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Health Education Research Advance Access originally published online on July 31, 2006
Health Education Research 2007 22(2):227-237; doi:10.1093/her/cyl066
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© The Author 2006. Published by Oxford University Press. All rights reserved. For permissions, please email: journals.permissions@oxfordjournals.org

Associations of social–environmental and individual-level factors with adolescent soft drink consumption: results from the SMILE study

Gert-Jan de Bruijn1,2,*, Stef P. J. Kremers1, Hein de Vries1, Willem van Mechelen3 and Johannes Brug2

1 Department of Health Education and Health Promotion, University of Maastricht, PO Box 616, NL-6200 MD Maastricht, The Netherlands
2 Department of Public Health, Erasmus University Medical Centre, Rotterdam, The Netherlands
3 Department of Public and Occupational Health and Institute for Research in Extramural Medicine, VU University Medical Centre, Amsterdam, The Netherlands

* Correspondence to: G. J. de Bruijn. E-mail: g.debruijn{at}erasmusmc.nl


   Abstract
Top
 Abstract
Introduction
 Methods
 Results
 Discussion
 Conflict of interest statement
 Acknowledgements
 References
 
Adolescent obesity is positively associated with soft drink consumption. We investigated the association of social–environmental and individual-level factors with soft drink consumption in a Dutch adolescent sample. Data were gathered in a longitudinal Dutch adolescent sample (n = 208, 62% girls). Soft drink consumption, social cognitions from the Theory of Planned Behaviour and parenting practices towards limited soft drink intake, and Big Five personality dimensions were assessed. Data were analyzed using three-step linear regression analyses. Effect sizes were used as the informational source for the explanatory value of the model. Interaction terms were computed to test the individual-environment interaction. Attitude and subjective norm were significantly associated with soft drink consumption. When controlling for social cognitions, the distal variables parenting practices and the personality dimension Agreeableness remained significantly associated with soft drink consumption. Agreeableness moderated the association of parenting practices with adolescent soft drink consumption. Standardized regression coefficients ranged from 0.16 to 0.24 and explained 14% of the variance in soft drink consumption, indicating a medium effect size. Stricter parenting practices were associated with less soft drink consumption and these effects were moderated by adolescent personality. The direct effects of practices and personality are noteworthy from a theoretical perspective. Implications for obesity prevention interventions are discussed.


   Introduction
Top
 Abstract
 Introduction
Methods
 Results
 Discussion
 Conflict of interest statement
 Acknowledgements
 References
 
Adolescent overweight and obesity prevalence rates are increasing in many countries [1–3] and present a major public health problem because of its association with cardiovascular disease risk factors, Type 2 diabetes [4–8] and psychosocial problems [9–11]. Additionally, obese youth tend to become obese adults [12, 13]. Although genetic factors are involved in the individual onset of obesity, they are not likely to be the main cause of its increasing prevalence in the last decades [14]. Weight gain occurs as a result of a positive energy balance, in which the energy intake (through diet) exceeds energy expenditure (mainly through physical activity) over a period of time. Thus, the increase in overweight and obesity prevalence rates in the last decades is largely behaviour related. Although evidence for specific behavioural factors that promote or protect against weight gain in children is more limited than in adults [15], several behaviours have been identified in youngsters, such as lack of physical activity, sedentary behaviour and the consumption of energy-dense snacks [14, 16]. Additionally, consumption of sugar-sweetened soft drink may promote weight gain in youngsters [17–19]. Although the association between weight gain and soft drink consumption is still debated [20–22], soft drink consumption may have additional health consequences, such as a decreased intake of milk and nutrients [23] and caries [24]. Hence, reducing adolescent soft drink consumption may be an important way to improve adolescent health.

When developing behavioural change interventions, insight into the behavioural determinants is needed [25]. Traditionally, health behaviour research has focused on identifying individual psychosocial determinants such as those used in the Theory of Planned Behaviour (TPB) [26]. The TPB proposes that behaviour is determined by one's intention to perform that particular behaviour. In turn, the intention is determined by three psychosocial concepts, namely, attitude, subjective norm and perceived behavioural control (PBC). The TPB is theorized to be a comprehensive model for explaining and predicting health behaviour, since extraneous or distal variables, such as the physical environment, the social context and personality, are thought to influence health behaviour through these social cognitions [26, 27]. However, recent studies suggest that the TPB is unable to fully account for such distal, relatively stable influences [28–30]. Consequently, (social) ecological models are increasingly being suggested to gain more insight into the influence of the social and physical environment on health behaviour [31]. Ecological models specify that distal individual-level factors interact with (social) environmental factors to influence health behaviour. Moreover, distal factors are hypothesized to directly affect health behaviour, thereby bypassing the proximal cognitive factors. In the present study, we investigated individual-level and social–environmental factors associated with adolescent soft drink consumption.

