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ORIGINAL RESEARCH COMMUNICATION |
1 From the Weight and Eating Disorders Program, University of Pennsylvania School of Medicine, Philadelphia (MSF); the New York Obesity Research Center, St LukesRoosevelt Hospital Center, Columbia University College of Physicians & Surgeons, New York (KLK, AP, SM, MAJ, JC, and SBH); the Department of Pediatrics, University of Colorado Health Sciences Center, Boulder, CO (SLJ); the Pediatric Unit, University of Verona, Verona, Italy (AP); the Weill Medical College, Cornell University, Ithaca, NY (PEM); and the Department of Biostatistics and Clinical Nutrition Research Center, University of Alabama at Birmingham (DBA).
2 Supported by NIH grant K08MH01530 (MSF), NIH grant P30DK056336 (DBA), and a Pilot & Feasibility Grant from the New York Obesity Research Center (MSF). 3 Address reprint requests to MS Faith, Weight and Eating Disorders Program, University of Pennsylvania School of Medicine, 3535 Market Street, 3rd Floor, Philadelphia, PA 19104-3309. E-mail: mfaith{at}mail.med.upenn.edu.
| ABSTRACT |
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Objective: We tested whether childrens total energy intake and macronutrient intake and their ability to compensate for earlier energy intake were associated with sociodemographic variables and anthropometric indexes. We also tested whether these behavioral traits aggregate among siblings.
Design: Thirty-two sibling pairs aged 37 y consumed a multi-item lunch preceded by a low-energy (12.55 kJ) or high-energy (627.60 kJ) preload drink. Mixed-models regression tested the associations between childrens energy intake, demographic variables, and anthropometric measures. An intraclass correlation coefficient quantified the family correlation of the measures of childrens eating.
Results: Children consumed significantly more total energy after consuming the low-energy preload (
± SD: 2237.39 ± 1176.45 kJ) than after consuming the high-energy preload (1601.18 ± 930.65 kJ). Compensation ability was unrelated to the childrens age, sex, or ethnicity. Total energy and macronutrient intake, but not compensation propensity, were associated among siblings.
Conclusions: The familial association of total energy and macronutrient intakes, independent of anthropometric measures, suggests genetic or home environmental influences specific to these behaviors. Short-term energy compensation, although very accurate within this sample, showed no significant familial correlation.
Key Words: Feeding behavior eating childhood obesity behavior genetics energy compensation
| INTRODUCTION |
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The average child will adjust energy intake at multi-item ad libitum meals in proportion to the amount of energy consumed
30 min earlier; this phenomenon is called "compensation" (3). However, there is considerable interchild variation in compensation ability: certain children are more sensitive to the foods that they consume than are others (4). Poorer energy compensation has been associated with elevated body weight in girls (4), and other studies have established associations between total energy intake, fat intake, and overweight among children (5, 6). Understanding the sources of individual differences in childrens eating patterns might guide novel interventions for healthier eating and might lead to better weight management (7).
One potential source of interchild differences in eating behavior is genetic variation. Suggestive evidence comes from the literature on animal studies (8), although less research has been conducted in humans. Most twin studies have been conducted among adults whose food intakes were measured by dietary records, and these studies support a genetic influence on food intake (9, 10). Using an adult twin design, we found that genetic variation accounted for
33% of the variation in participants total ad libitum energy intakes in the laboratory (11). These collective findings lead us to suspect that genetic variations may contribute to differences in childrens eating behavior. This question has not yet been addressed through the study of related children.
The sibling design of the present study can address one of the most basic questions concerning variation in childrens eating behaviornamely, its family correlation, or "familiality." Estimating familiality is traditionally the first step in documenting potential genetic bases for a trait among humans, and, in classic genetic doctrine, it precedes the formal partitioning of genetic and environmental influences on traits (12). We report here the results of a study of the familiality of childrens eating behavior.
