a meal, the amount of food consumed increases. Since these studies were correlational in nature, causation cannot be established. The present study attempted to address this issue by manipulating the social conditions involved in food intake. Subjects were asked to record their dietary intake for three periods of 5 days each, eating exclusively alone, eating exclusively with others actively eating with them, or eating as they normally
Alcohol
Humans
would (with or without others present). By manipulating only the social settings of the subjects' eating episodes, group influence effects may be clearly differentiated and compared. METHOD The methods are similar to those employed in a previous study ( 11 ).
Subjects Twelve male and 26 female undergraduate psychology students participated in this study. The subjects averaged 22.8 years (range 18-46) of age, 61.5 kilograms (range 46.4-87.8), and 1.63 meters (range 1.55-1.93). The subjects received research participation credits and a nutritional summary based on their reported food intake. Two males and six females were excused from this experiment due to either illness or inability to follow the procedures. The remaining l0 males and 20 females completed the study.
Procedure The subjects completed a questionnaire asking general demographic questions and questions regarding their health and eating behavior. The questionnaire asked for the names of two people the subject would eat with during the experiment who could be contacted later to verify the diary entries. The subjects also completed an informed consent form. During the initial meeting, subjects were told that the experiment dealt with nu-
Supported in part by Grant DK 39881 from the National Institute of Diabetes and Digestiveand Kidney Diseases and from a grant from the National Livestock and Meat Board. J Requests for reprints should be addressed to John M. de Castro, Department of Psychology, Georgia State University, Atlanta, GA 30303.
749
750 trition and eating behavior. They were asked if they were willing to complete a diet diary for 15 days in periods of 5 days each, with each period beginning on Wednesday and ending on Sunday. During these periods, they were instructed to eat as they normally would, to eat exclusively alone, or to eat exclusively with others present and eating with them. Subjects were strongly urged to adhere to the condition requirements, but in the event that the subject could not satis~ the condition's requirements, i.e., the subject had to eat with others when in the alone condition, they could do so, but note the episode in their diary. The subjects were also instructed to not skip a meal or snack if they could not comply with the conditions. The subjects were allowed to eat whatever and whenever they wanted. Only the social conditions were manipulated. The subjects recorded their food intake in a small pocketsized diet diary (8 × 18 cm). The food entries were recorded in great detail (whether fried, baked, etc., and whether cups, ounces, etc.). In addition to recording the food entries, the subjects reported the time and day of the eating bout, the number of people eating with the subject, and whether the eating episode was a meal or a snack. Premeal and postmeal seven-point Likert scales were used to rate the subject's hunger, depression, anxiety, thirst, the attractiveness of the meal, and the subject's comfort level of the people eating with the subject, and how important to the subject the people present were. Subjects completed a l-day trial diary, and met with the experimenter to ask questions, to correct any problems, and to check for accuracy. Subsequently, the subjects completed a 5day diary either eating alone, with people, or normally; the order of the conditions was determined randomly. After each diary was completed, the subject met with the experimenter on the following Monday to check for accuracy and to answer questions. Each diary was then verified by a person who had eaten with the individual during that interval. The people eating with the subject were asked if they remembered what the subject ate at a specific meal. If the person could not remember, then the experimenter prompted the person, for example, by asking if the subject had eaten beef or chicken. If they still could not answer, the experimenter asked if the subject had eaten a hamburger or a steak. To further check for accuracy, the subjects were asked about problems they may have had during the previous period, and whether they had recorded accurately. The subjects were told that regardless of their responses, they would still gain full research participation credits for the experiment. It was further emphasized that accuracy in recording would provide a much more accurate nutritional analysis. This procedure has been employed in other related studies (10),
Data Analy.~is A computer analysis of the nutrient composition of the recorded food was performed once the diaries were completed and verified. The experimenter coded the food items reported in the diaries using a computer file of over 3500 predetermined assigned codes. This computer file was created using the U.S. Department of Agriculture Handbooks number 6 and 456 of the Nutritive Value of American Foods. Complex food items were broken down into their component parts. For example, a grilled cheese sandwich was broken down into bread, cheese, and butter. In addition, water and other liquids were coded. Once the food items were entered, the computer summed the nutrient compositions of the food items recorded for each meal. The diet diaries were analyzed separately for each of the three conditions (normally, alone, and with others present). In addition, meals eaten during the normal condition were separated into those eaten with others present and those eaten alone.
REDD AND r)~! CASTRO Means were calculated for each individual in each condition and averaged to produce group means in each condition. Means were calculated for meal size (kcals), meal composition (carbohydrates, protein, fat, alcohol, and water), intermeal intervals, and premeal and postmeal stomach contents, and self-ratings (attractiveness of the meal, hunger, anxiety, depression, how comfortable the subject was with the people present, and importance of the people present). Stomach caloric content was estimated using a computer model. The reported intake was entered in the model and was estimated to empty from the stomach at a rate proportional to the square root of the caloric content of the stomach in little calories (S) per minute (11,17-19). S-S
5xS 5
The total caloric content of the stomach and the proportion of each macronutrient estimated to be present at the beginning and at the end of each meal was included in the subsequent analyses. Repeated measures ANOVAs were used to analyze the differences in the meal characteristics and the self-ratings among the three conditions. Repeated measures ANOVAs were also used to determine if the sequences of the conditions had any effect on food intake. If statistically significant differences were found, then the data were further analyzed using correlated ttests. Meal size, meal compositions (carbohydrates, fats, and proteins), number of people, intermeal intervals, and stomach contents and self-reported hunger, depression, and anxiety were intercorrelated using Pearson Product Moment Correlations. Correlations were calculated using all intermeal intervals except overnight fasts. Mean correlations across the groups were calculated using r to z transformed coefficients (3). RESULTS
Subject Compliance Subject compliance was checked by verifying at least two meals eaten with the two people listed by the subject. All the people contacted, without exception, verified that the diary entries for the meals reportedly eaten with them were accurate. In addition, subjects reported an average of 0.12 other people present per meal during the alone period, 1.7 others present during the with-people period, and 1.3 others present during the normal period, t(29) = 3.13, p < 0.01 ; with-people condition vs. normal condition. The subjects were instructed not to miss meals in order to comply with the instructions. In fact, the number of meals per day in each condition did not significantly differ, with 3.2, 3.1, and 3.0 meals per day reported for the Normal, With People, and Alone conditions, respectively.