One of the most influential social–environmental factors for adolescents are their parents [32], who play an important role in adolescents' health behaviours [33–35]. A type of parental influence that is receiving increased scientific attention is parenting practices [36–38]. In contrast to general parenting styles, practices refer to content-specific acts of parenting [39]. Parents may try to influence their children's food intake by setting house rules: telling their children what to eat and when to eat it when they are at home. Child-feeding practices may be regarded as environmental factors in childhood and adolescent obesity [40] and the effects of those practices have been studied in relation to various health behaviours [38, 41–45]. However, those studies have yielded mixed results with strict practices having either a positive or a negative effect on adolescent health behaviour. These adverse results suggest that additional factors may moderate the influence of practices. For instance, the contextual influence of a general parenting style is assumed to moderate the association between parenting practices and adolescent outcomes [39]. Beyond additional social–environmental factors, individual-level factors may also influence the association between parenting practices and adolescent outcomes. Indeed, in Darling and Steinberg's theoretical model on parental influences, adolescent personality is theorized to moderate the association between practices and adolescent outcomes. However, empirical studies that investigate such moderating factors are lacking. Thus, there is a need to further investigate how individual-level factors, i.e. adolescent personality, interact with social–environmental factors, i.e. parenting practices, and subsequently influence adolescent health behaviour.

In current personality research, consensus has emerged on the structure of personality, which can be adequately described in terms of five broad dimensions [46]. These dimensions are commonly labelled conscientiousness, Agreeableness, extraversion, Emotional Stability and Openness to Experience and appear universal across cultures [47] and robust irrespective of language, factor analytical techniques and method of assessment [46, 48, 49]. Empirical evidence indicates that these dimensions are associated with health behaviour in both adolescents and adults [50–55]. Moreover, direct effects of these personality dimensions are found even when controlling for social cognitions and intention [27–29] and suggest that combining personality and social cognitions may provide a more sufficient account of the determinants of health behaviour [28].

The purpose of the present study was two-fold. First, we investigated the influence of the distal variables adolescent personality and parenting practices with adolescent soft drink consumption. Based on the proposed theoretical relations of the TPB [26], we hypothesized that the influences of personality and parenting practices on adolescent soft drink were mediated by behaviour-specific cognitions and intention. Second, we investigated if adolescent personality moderated the association between practices and adolescent soft drink consumption.


   Methods
Top
 Abstract
 Introduction
 Methods
Results
 Discussion
 Conflict of interest statement
 Acknowledgements
 References
 
Subject and procedures
For the present study, data from the Study on Medical Information and Lifestyle in Eindhoven (SMILE), an ongoing prospective cohort study, were analysed. This study is a joint project of Maastricht University and 23 General Practitioners (GPs) from nine Family Practice Centres in Eindhoven, a city of about 200.000 inhabitants located in the southern part of the Netherlands. All patients of 12 years and older registered at those GPs are requested every 6 months to complete self-administered questionnaires at home. Anonymity is guaranteed and participant addresses are obtained through the GPs. Respondents are informed that GPs would not be notified about participation. For respondents in the age bracket 12–18, informed consent is required from both the respondent and their parents. Only those adolescents of whom informed consent was received from themselves and their parents are included in the study. In addition to the questionnaire sent, an explanatory covering letter and a reply-paid envelope is included. In case of non-response to the initially mailed questionnaire, a reminder is sent 2 weeks later.

In May 2003 and November 2003, 12- to 18-year-olds were asked to complete a self-administered questionnaire at home. In May 2003 and November 2003, 476 [263 girls and 213 boys; mean age = 15.0 years (SD = 2.1)] and 507 [303 girls and 204 boys; mean age = 14.9 years (SD = 2.0)] adolescents, respectively, completed these questionnaires. Respondents who completed both surveys (n = 241; 51%) were included in the present study. Deletion of cases with missing values on key variables left a total sample of 208 [80 boys and 128 girls; mean age = 15.2 years (SD = 1.9)]. Attrition analyses showed that girls were more likely (OR = 1.6, P = 0.017) to respond to both questionnaires. No significant differences for drop-out were observed for age and personality dimensions.

