The Behavioral Environment

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Dietary intake and the energy spent on activity represent the only discretionary components of energy intake and expenditure. Over the past 30 years, important changes have occurred in family eating patterns, including greater consumption of fast food, carbonated beverages, and preprepared meals. At the same time, some children are less physically active because of increased use of cars, concern for neighborhood safety, and decreased opportunity for physical activity at school or on the way to school. Children's time spent watching television and playing video games has increased. Both the food intake and physical activity of young children are strongly influenced by their parents. Although the literature that examines the effect of the potential risk factors for overweight in children is growing, as yet there are few longitudinal studies of adequate duration and rigor that have identified causal factors.

1 Breastfeeding

Breastfeeding may protect against subsequent overweight in childhood. Two review papers on this topic published in 1999 and in 2001 both failed to find conclusive evidence of a significant positive effect of breastfeeding (79,94). Nonetheless, well-designed studies with large samples that controlled for potential confounders have demonstrated a protective effect of breastfeeding against overweight in childhood. Von Kries et al. (95) found that infants who were ever breastfed were 0.75 (95% Confidence Interval [CI] = 0.57-0.98) times as likely to be overweight (BMI >90th centile German schoolchildren) at 5-6 years of age. Gillman et al. (96) found that infants who were exclusively or mostly breastfed were 0.78 (95% CI = 0.66-0.81) times as likely to be overweight (BMI >95th centile NCHS/CDC reference) at 9-14 years of age. Both studies found a dose-response effect: breastfeeding of at least 6 or 7 months was more protective than <3 months. Although the Hediger et al. study (NHANES III) found an insignificant protective effect (odds ratio for mostly breastfeeding was 0.84, 95% CI = 0.62-1.13) (97), the small sample size of overweight children in this study may have influenced their ability to document a protective effect. Only ~300 of ~2700 children were overweight by 3-5 years of age (97).

2 Dietary Practice

The behavior and practices of parents strongly influences the dietary intake of their children. During childhood, parental intake of carbohydrate, fat, and energy appears to account for 23-97% of the variance in children's intakes of these nutrients (98). However, family resemblances in nutrient intake may reflect familial resemblances only in body size, since we do not know from this study whether actual BMIs were comparable. This possibility is strengthened by the observation that the relationship between the preferences of parents and children for specific sets of foods does not differ substantially from the relationship between the preferences of children and unrelated adults (99). Resemblances in nutrient intake, when they occur, appear to reflect a common environment rather than a genetically mediated preference for macronutrients (100).

The family environment has the potential to promote healthy eating. For example, children who eat meals with their family consume more fruits and vegetables, have a more nutritionally dense diet with less of their energy intake derived from fat, and drink fewer carbonated and sugared beverages than do children who do not eat with their families (101). A recent summary of the intakes of youth ages 2-19 years, based on the U.S. Department of Agriculture's 1989-1991 Continuing Surveys of Food Intakes by Individuals, documented that American children have a widespread need for improvement in their diets (102). Indeed, the authors found that only 1% of youth met their recommended intake of nutrients, and membership in this small group was associated with excess intake, especially of fat (102). Daily consumption of three meals of approximately equal energy content may be another potential strategy to reduce adiposity in children (103). Further evidence that food patterns may contribute to overweight is the association between incident overweight and the consumption of sugar-sweetened drinks in school children (104). Interestingly, the available trend data on energy intake among children suggest that it has not changed over ~20 years: total energy intake did not change between 1973 and 1994 among 10-year-old children in the Bogalusa Heart Study (105).

3 Child Feeding Practices

Children appear quite capable of self-regulating their dietary intake in unsupervised settings. Their meal-to-meal variation in energy intake is substantial, but variation in day-to-day energy intake is considerably lower (106). Certain parent-child interactions at feeding may disrupt the ability of the child to regulate intake; the more parents encourage the consumption of certain foods, the less likely children are to eat them (107). Conversely, restricting access to certain foods appears to encourage their consumption when children have access to them (107). In a setting in which the ability of children to adjust their food intake in response to the caloric density of the diet was measured, children whose mothers were more controlling of their food intake were less capable of self-regulation of food intake, and these children had greater body fat stores (108). However, whether the lack of self-regulation is a cause or a consequence of the mother's control remains uncertain. At least two longitudinal studies of children suggest that parental control of child intake is not associated with energy intake and the development of overweight (109,110).

Interestingly, lack of knowledge about a child's intake is also associated with impairment of self-regulation. As a recent Danish study showed (111), the risk of subsequent overweight among 9- to 10-year-old children was not increased by their frequency of consuming sweets or when the mother accepted the child's consumption of sweets, but it was increased significantly if the mother lacked knowledge about her offspring's sweet-eating habits independent of their degree of fatness in childhood, their gender, and their social background.

Parental neglect has predicted overweight in young adulthood independent of gender, age, socioeconomic status, and childhood BMI (112). Additional observations have suggested that psychosocial stress is associated with rapid rates of weight gain (113,114). The greater effects of stress on weight gain in girls may emphasize both their biologic susceptibility to obesity in early puberty, as well as the adverse social effects that obesity imposes on them. In the studies cited, the retrospective collection of data represents a potential source of bias. In addition, these observations do not eliminate the possibility that rapid weight gain caused an increase in psychosocial problems rather than the reverse.

4 Physical Activity and Sedentary Behavior

Both activity and inactivity appear to affect the risk of obesity and its complications in childhood and adolescence. In preschoolers, for example, the energy spent on activity appears inversely related to fatness (115). Whether decreased nonbasal energy expenditure on physical activity increases the risk for developing childhood or adolescent overweight has not been clearly established. As shown in Table 4, the proportion of energy spent on activity, expressed as a ratio of total energy expenditure, appears to increase from infancy and early childhood to adolescence. One explanation for this intuitively unlikely result is that lower nonbasal energy expenditure of early childhood may reflect increased time spent sleeping rather than a decrease in the energy spent on activity.

Not surprisingly, the prevalence of physical activity and sedentary behavior, as well as factors associated with activity levels and overweight in children and adolescents, has varied by study. Unfortunately, the lack of consistency in measures or categories of physical activity and sedentary activity and differences in the samples or populations studied make comparing them difficult. In general, physical activity among children appears to have decreased significantly. For example, research in the United Kingdom among 0- to 14-year-old children found that between 1985 and 1992 the average distance walked annually decreased by 20% and the distance cycled by 26%, while the distance traveled by car increased 40% (121). In their review of studies, Parsons et al. found no association between activity before children are old enough to walk and later fatness; no consistent association was found between activity after children could walk and subsequent fatness (79). Analyses of the NHANES III data found that 80% of 8- to 16-year-old children participated in three or more episodes of vigorous activity each week; rates were lower in non-Hispanic black and Mexican-American girls (122). The Youth Risk Behavior Survey for 1999 and the National Longitudinal Study of Adolescent Health data for 1996 found that approx-

Table 4 Ratio (SD) of Mean Energy Expenditure of Activity to Mean Resting Energy Expenditure for Children and Adolescents. Data Expressed as the Ratio of Total Energy Expenditure (kcal/day)/Resting or Sleeping Energy Expenditure (kcal/day) Metabolic Ratea

Age group (Ref.)

Males

Combinedb

Females

Infants (116)

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