Overall Intakes Table 1 shows the mean minimum and maximum nutrient compositions of the overall food intakes during the alone, normal, and with-people conditions. Differences in the overall intakes (kcals) among the three conditions were found, F(2, 56) = 3.35, p < 0.05. During both the normal and with-people conditions, overall intakes were significantly higher than in the alone condition, t(28) = 2.26, p < 0.05; t(28) = 2.08, p < 0.05, respectively. Differences in the overall intakes of fat during the three conditions were found, F(2, 56) = 3.15, p < 0.05. Fat intake was significantly higher in the normal condition than in the alone condition, t(28) = 2.15, p < 0.05. Alcohol intakes also
SOCIAL F A C I L I T A T I O N
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TABLE 1 MEAN + SEM FOR DAILY NUTRIENT INTAKE Daily Nutrient Intake Nutrients kcal Carb (g) Fat (g) Protein (g) Sodium (mg) Calcium (mg) Phosph (mg) Vit B-6 (#mg) Vit B-I (mg) Vit A (IU) Magn (mg) Iron (mg) Ribofl (umg) Vit C (mg) Vit E (IU) Vit BI2 (#g) Foilc (#g) Alcohol (g) Cholesterol (mg) Grams Grams H20 Sugar (g) Zinc (rag)
Alone 1774.7 193.3 80.2 72.6 2473.8 836.4 1150.8 1265.6 18.6 4749.1 187.2 15.8 1653.3 71.5 13.5 4.5 164.6 2.0 312.5 1793.1 1472.5 96.6 9.0
Normal 116.0 13.8 6.0 4.7 10.7 80.5 80.5 157.9 1.6 614.3 14.4 3.3 152.3 9.0 1.9 1.1 25.2 0.9 28.9 121.5 104.8 8.3 0.7
2021.0 215.1 90.9 81.9 3174.3 826,6 1239.2 1474.4 21.9 4297.2 204.2 16.3 1725.9 78.6 15.1 4.5 157.8 5.6 341.3 1884.8 1523.7 93.3 11.9
With 134.7 17.2 6.6 5.6 223.6 63.0 77.2 171.0 1.6 461.1 14.3 1.8 146.5 8.3 2.0 0.5 18.5 2.0 34.2 142.4 118.3 9.7 1.3
1986.9 206.9 91.0 79.9 3194.6 972.1 1293.2 1270.6 20.3 4773.6 205.8 13.7 1757.6 88,7 15,1 4.7 188.2 6.0 365.4 1989.1 1625.6 100.7 11.4
112.7" 12.8 6.3* 9.9 206.4":~ 91.0 82.4i151.6 1.8 414.9 12.2 1.4 139.0 12.6 1.9 0.6 18.7 2.1 "t" 24.8 112.6J99.0t 8.8 1.3"
This table shows the amounts of nutrients eaten in each condition. Statistical significance was determined by ANOVAs and correlated t-tests. (g = grams, mg= milligrams, IU = international units, ug = micrograms). * Alone vs. normal p < 0.05. i Alone vs. with p < 0.05. Normal vs. with p < 0.05.
differed among the conditions, F(2, 56) = 2.35, p < 0.05. Alcohol intake was significantly higher during the with-people condition than during the alone condition, t(28) = 2.35, p < 0.05. Micronutrient compositions of the meals also differed, especially with respect to sodium, F(2, 56) = 7.91, p < 0.01, and zinc intakes, F(2, 56) = 3.56, p < 0.05. Sodium intake was significantly higher in both the normal condition and the withpeople condition than in the alone condition, t(28) = 4.08, p < 0.01; t(28) = 3.56, p < 0.01; respectively. Daily zinc intakes were significantly higher in the normal condition than in the alone condition, t(28) = 2.25, p < 0.05.
Water intake per meal also varied among the three conditions, F(2, 56) = 4.75, p < 0.05. Water intake was significantly higher in the with-people condition than in the normal condition, t(28)
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Meal Characteristics The subjects ate an average of 668 kcals per meal in the withpeople condition, 658 kcals in the normal condition, and 605 kcals in the alone condition (see Fig. 1). These differences in meal size were not statistically significant, However, when meal size was expressed by weight, a significant difference in the a m o u n t of grams eaten per meal was evident among the conditions, F(2, 56) = 4.70, p < 0.05. Meals eaten during the withpeople condition were larger than meals eaten in the normal condition, t(28) = 2.76, p < 0.05, and the alone condition, t(28) = 2.32, p < 0.05. Overall food intakes for males were significantly different from females' overall food intakes, F(I, 26) = 6.18, p < 0.05. Males ate more than females overall, t(26) = 2.69, p < 0.05, yet the intakes did not significantly differ in the different social conditions.
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FIG. 1. Meal characteristics. This figure shows the average meal size in kcals for the normal, alone, and with people conditions, and the normal conditions separated into meals eaten alone and with others present. Each bar shows the carbohydrate, protein, fat, alcohol, and premeal stomach contents for each condition.
752
REDD AND DE CASTRO Postmell Self-rlttlngs
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FIG. 2. Premeal and postmeal self-ratings. This figure shows the mean premeal and postmeal self-ratings for attractiveness of the meal, hunger, thirst, elation, anxiety, importance of the other people present, and how comfortable the subject is with the other people present. The first bar in each group represents the normal condition; the second bar the alone condition, and the last bar the with people condition. For the importance and comfort self-ratings, the first bar represents the normal condition and the last bar represents the with people condition for each self-rating (the alone condition was not included).
= 2.92. p < 0.05, and the alone condition, t(28) = 2.23, p < 0.05. Results for the premeal self-rating scales for the three conditions are presented in Fig. 2. Both premeal and postmeal ratings of depression differed, F(2, 56) = 3.37, p < 0.05; F(2, 56) = 3.44, p < 0.05, respectively. Subjects reported greater depression (decreased elation) in the alone condition than in the with-people condition, both before the meal, t(28) = 2.66, p < 0.05, and after the meals, t(28) = 2.82, p < 0.05. Subjects reported a higher importance of people eating with them in the with-people condition both before the meal, t(18) = 2.94, p < 0.01, and after the meal, t(18) = 3.26, p < 0.01, than in the normal condition. Subjects also rated the attractiveness of the meal differently among the conditions, F(2, 56) = 5.49, p < 0.05. The meals in the with-people condition were rated as more attractive than meals in the alone condition, t(28) = 3.43, p < 0.05.