The goal of this study was to elucidate the sources of variation in young childrens energy compensation tendency, total energy intake, and macronutrient intake. Our specific aims were to test the compensation phenomenon among an ethnically diverse sample of young siblings, to test whether variations in child eating measures were associated with sociodemographic variables and anthropometric indexes, and to test the familiality of energy compensation, total energy intake, and macronutrient intake. We predicted that children would eat more after the low-energy preload than after the high-energy preload, that poorer compensation and increased total energy intake would be associated with elevated body fat indexes, and that eating measures would correlate among siblings. We did not have a priori hypotheses concerning the sociodemographic variables or their associations with childrens eating outcomes.
| SUBJECTS AND METHODS |
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3 y apart in age) who were free of food allergies that would preclude participation. No restrictions were made with respect to child or parent weight status. Each laboratory visit lasted
90 min and occurred around the lunch hour (ie,
11301300). We instructed families not to eat for at least 2 h before coming to the laboratory. As in previous research in children (4), families made 2 visits to the laboratory to consume a low-energy preload beverage and a high-energy preload beverage, respectively. The preload beverages were presented in a randomized order, such that children were assigned to one of 3 conditions. In the low/low and high/high condition, both siblings received low-energy preloads on visit 1 and high-energy preloads on visit 2; in the high/high and low/low condition, both siblings received high-energy preloads on visit 1 and low-energy preloads on visit 2; and in the low/high and high/low condition, one member of a sibling pair received a low-energy preload and the other sibling received a high-energy preload on visit 1, and the order of the preloads was switched for visit 2. This study received full approval from the Institutional Review Board of St LukesRoosevelt Hospital.
Visit 1
Families were greeted by staff and acclimated to the feeding laboratory. After completing the informed consent form, mothers were provided a questionnaire to be completed while children consumed the liquid preload and lunch test meals. Mothers sat within 5 ft of their children in the same room. The questionnaire packet included demographic items and items concerning maternal eating attitudes and feeding styles.
Approximately 15 min after arrival, siblings were seated at a round table where they were served either a low-energy or high-energy preload fruit drink. Recipes for low- and high-energy preloads were adapted from Johnson and Birch (4). These cherry-flavored carbohydrate liquid preloads were matched for volume, mass (173 g), and sensory properties and were served in clear plastic cups with straws through the lids. All drinks were served at room temperature. Children were verbally encouraged to finish the entire preload drink within a few minutes, drinking until they heard a "slurpy sound" in the cup when they sucked on the straw. The fundamental difference between the 2 preloads was the total energy content, which was achieved by serving either standard cherry Kool-Aid (Kraft General Foods, White Plains, NY) that we supplemented with maltodextran (high-energy preload) or Kool-Aid manufactured with aspartame artificial sweetener (low-energy preload). The high- and low-energy preloads contained 628 kJ (150 kcal) and 12.5 kJ (3 kcal), respectively.
After consuming the preload, siblings were allowed a 25-min interim play period, during which the staff postweighed the plastic cups for determination of preload consumption (ie, weight of the cup plus drink before consumption minus weight of the cup plus drink after consumption). Children were then invited to sit at the table for the ad libitum lunch. They were served a multi-item lunch that included macaroni and cheese, canned string beans, string cheese, graham crackers, green grapes, baby carrots, and whole milk (4). These items, which provided 3353 kJ of energy (
800 kcal), are summarized in Table 1
. This corresponds to
5065% of the total daily energy requirements for 4- to 7-y-old children (13). Research assistants read stories to the children while they ate lunch. Children were free to consume the foods they wished and could ask for additional servings. Research assistants were instructed to ask children if they wanted more of a particular food once they finished the amount on their plate. Siblings were instructed not to share their foods with one another and could not eat off each others plates. We let siblings sit together to enhance their comfort and to simulate the more typical conditions under which they eat at home. Mothers sat off to the side during these lunch meals, and interactions with their children were discouraged.
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Visit 2
Visit 2 was identical to the first visit with a few exceptions. First, parents were not given questionnaires to complete, but they had access to a variety of magazines to read. Second, each child received the preload drink that they had not consumed on the first visit. Childrens weight, height, and waist circumference measures were taken on this visit.
Measurements
Energy compensation tendency
Childrens energy compensation tendency was quantified by a compensation index called COMPX (4), which measures the difference in energy intake between the 2 multi-item test meals divided by the difference in preload consumption:
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Total energy intake
We computed the mean total energy intake across the 2 test meal lunches, by using the manufacturers information and food labels. Energy intake from the preloads was not included in this calculation. We used a similar calculation in a previous preloading study with adults (11).