Correlational Analysis The correlations between meal size and estimated premeal stomach contents were not significantly different among the conditions. Therefore, changing the social conditions in which subjects ate did not change the influence of estimated premeal stomach contents on meal size. Correlations of the n u m b e r of people present and meal size were strong and positive in both conditions and were significantly different between the normal and with-people conditions, t(28) = 3.05, p < 0.05 (see Fig. 3). (Since subjects ate alone in the alone condition, the correlations between the n u m b e r of people and meal size were not considered relevant.) This difference may
be due to frequent inclusion of alone (zero people present) in the normal condition, while only infrequent in the with-people condition. Since all other correlations were not different, this suggests that the manipulation of the social conditions did not disrupt other regulatory mechanisms.
Analysis of the Normal Condition Approximately 2~ of the meals eaten during the normal condition had other people present averaging 2.06 people per meal. When separating the normal condition into meals eaten with people and meals eaten alone, meal size was found to be significantly larger when meals were eaten with others present (771.5 kcals versus 485.4 kcals), t(17) = 5.62, p < 0.01. Carbohydrate, t(17) = 3.61, p < 0.01, protein, t(17) = 5.13, p < 0.01, fat, t(17) = 6.43, p < 0.01, and water, t(17) = 5.40, p < 0.01, ingestion were also significantly higher when eating with others than when eating alone. Although estimated premeal stomach contents did not differ significantly, the larger social meal sizes result in higher estimated postmeal stomach contents with others present, t(17) = 5.60, p < 0.01. The percentages of carbohydrates in the meal and the percentages of fat in the meals were significantly different such that when eating alone, subjects ate a higher percentage of carbohydrates in the meal, t(9) = 2.69, p < 0.05, and when eating with others, subjects ate a higher percentage of fat, t(17) = 2.63, p < 0.01. Although postmeal intervals were not significantly different, premeal intervals were significantly longer when the subjects ate with others, t(7) = 3.88, p < 0.05. Duration of intake was longer when subjects ate with others than when they ate alone, t(17) =
SOCIAL FACILITATION
753
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FIG. 3. Correlations between number of people present, premeal hunger ratings, meal time, and pre-lMI with meal size. The solid bars represent the normal condition. The open bars represent the alone condition. The hatched bars represent the with people condition. The first group of bars show the correlation between the number of people present and meal size. The second group of bars show the premeal hunger rating and the meal size correlation. The third group of bars show the correlation of the meal time and meal size.The fourth group of bars show the correlation of premeal intermeal interval and meal size. *p < 0.05.
3.50, p < 0.01. The rate of the intake was not significantly different between the meals eaten with others or eaten alone. DISCUSSION Previous observational research has shown that subjects consume more food when they eat in a group than when they eat alone. The present study manipulated the social conditions at meals to determine if having other people present causes an increase in food intake. The results showed that the subjects consumed more total food energy and increased their meal sizes when following instructions to eat with other people present than to eat alone. During the period when subjects were instructed to eat as they normally would, they consumed larger meals when they ate with others than when alone. These results suggest that the presence of other people is, indeed, the cause of heightened intake. The data suggest that the presence of other people not only increases the intake overall but also alters the composition of the meal. In particular, the proportion of calories accounted for by fat was higher in the normal condition than in the alone condition. Also, the percentage of fat consumed in the normal condition was significantly higher when the subjects ate with others. This suggests that the meals eaten alone were proportionally lower in fat regardless of whether they occurred by instruction or not.
Alcohol intake was strikingly different in the alone and withpeople conditions. Alcohol intake was much larger in the withpeople condition than in the alone condition. This social facilitation of alcohol intake also has been previously shown in an observational study (8,12). The present study, because of its manipulative nature, demonstrates a causal connection between the presence of other people and the intake of alcohol. Sodium intake was higher in both the normal and with-people conditions than in the alone condition. This increase in sodium intake may be responsible for the increase in water intake that occurred in the with-people condition. Increasing sodium concentrations in the body requires increasing water concentrations and, thus, promotes water intake to maintain homeostasis in the body (14). The increased water intake may simply be due to the higher amounts of solids ingested in these meals (4). The normal condition data compares favorably with data from a similar study. In the de Castro and de Castro study, subjects completed a 7-day diet diary with no manipulations (14). The subjects ate as they normally would, de Castro and de Castro reported that larger meals were ingested when subjects ate with other people present. When the meals in the normal condition in the present study were separated into meals eaten alone and meals eaten with others, larger meals were ingested when eating with others present, de Castro and de Castro found that these social meals were 44% larger than meals eaten alone. These results compare favorably with the normal condition in the present study where meals eaten with others were 60% larger than meals eaten alone. All the amounts of macronutrients ingested were larger when people were present at a meal in both the normal condition and the de Castro and de Castro study. In the present study, the with-people condition was very similar to the normal condition, but dissimilar to the alone condition. These similarities may be due to the number of people eating with the subject; in the with-people condition, an average of 1.7 people ate with the subject, while an average of 1.3 people ate with others in the normal condition. Other research reports that the amount of food ingested is a power function of the number of the people present (7). The log of the meal size increases in a linear fashion as the log of the number of people present at a meal increases. An increase of people present at a meal from no other people present (0) to 1.3 people present at a meal would be expected to show a large effect on meal size, but an increase from 1.3 to 1.7 people present at a meal would show only a small, perhaps undetectable, effect. Meals eaten during the normal and with-people conditions were only slightly (10%) larger than meals eaten in the alone condition. However, in the normal condition, meals eaten with people present were 60% larger than those eaten alone. Meals in the alone condition were 20% larger than meals eaten alone in the normal condition, and meals in the normal condition that were eaten with others present were 13% larger than meals eaten in the with-people condition. These differences in food intake between the conditions suggest that either the subjects may be compensating, i.e., decreasing meal sizes in the with-people condition and increasing meal sizes in the alone condition, thus bringing overall intake to more normal levels, or that by separating normal meals eaten alone or with people, the impact of social facilitation on eating may be overestimated. Ratings of subjective states indicated that subjects were not hungrier when they ate with other people present. Premeal and postmeal subjective hunger ratings were not significantlydifferent among the three conditions. Hence, larger meals eaten in social conditions occurred under equivalent conditions of subjective hunger. In contrast, subjects reported greater depression when eating in the alone condition than they reported in the withpeople condition. In addition, in the with-people condition, the
754
R E D D A N D 19~: C A S T R O
persons eating with the subjects were rated as more important than in the normal condition. Perhaps the instruction caused the importance to increase; subjects rated their eating companions as more important because of the necessity to eat with someone to comply with the instructions. The present study's results depend on the veracity of the subjects' recordings, lfthe diaries were not truthfially and faithfully completed, then the results would also be inaccurate. However, many factors suggest that the diaries were completed accurately and truthfully. Verification of the diary entries helped to ensure accurate recording. At least two meals from each diary were verified by at least one of the persons listed on the questionnaire. In addition to verification of the meals by people eating with the subject, the subjects were asked if they completed the diaries accurately and they were told there would be no penalty for indicating inaccuracy. All the subjects reported that the diaries were completed truthfully and accurately. Furthermore, the subjects seemed to anticipate the nutritional analyses and were aware that the analyses were dependent on their accuracy of recording. Other problems of the diet diary method may be more intrinsic, i.e., the subjects may unknowingly complete the diary inaccurately. These problems may include inaccurate recall, inaccurate recording, and modified eating behavior due to recording (1,13,22). In the present study, subjects recorded food items in terms they understood, recorded food intake immediately after eating, and met with the experimenter weekly to discuss any problems. Recording of intake occurred over an extended period of time (15 days) to control for the possible altering of behavior. The order of the conditions was randomly determined for each subject, and the subjects acted as their own controls by completing a diary for a 5-day period eating as they normally would. Analysis of the sequence of the orders showed that the order of the conditions did not affect meal size.
The data further suggest that the subjects accurately completed the diaries. Subjects recorded meals that did not comply with the experimental instructions. Subjects reported meals eaten with others present in the alone condition. The number of meals did not differ significantly among the conditions. The subjects" recordings of meals that did not comply with the instructions and the similar meal frequencies (number of meals) in the conditions suggest that all the meals were recorded. Additionally, if the subjects were lying and not recording that the people were present during the alone condition or adding fictitious companions to the with-people condition, then the differences between the conditions would be reduced and an effect of the manipulation would be expected. Hence, noncompliance would be expected to decrease condition effects, not produce them, The results of the present study strongly suggest that the presence of other people at meals causes an increase in food intake at meals. Since only one factor was manipulated (the social setting with regard to people present), the differences can be attributed to the social effect others have on meal size. These results have implications for further research. Eating alone may be an effective dieting strategy if people present at a meal cause an increased meal size which results in higher overall intakes. A difference of 212 kcals were consumed per day between the with-people condition (1987 kcals) and the alone condition ( 1775 kcals). Over periods of months, this could result in a significant weight loss. Eating alone also causes a decrease in fat and salt consumption. Both these factors have been linked to high cholesterol and heart disease. A reduction of these factors have been suggested to reduce the risk of heart disease. Additionally, geriatric patients may benefit from eating with others as well, such as eating in a community dining room. This might promote a decrease in depression levels and an increase in the amount of food consumed.
REFERENCES 1. Balough, M.; Kahn, H. A.; Medalie, J. H. Random repeat 24-hour dietary recalls. Am. J. Clin. Nutr. 24:304-310; 1971. 2. Bayer, E. Beitrage zur zweikomponenten theorie des hungers. Z. Psychol. 112:1-54; 1929. 3. de Castro, J. M. Meal pattern correlations: Facts and artifacts. Physiol. Behav. 26:795-798; 1975. 4. de Castro, J. M. Physiological, environmental, and subjective determinants of food intake in humans: A meal pattern analysis. Physiol. Behav. 44:651-659; 1988. 5. de Castro, J. M. Social facilitation of duration and size but not rate of the spontaneous meal intake of humans. Physiol. Behav. 47:11291135; 1990. 6. de Castro, J. M. Social facilitation of the spontaneous meal size of humans occurs on both weekdays and weekends. Physiol. Behav. 49:1289-1291; 1990. 7. de Castro, J. M.; Brewer, E. M. The amount eaten in meals by humans is a power function of the number of people present. Physiol. Behav. 51:121-125; 1992. 8. de Castro, J. M.; Brewer, M.; Elmore, D.; Orozco, S. Social facilitation of the spontaneous meal size of humans is independent of time, place, alcohol, or snacks. Appetite 15:89-101; 1990. 9. de Castro, J. M.; de Castro, E. M. The presence of other people is associated with enlarged meal sizes and disruption of postprandial regulation in the spontaneous eating patterns of humans. Am. J. Clin. Nutr. 50:237-247; 1989. 10. de Castro, J. M.; Elmore, D. K. Subjective hunger relationships with meal patterns in the spontaneous feeding behavior in humans: Evidence for causal connection. Physiol. Behav. 43:159-165; 1988. 11. de Castro, J. M.; McCormick, J. N.; Pederson, M.; Kreitzman, S. N. Spontaneous human meal patterns are related to preprandial factors regardless of natural environmental constraints. Physiol. Behav. 38:25-29; 1986,
12. de Castro, J, M.; Orozco, S. Moderate alcohol intake on the spontaneous eating patterns of humans: Evidence of unregulated supplementation. Am. J. Clin. Nutr. 52:246-253; 1990. 13. Fidanza, F. Sources of error in dietary, surveys. Bibl. Nutr. Diet. 64: 19-28; 1974. 14. Fitzsimons, J. T. Drinking in rats depleted of body fluid without increase in osmotic pressure. J. Physiol. (Lond.) 159:297-309; 1961. 15. Harlow, H. F. Social facilitation of feeding in the albino rat. J. Genet. Psychol. 43:211-221: 1932. 16. Harlow, H. F.: Yudin, H. C. Social behavior in primates. I. Social facilitation of feeding in the monkey and its attitudes to ascendence and submission. J. Comp. Psychol. 16:171-185: 1933. 17. Hopkins, A. The pattern of gastric emptying: A new view of old results. J. Physiol. (kond.) 182:144-150; 1966. 18. Hunt, J. N.; Knox, M. T, Regulation of gastric emptying. In: Code, C. F.; Heidel, W., eds. Handbook of physiology: Alimentary Canal, vol. 4: Motility. Washington, DC: American Physiological Society: 1968:1917-1935. 19. Hunt, J. N.; Stubbs, D. F. The volume and content of meals as determinants of gastric emptying. J. Physiol. (Lond.) 245:209-225: 1975. 20. James, W. T. Social facilitation of eating behavior in puppies after satiation. J, Comp. Physiol. PsychoL 46:427-428; 1953. 21. James, W. T. The development of social facilitation of eating in puppies. J. Genet. Psychol. 96:123-127; 1960. 22. Krantzler, N. J.; Mullen, B. J.; Schutz, H. G.; Grivetti, L. E.; Holden, C. A.; Meiselman, H. L. Validity of telephoned diet recalls and records for assessment of individual intake. Am. J. Clin. Nutr. 36:12341242; 1982. 23. Ross, S.; Ross, J. G. Social facilitation of feeding behavior in dogs. I. Group and solitary feeding. J. Genet. Psychol. 74:97-108; 1949.