Macronutrient intake
We computed the mean percentage intake of fat, carbohydrate, and protein across the 2 test meal lunches, by using the manufacturers information and food labels.
Anthropometric indexes
Childrens weight and height were measured with the use of a digital scale and a stadiometer, respectively, and converted to body mass index (BMI; in kg/m2). BMI values, in turn, were converted to BMI z scores and percentiles according to the Centers for Disease Control and Prevention growth charts (14). Child waist circumference was measured in the standing position from midway between the last rib and the iliac crest (15). Because of their young ages, a number of children would not allow us to collect these measures. These children either were intimidated by this particular aspect of the protocol or were tired, or they equated the equipment with a doctors office. We honored these requests. BMI and waist circumference were calculated on 50 and 34 children, respectively, representing 78% and 53% of the total sample.
Mothers were asked to self-report their weights and heights, which we converted to BMI. BMI was calculated for the 38 mothers (59% of the sample) who reported this information.
Sociodemographic measures
The following information was obtained from the mothers: their educational level (
high school, college, or graduate or professional school), marital status (never married, married, or separated), and current employment status (employed or unemployed). Child sex was coded as 0 = boys and 1 = girls.
Statistical analysis
Descriptive data are presented as means ± SDs. Associations among child eating measures, demographic variables, child anthropometric indexes, and maternal BMI were tested by mixed-models regression. These analyses tested the associations among variables while adjusting for the dependency in the data structure due to family membership (16, 17). Specifically, we regressed the given outcome variable onto individual predictors at the same time as we controlled for family membershipie, the random-effects variable.
Familial aggregation among eating measures was estimated by an intraclass correlation coefficient (
; 18), which quantifies variation between families relative to total variation. Higher
scores (approaching 1) imply a stronger familial resemblance. For 5 eating outcomes (COMPX, total energy intake, percentage fat intake, percentage carbohydrate intake, and percentage protein intake), we computed
according to 3 categories of statistical adjustment. First, we analyzed unadjusted food intake scores. Second, we adjusted scores for child age, sex, and ethnicity (dummy-coded 0 = whites and 1 = nonwhites). Third, we adjusted scores for child age, sex, ethnicity, and BMI. Statistical adjustments were achieved through multiple regression (19) to ensure that any familial association for eating measures was not simply due to common sibling demographics or anthropometric indexes. Data were analyzed by using SPSS software (version 11; SPSS Inc, Chicago).
| RESULTS |
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Associations between maternal BMI and childrens anthropometric indexes
Maternal BMI was not associated with childrens BMI (P = 0.62), BMI z score (P = 0.88), or waist circumference (P = 0.46) when tested by mixed-models regression.
Familiality of childrens eating measures
There was significant familial aggregation for total energy intake (
= 0.39, P < 0.05), percentage fat intake (
= 0.66, P < 0.001), percentage carbohydrate intake (
= 0.67, P < 0.001), and percentage protein intake (
= 0.61, P < 0.001), but not for COMPX scores (
= 0.11, P = 0.29; Table 4
). The similarity of within-family and between-families total energy intakes is depicted in Figure 2
. These findings remained the same when eating measures were adjusted for childrens age, sex, ethnicity, and BMI (Table 4
). Thus, the familial resemblance for food intake was not entirely attributable to similarities in child anthropometric indexes.
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| DISCUSSION |
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That genetic variations may partially underlie the familial correlation of child energy intake is consistent with twin studies that suggest a heritable component to the intake of specific foods and the preference for specific tastes. There is evidence for genetic influences on the reported intake of spicy foods (20), sweet foods (21), salty foods (22), fruit and fruit juices (21), vegetables (23), and meats and dairy products (23). Indeed, positional cloning of the genes for taste sensitivity to phenylthiocarbamide (PTC) was recently reported (24). Reviews of this literature are provided elsewhere (25, 26). Also noteworthy are studies documenting genetic bases for energy intakes and preferences for sweet foods in animals (8, 27). At the same time, research by Birch and Davison (28), Rozin (29), and others suggests the importance of sociocultural and familial influences on food preferences and aversions. These publications set the backdrop for future studies that will test genetic and environmental influences on childrens eating behavior.