0031-9384/92$5.00 + .00 Copyright © 1992PergamonPressLtd.
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Social Facilitation of Eating: Effects of Social Instruction on Food Intake M A R I E R E D D A N D J O H N M. DE C A S T R O 1
Department of Psychology and Psychological Sciences Program, Georgia State University, Atlanta, GA 30303 Received 7 J u n e 1991 REDD, M. AND J. M. DE CASTRO. Socialfacilitation of eating: Effects of social instruction onfood intake. PHYSIOL BEHAV 52(4) 749-754, 1992.--To investigatewhether social influencescause increasesin eating behavior, thirty undergraduatepsychology students completed a diet diary for three 5-day periods. Subjects were instructed to either eat alone or eat with other people, actively eating with them for two of these periods. For the third period, subjects were instructed to eat as they normally would (with or without other people present). When instructed to eat with others present, subjects overall consumed more food, water, sodium, and alcohol than when they were instructed to eat alone. In the normal condition, food intake was 60% higher when the subjects ate with others present than when they ate alone. These results suggest that social facilitation has a causal influence on eating which increases food intake. Meals
Eating
Carbohydrate
Socialfacilitation
Sodium
SOCIAL conditions associated with food intake have a powerful effect on the amount of food ingested both in animals (2,15,16,20,21,23) and in humans (9). The number of people eating with the subject influences the amount of food eaten in a meal. This social influence is the best predictor of how much food an individual will consume, independent of when the individual eats, where the individual eats, whether the eating episode is a meal or a snack, and whether the eating episode occurs on a weekday or a weekend (6,8). As the number of people eating with an individual increases, the amount of food consumed increases incrementally as a power function. Subjects eat an average of 28% more when one other person is present at a meal than when they eat alone; with two people present, 41% more; with three people present, 53% more; and 53%, 71%, and 76% more when eating with four, five, or six people present, respectively (7). This results in a strong positive correlation between the number of people eating with an individual and the size of the meal ingested (9). People appear to eat more when other people are present because, in social meals, people tend to eat for a longer period of time (5). Previous correlational studies have shown that when other people are present at a meal, the amount of food consumed increases. Since these studies were correlational in nature, causation cannot be established. The present study attempted to address this issue by manipulating the social conditions involved in food intake. Subjects were asked to record their dietary intake for three periods of 5 days each, eating exclusively alone, eating exclusively with others actively eating with them, or eating as they normally
Alcohol
Humans
would (with or without others present). By manipulating only the social settings of the subjects' eating episodes, group influence effects may be clearly differentiated and compared. METHOD The methods are similar to those employed in a previous study ( 11 ).
Subjects Twelve male and 26 female undergraduate psychology students participated in this study. The subjects averaged 22.8 years (range 18-46) of age, 61.5 kilograms (range 46.4-87.8), and 1.63 meters (range 1.55-1.93). The subjects received research participation credits and a nutritional summary based on their reported food intake. Two males and six females were excused from this experiment due to either illness or inability to follow the procedures. The remaining l0 males and 20 females completed the study.
Procedure The subjects completed a questionnaire asking general demographic questions and questions regarding their health and eating behavior. The questionnaire asked for the names of two people the subject would eat with during the experiment who could be contacted later to verify the diary entries. The subjects also completed an informed consent form. During the initial meeting, subjects were told that the experiment dealt with nu-
Supported in part by Grant DK 39881 from the National Institute of Diabetes and Digestiveand Kidney Diseases and from a grant from the National Livestock and Meat Board. J Requests for reprints should be addressed to John M. de Castro, Department of Psychology, Georgia State University, Atlanta, GA 30303.
749
750 trition and eating behavior. They were asked if they were willing to complete a diet diary for 15 days in periods of 5 days each, with each period beginning on Wednesday and ending on Sunday. During these periods, they were instructed to eat as they normally would, to eat exclusively alone, or to eat exclusively with others present and eating with them. Subjects were strongly urged to adhere to the condition requirements, but in the event that the subject could not satis~ the condition's requirements, i.e., the subject had to eat with others when in the alone condition, they could do so, but note the episode in their diary. The subjects were also instructed to not skip a meal or snack if they could not comply with the conditions. The subjects were allowed to eat whatever and whenever they wanted. Only the social conditions were manipulated. The subjects recorded their food intake in a small pocketsized diet diary (8 × 18 cm). The food entries were recorded in great detail (whether fried, baked, etc., and whether cups, ounces, etc.). In addition to recording the food entries, the subjects reported the time and day of the eating bout, the number of people eating with the subject, and whether the eating episode was a meal or a snack. Premeal and postmeal seven-point Likert scales were used to rate the subject's hunger, depression, anxiety, thirst, the attractiveness of the meal, and the subject's comfort level of the people eating with the subject, and how important to the subject the people present were. Subjects completed a l-day trial diary, and met with the experimenter to ask questions, to correct any problems, and to check for accuracy. Subsequently, the subjects completed a 5day diary either eating alone, with people, or normally; the order of the conditions was determined randomly. After each diary was completed, the subject met with the experimenter on the following Monday to check for accuracy and to answer questions. Each diary was then verified by a person who had eaten with the individual during that interval. The people eating with the subject were asked if they remembered what the subject ate at a specific meal. If the person could not remember, then the experimenter prompted the person, for example, by asking if the subject had eaten beef or chicken. If they still could not answer, the experimenter asked if the subject had eaten a hamburger or a steak. To further check for accuracy, the subjects were asked about problems they may have had during the previous period, and whether they had recorded accurately. The subjects were told that regardless of their responses, they would still gain full research participation credits for the experiment. It was further emphasized that accuracy in recording would provide a much more accurate nutritional analysis. This procedure has been employed in other related studies (10),
Data Analy.~is A computer analysis of the nutrient composition of the recorded food was performed once the diaries were completed and verified. The experimenter coded the food items reported in the diaries using a computer file of over 3500 predetermined assigned codes. This computer file was created using the U.S. Department of Agriculture Handbooks number 6 and 456 of the Nutritive Value of American Foods. Complex food items were broken down into their component parts. For example, a grilled cheese sandwich was broken down into bread, cheese, and butter. In addition, water and other liquids were coded. Once the food items were entered, the computer summed the nutrient compositions of the food items recorded for each meal. The diet diaries were analyzed separately for each of the three conditions (normally, alone, and with others present). In addition, meals eaten during the normal condition were separated into those eaten with others present and those eaten alone.
REDD AND r)~! CASTRO Means were calculated for each individual in each condition and averaged to produce group means in each condition. Means were calculated for meal size (kcals), meal composition (carbohydrates, protein, fat, alcohol, and water), intermeal intervals, and premeal and postmeal stomach contents, and self-ratings (attractiveness of the meal, hunger, anxiety, depression, how comfortable the subject was with the people present, and importance of the people present). Stomach caloric content was estimated using a computer model. The reported intake was entered in the model and was estimated to empty from the stomach at a rate proportional to the square root of the caloric content of the stomach in little calories (S) per minute (11,17-19). S-S
5xS 5
The total caloric content of the stomach and the proportion of each macronutrient estimated to be present at the beginning and at the end of each meal was included in the subsequent analyses. Repeated measures ANOVAs were used to analyze the differences in the meal characteristics and the self-ratings among the three conditions. Repeated measures ANOVAs were also used to determine if the sequences of the conditions had any effect on food intake. If statistically significant differences were found, then the data were further analyzed using correlated ttests. Meal size, meal compositions (carbohydrates, fats, and proteins), number of people, intermeal intervals, and stomach contents and self-reported hunger, depression, and anxiety were intercorrelated using Pearson Product Moment Correlations. Correlations were calculated using all intermeal intervals except overnight fasts. Mean correlations across the groups were calculated using r to z transformed coefficients (3). RESULTS
Subject Compliance Subject compliance was checked by verifying at least two meals eaten with the two people listed by the subject. All the people contacted, without exception, verified that the diary entries for the meals reportedly eaten with them were accurate. In addition, subjects reported an average of 0.12 other people present per meal during the alone period, 1.7 others present during the with-people period, and 1.3 others present during the normal period, t(29) = 3.13, p < 0.01 ; with-people condition vs. normal condition. The subjects were instructed not to miss meals in order to comply with the instructions. In fact, the number of meals per day in each condition did not significantly differ, with 3.2, 3.1, and 3.0 meals per day reported for the Normal, With People, and Alone conditions, respectively.
Overall Intakes Table 1 shows the mean minimum and maximum nutrient compositions of the overall food intakes during the alone, normal, and with-people conditions. Differences in the overall intakes (kcals) among the three conditions were found, F(2, 56) = 3.35, p < 0.05. During both the normal and with-people conditions, overall intakes were significantly higher than in the alone condition, t(28) = 2.26, p < 0.05; t(28) = 2.08, p < 0.05, respectively. Differences in the overall intakes of fat during the three conditions were found, F(2, 56) = 3.15, p < 0.05. Fat intake was significantly higher in the normal condition than in the alone condition, t(28) = 2.15, p < 0.05. Alcohol intakes also
SOCIAL F A C I L I T A T I O N
751
TABLE 1 MEAN + SEM FOR DAILY NUTRIENT INTAKE Daily Nutrient Intake Nutrients kcal Carb (g) Fat (g) Protein (g) Sodium (mg) Calcium (mg) Phosph (mg) Vit B-6 (#mg) Vit B-I (mg) Vit A (IU) Magn (mg) Iron (mg) Ribofl (umg) Vit C (mg) Vit E (IU) Vit BI2 (#g) Foilc (#g) Alcohol (g) Cholesterol (mg) Grams Grams H20 Sugar (g) Zinc (rag)
Alone 1774.7 193.3 80.2 72.6 2473.8 836.4 1150.8 1265.6 18.6 4749.1 187.2 15.8 1653.3 71.5 13.5 4.5 164.6 2.0 312.5 1793.1 1472.5 96.6 9.0
Normal 116.0 13.8 6.0 4.7 10.7 80.5 80.5 157.9 1.6 614.3 14.4 3.3 152.3 9.0 1.9 1.1 25.2 0.9 28.9 121.5 104.8 8.3 0.7
2021.0 215.1 90.9 81.9 3174.3 826,6 1239.2 1474.4 21.9 4297.2 204.2 16.3 1725.9 78.6 15.1 4.5 157.8 5.6 341.3 1884.8 1523.7 93.3 11.9
With 134.7 17.2 6.6 5.6 223.6 63.0 77.2 171.0 1.6 461.1 14.3 1.8 146.5 8.3 2.0 0.5 18.5 2.0 34.2 142.4 118.3 9.7 1.3
1986.9 206.9 91.0 79.9 3194.6 972.1 1293.2 1270.6 20.3 4773.6 205.8 13.7 1757.6 88,7 15,1 4.7 188.2 6.0 365.4 1989.1 1625.6 100.7 11.4
112.7" 12.8 6.3* 9.9 206.4":~ 91.0 82.4i151.6 1.8 414.9 12.2 1.4 139.0 12.6 1.9 0.6 18.7 2.1 "t" 24.8 112.6J99.0t 8.8 1.3"
This table shows the amounts of nutrients eaten in each condition. Statistical significance was determined by ANOVAs and correlated t-tests. (g = grams, mg= milligrams, IU = international units, ug = micrograms). * Alone vs. normal p < 0.05. i Alone vs. with p < 0.05. Normal vs. with p < 0.05.
differed among the conditions, F(2, 56) = 2.35, p < 0.05. Alcohol intake was significantly higher during the with-people condition than during the alone condition, t(28) = 2.35, p < 0.05. Micronutrient compositions of the meals also differed, especially with respect to sodium, F(2, 56) = 7.91, p < 0.01, and zinc intakes, F(2, 56) = 3.56, p < 0.05. Sodium intake was significantly higher in both the normal condition and the withpeople condition than in the alone condition, t(28) = 4.08, p < 0.01; t(28) = 3.56, p < 0.01; respectively. Daily zinc intakes were significantly higher in the normal condition than in the alone condition, t(28) = 2.25, p < 0.05.