Increased total energy intake was associated with increased waist circumference. This association may have important health implications, given the metabolic health risks associated with abdominal body fat (30, 31). A growing literature has linked obesity to certain metabolic complications in children, and waist circumference cutoffs have been established to delineate elevated cardiovascular disease risk factors in young children (32). The role of dietary practices in the development of this relation remains unknown, but it may be important to an understanding of the development of metabolic syndrome during growth and to the prevention of obesity-related comorbidities (33).
With respect to energy compensation, the participating children were very sensitive to the preload manipulation, compensating on average for 104% of the energy difference. However, individual variations in this trait were quite high. This finding replicates past studies with children (3, 4) and also provides evidence for short-term energy compensation in an ethnically mixed sample. COMPX scores were unrelated to age, sex, or ethnicity, which suggests that an energy compensation ability may not be unique to one demographic subgroup. That this protocol was implemented among siblings further suggests the feasibility of this trait as a focus of behavior genetics studies.
Curiously, COMPX scores were unrelated to childrens anthropometric indexes. This null finding may be due to our assessment of short-term rather than long-term energy compensation (34) or to our sample size. The sample size issue is noteworthy, because our power to detect small associations was limited. For example, to detect a familial correlation as small as
= 0.20 (95% CI: 0.05, 0.35) in the population, 267 families would be necessary to have 80% power at
= 0.05 (35). The present sample size was not powered to detect such effects.
Another reason for the lack of familial correlation for COMPX might be model misspecificationthat is, our model may not have included other pertinent variables that were necessary for the expected association to be observed. Birch and Fisher (36) found that poorer compensation was predictive of elevated body weight in girls, but only as part of a larger model that included maternal perceptions of daughter weight, feeding restriction, and other variables. Finally, the finding may reflect a genuine null association, given the paucity of data on this topic. The relation between short- and long-term energy compensation and childrens weight status is ideally tested via longitudinal design.
COMPX scores showed no significant familial association, which suggests that energy compensation may be influenced primarily by nonshared or random environmental influences. The exact identity of such influences is unknown, but the literature may provide material for speculation about several. Two putative factors are parental feeding control (4) and the intake of high-sugar beverages (37, 38), which may disrupt childrens energy compensation ability and thereby promote obesity.
Maternal BMI was unrelated to childrens anthropometric measurements in the present sample, a finding that is inconsistent with prior research (39). This null finding most likely relates to our relatively small sample size for these analyses. It is noteworthy that maternal BMI was marginally associated with childhood BMI (P = 0.07) and childrens BMI z scores (P = 0.10) in ordinary regression analyses that did not adjust for family membership (unreported analyses).
These results must be interpreted in light of study limitations. First, our sample size was insufficient to conduct in-depth ethnicity- and sex-specific analyses. Second, this study was not designed to disentangle genetic and environmental influences on eating traits, an issue that we are currently investigating (40). Third, we did not test specific environmental variables that may be correlated with measures of childrens energy intake. Future studies will address this possibility. Fourth, the findings cannot be generalized beyond the specific compensation protocol, preloads, and lunch provisions used here. We chose the specific carbohydrate liquid preload on the basis of previous research with children (4). Fifth, the present study examined only the regulation of short-term energy intake, and therefore findings may not be generalizable to longer-term regulation (34). Sixth, we did not ascertain body-composition measures in this sample. Seventh, the study examined familial correlations only among siblings. Although these data constitute an acceptable test of familiality (12), it would be more informative to have data on parent-child associations as well. Such analyses have been conducted for reported food intake (41). Eighth, this study did not include measures of habitual physical activity levels (42), which may introduce variation in short-term regulation.
In sum, variations in childrens ad libitum total and macronutrient intakes can be explained in part by family membership. The extent to which genes or shared environmental factors induce this familial association should be tested. A burgeoning literature has begun to document the molecular bases of human eating behavior (43, 44), and other investigators are exploring the unfolding nature of feeding dynamics in parents and their children (45). The merger of laboratory feeding protocols with genetics designs may provide new insights into the development of childrens eating practices and childhood obesity.
| ACKNOWLEDGMENTS |
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| REFERENCES |
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