Water intake per meal also varied among the three conditions, F(2, 56) = 4.75, p < 0.05. Water intake was significantly higher in the with-people condition than in the normal condition, t(28)
Stomach C o n t ~ t Alcohol 7oo
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Meal Characteristics The subjects ate an average of 668 kcals per meal in the withpeople condition, 658 kcals in the normal condition, and 605 kcals in the alone condition (see Fig. 1). These differences in meal size were not statistically significant, However, when meal size was expressed by weight, a significant difference in the a m o u n t of grams eaten per meal was evident among the conditions, F(2, 56) = 4.70, p < 0.05. Meals eaten during the withpeople condition were larger than meals eaten in the normal condition, t(28) = 2.76, p < 0.05, and the alone condition, t(28) = 2.32, p < 0.05. Overall food intakes for males were significantly different from females' overall food intakes, F(I, 26) = 6.18, p < 0.05. Males ate more than females overall, t(26) = 2.69, p < 0.05, yet the intakes did not significantly differ in the different social conditions.
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752
REDD AND DE CASTRO Postmell Self-rlttlngs
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FIG. 2. Premeal and postmeal self-ratings. This figure shows the mean premeal and postmeal self-ratings for attractiveness of the meal, hunger, thirst, elation, anxiety, importance of the other people present, and how comfortable the subject is with the other people present. The first bar in each group represents the normal condition; the second bar the alone condition, and the last bar the with people condition. For the importance and comfort self-ratings, the first bar represents the normal condition and the last bar represents the with people condition for each self-rating (the alone condition was not included).
= 2.92. p < 0.05, and the alone condition, t(28) = 2.23, p < 0.05. Results for the premeal self-rating scales for the three conditions are presented in Fig. 2. Both premeal and postmeal ratings of depression differed, F(2, 56) = 3.37, p < 0.05; F(2, 56) = 3.44, p < 0.05, respectively. Subjects reported greater depression (decreased elation) in the alone condition than in the with-people condition, both before the meal, t(28) = 2.66, p < 0.05, and after the meals, t(28) = 2.82, p < 0.05. Subjects reported a higher importance of people eating with them in the with-people condition both before the meal, t(18) = 2.94, p < 0.01, and after the meal, t(18) = 3.26, p < 0.01, than in the normal condition. Subjects also rated the attractiveness of the meal differently among the conditions, F(2, 56) = 5.49, p < 0.05. The meals in the with-people condition were rated as more attractive than meals in the alone condition, t(28) = 3.43, p < 0.05.
Correlational Analysis The correlations between meal size and estimated premeal stomach contents were not significantly different among the conditions. Therefore, changing the social conditions in which subjects ate did not change the influence of estimated premeal stomach contents on meal size. Correlations of the n u m b e r of people present and meal size were strong and positive in both conditions and were significantly different between the normal and with-people conditions, t(28) = 3.05, p < 0.05 (see Fig. 3). (Since subjects ate alone in the alone condition, the correlations between the n u m b e r of people and meal size were not considered relevant.) This difference may
be due to frequent inclusion of alone (zero people present) in the normal condition, while only infrequent in the with-people condition. Since all other correlations were not different, this suggests that the manipulation of the social conditions did not disrupt other regulatory mechanisms.
Analysis of the Normal Condition Approximately 2~ of the meals eaten during the normal condition had other people present averaging 2.06 people per meal. When separating the normal condition into meals eaten with people and meals eaten alone, meal size was found to be significantly larger when meals were eaten with others present (771.5 kcals versus 485.4 kcals), t(17) = 5.62, p < 0.01. Carbohydrate, t(17) = 3.61, p < 0.01, protein, t(17) = 5.13, p < 0.01, fat, t(17) = 6.43, p < 0.01, and water, t(17) = 5.40, p < 0.01, ingestion were also significantly higher when eating with others than when eating alone. Although estimated premeal stomach contents did not differ significantly, the larger social meal sizes result in higher estimated postmeal stomach contents with others present, t(17) = 5.60, p < 0.01. The percentages of carbohydrates in the meal and the percentages of fat in the meals were significantly different such that when eating alone, subjects ate a higher percentage of carbohydrates in the meal, t(9) = 2.69, p < 0.05, and when eating with others, subjects ate a higher percentage of fat, t(17) = 2.63, p < 0.01. Although postmeal intervals were not significantly different, premeal intervals were significantly longer when the subjects ate with others, t(7) = 3.88, p < 0.05. Duration of intake was longer when subjects ate with others than when they ate alone, t(17) =
SOCIAL FACILITATION
753
~WITH ~ALONE
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FIG. 3. Correlations between number of people present, premeal hunger ratings, meal time, and pre-lMI with meal size. The solid bars represent the normal condition. The open bars represent the alone condition. The hatched bars represent the with people condition. The first group of bars show the correlation between the number of people present and meal size. The second group of bars show the premeal hunger rating and the meal size correlation. The third group of bars show the correlation of the meal time and meal size.The fourth group of bars show the correlation of premeal intermeal interval and meal size. *p < 0.05.
3.50, p < 0.01. The rate of the intake was not significantly different between the meals eaten with others or eaten alone. DISCUSSION Previous observational research has shown that subjects consume more food when they eat in a group than when they eat alone. The present study manipulated the social conditions at meals to determine if having other people present causes an increase in food intake. The results showed that the subjects consumed more total food energy and increased their meal sizes when following instructions to eat with other people present than to eat alone. During the period when subjects were instructed to eat as they normally would, they consumed larger meals when they ate with others than when alone. These results suggest that the presence of other people is, indeed, the cause of heightened intake. The data suggest that the presence of other people not only increases the intake overall but also alters the composition of the meal. In particular, the proportion of calories accounted for by fat was higher in the normal condition than in the alone condition. Also, the percentage of fat consumed in the normal condition was significantly higher when the subjects ate with others. This suggests that the meals eaten alone were proportionally lower in fat regardless of whether they occurred by instruction or not.
Alcohol intake was strikingly different in the alone and withpeople conditions. Alcohol intake was much larger in the withpeople condition than in the alone condition. This social facilitation of alcohol intake also has been previously shown in an observational study (8,12). The present study, because of its manipulative nature, demonstrates a causal connection between the presence of other people and the intake of alcohol. Sodium intake was higher in both the normal and with-people conditions than in the alone condition. This increase in sodium intake may be responsible for the increase in water intake that occurred in the with-people condition. Increasing sodium concentrations in the body requires increasing water concentrations and, thus, promotes water intake to maintain homeostasis in the body (14). The increased water intake may simply be due to the higher amounts of solids ingested in these meals (4). The normal condition data compares favorably with data from a similar study. In the de Castro and de Castro study, subjects completed a 7-day diet diary with no manipulations (14). The subjects ate as they normally would, de Castro and de Castro reported that larger meals were ingested when subjects ate with other people present. When the meals in the normal condition in the present study were separated into meals eaten alone and meals eaten with others, larger meals were ingested when eating with others present, de Castro and de Castro found that these social meals were 44% larger than meals eaten alone. These results compare favorably with the normal condition in the present study where meals eaten with others were 60% larger than meals eaten alone. All the amounts of macronutrients ingested were larger when people were present at a meal in both the normal condition and the de Castro and de Castro study. In the present study, the with-people condition was very similar to the normal condition, but dissimilar to the alone condition. These similarities may be due to the number of people eating with the subject; in the with-people condition, an average of 1.7 people ate with the subject, while an average of 1.3 people ate with others in the normal condition. Other research reports that the amount of food ingested is a power function of the number of the people present (7). The log of the meal size increases in a linear fashion as the log of the number of people present at a meal increases. An increase of people present at a meal from no other people present (0) to 1.3 people present at a meal would be expected to show a large effect on meal size, but an increase from 1.3 to 1.7 people present at a meal would show only a small, perhaps undetectable, effect. Meals eaten during the normal and with-people conditions were only slightly (10%) larger than meals eaten in the alone condition. However, in the normal condition, meals eaten with people present were 60% larger than those eaten alone. Meals in the alone condition were 20% larger than meals eaten alone in the normal condition, and meals in the normal condition that were eaten with others present were 13% larger than meals eaten in the with-people condition. These differences in food intake between the conditions suggest that either the subjects may be compensating, i.e., decreasing meal sizes in the with-people condition and increasing meal sizes in the alone condition, thus bringing overall intake to more normal levels, or that by separating normal meals eaten alone or with people, the impact of social facilitation on eating may be overestimated. Ratings of subjective states indicated that subjects were not hungrier when they ate with other people present. Premeal and postmeal subjective hunger ratings were not significantlydifferent among the three conditions. Hence, larger meals eaten in social conditions occurred under equivalent conditions of subjective hunger. In contrast, subjects reported greater depression when eating in the alone condition than they reported in the withpeople condition. In addition, in the with-people condition, the
754
R E D D A N D 19~: C A S T R O
persons eating with the subjects were rated as more important than in the normal condition. Perhaps the instruction caused the importance to increase; subjects rated their eating companions as more important because of the necessity to eat with someone to comply with the instructions. The present study's results depend on the veracity of the subjects' recordings, lfthe diaries were not truthfially and faithfully completed, then the results would also be inaccurate. However, many factors suggest that the diaries were completed accurately and truthfully. Verification of the diary entries helped to ensure accurate recording. At least two meals from each diary were verified by at least one of the persons listed on the questionnaire. In addition to verification of the meals by people eating with the subject, the subjects were asked if they completed the diaries accurately and they were told there would be no penalty for indicating inaccuracy. All the subjects reported that the diaries were completed truthfully and accurately. Furthermore, the subjects seemed to anticipate the nutritional analyses and were aware that the analyses were dependent on their accuracy of recording. Other problems of the diet diary method may be more intrinsic, i.e., the subjects may unknowingly complete the diary inaccurately. These problems may include inaccurate recall, inaccurate recording, and modified eating behavior due to recording (1,13,22). In the present study, subjects recorded food items in terms they understood, recorded food intake immediately after eating, and met with the experimenter weekly to discuss any problems. Recording of intake occurred over an extended period of time (15 days) to control for the possible altering of behavior. The order of the conditions was randomly determined for each subject, and the subjects acted as their own controls by completing a diary for a 5-day period eating as they normally would. Analysis of the sequence of the orders showed that the order of the conditions did not affect meal size.
The data further suggest that the subjects accurately completed the diaries. Subjects recorded meals that did not comply with the experimental instructions. Subjects reported meals eaten with others present in the alone condition. The number of meals did not differ significantly among the conditions. The subjects" recordings of meals that did not comply with the instructions and the similar meal frequencies (number of meals) in the conditions suggest that all the meals were recorded. Additionally, if the subjects were lying and not recording that the people were present during the alone condition or adding fictitious companions to the with-people condition, then the differences between the conditions would be reduced and an effect of the manipulation would be expected. Hence, noncompliance would be expected to decrease condition effects, not produce them, The results of the present study strongly suggest that the presence of other people at meals causes an increase in food intake at meals. Since only one factor was manipulated (the social setting with regard to people present), the differences can be attributed to the social effect others have on meal size. These results have implications for further research. Eating alone may be an effective dieting strategy if people present at a meal cause an increased meal size which results in higher overall intakes. A difference of 212 kcals were consumed per day between the with-people condition (1987 kcals) and the alone condition ( 1775 kcals). Over periods of months, this could result in a significant weight loss. Eating alone also causes a decrease in fat and salt consumption. Both these factors have been linked to high cholesterol and heart disease. A reduction of these factors have been suggested to reduce the risk of heart disease. Additionally, geriatric patients may benefit from eating with others as well, such as eating in a community dining room. This might promote a decrease in depression levels and an increase in the amount of food consumed.
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