Measures
Big Five personality measures were assessed in the May 2003 questionnaire, while measures of soft drink consumption, soft drink related cognitions and parenting practices and demographics were assessed in the November 2003 questionnaire. Based on a validated questionnaire for dietary intake [56, 57], soft drink consumption was assessed by asking respondents to indicate on how many days a week (range from never to 7 days per week) they drank sugar-containing soft drinks, such as regular cola and sprite. Additionally, respondents were asked to indicate how many glasses (150 ml), cans (330 ml) and/or bottles (500 ml) they drank on such a day. Multiplying frequency and usual amount and dividing that score by 7 computed an average score for consumed soft drinks in millilitres per day.

Intention was assessed by the item ‘I intend to consume a limited amount of soft drink in the next six months’ on a five-point Likert scale (+2 = yes definitely, –2 = no definitely not). Attitude was measured as the average of two items ({alpha} = 0.59) on five-point Likert scales assessing the good-bad aspect (+2 = very good, –2 = very bad) and the pleasant–unpleasant aspect (+2 = very pleasant, –2 = very unpleasant) of the preceding statement ‘I believe consuming a limited amount of soft drink is’. Subjective norm was measured as the average of two items ({alpha} = 0.63) on five-point Likert scales (+2 = yes definitely, –2 = no definitely not) regarding parents and peers (my parents/peers think I should consume a limited amount of soft drink). PBC was measured as the average of two items ({alpha} = 0.80) on five-point Likert scales assessing the easy–difficult aspect (+2 = very easy, –2 = very difficult) and the succeed–fail aspect (+2 = very likely to succeed, –2 = very likely to fail) of the preceding statement ‘When I want to limit the amount of soft drink I consume, this will be/I think I will be’.

Based on previous work by Cullen [58], we assessed parenting practices with eight items ({alpha} = 0.87) regarding soft drink consumption. Respondents were asked to indicate on a five-point Likert scale to what extent they agreed (0 = totally agree, 5 = totally disagree) with such statements as ‘my mother/father tells me what kind of soft drink I am allowed to consume; my mother/father tells me how much soft drink I may consume; my mother/father tells me when I may drink soft drink; my mother/father allows me to drink soft drink in the weekend’. A total score was computed by summing the scores in such a way that a higher score meant respondents perceived more strict practices.

Big Five dimensions were assessed using a shortened version of a Dutch translation of Goldberg's adjective 100 list [59] that has shown good reliability in 12- to 18-year-olds [60]. Respondents indicated on a 7-point scale to what extent they agreed (0 = totally disagree, 7 = totally agree) with such statements as ‘I am creative; I am quiet; I am helpful; I am careless’. Internal consistency analyses revealed good psychometric properties (extraversion {alpha} = 0.83, Agreeableness {alpha} = 0.80, Conscientiousness {alpha} = 0.84, Emotional Stability {alpha} = 0.81, Openness to Experience {alpha} = 0.72). Scale scores were computed by summing the scores on the respective scales, in such a way that a higher score meant one was, for instance, more agreeable or more open to experience.

Analyses
Spearman (gender) and Pearson correlations were computed. Additionally, scores for the study variables were checked for normal distributions using tests for skewness and kurtosis [61]. Second, based on the recommendations by Fox [62], regression diagnostics for outliers were conducted. Influence scores were investigated using Cook's distance [63] with cases in which Cook's distance is >1 indicative of outliers.

In order to test the hypothesis that social-cognitive factors would mediate the effect of parenting practices and personality on adolescent soft drink consumption, we followed a stepwise approach based on the recommendations by Baron and Kenny [64]. In the first step, parenting practices and personality must be associated with the social-cognitive variables studies (A–B, see Fig. 1). In the second step, parenting practices and personality must be associated with soft drink consumption (A–D). In the third step, social-cognitive factors must be associated with intention (B–C), while in the fourth step intention must be associated with soft drink consumption (C–D). The final step tests if the association between parenting practices and personality with soft drink consumption (A–D) is reduced when the social-cognitive variables and intention are added to the model. In the case of perfect mediation, the effect of parenting practices and personality will become zero. If the effect of parenting practices and personality with soft drink consumption reduces but remains statistically significant, social-cognitive variables are partial mediators. In case of a significant association between one or more personality dimensions with soft drink consumption, the other personality dimensions were additionally controlled for in the final multivariate regression analysis. Finally, if age and/or gender were significantly associated with soft drink consumption in the univariate analysis, we adjusted for these demographics in the multivariate analysis.


Figure 1
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  		Fig. 1. Theorized relations in the theory of planned behaviour.

 
We used the magnitude of the effect size as a source of information for the explanatory value of the full model. Effect sizes (f2) were computed by dividing the amount of explained variance (r2) by the amount of error variance (1 – r2). Based on Cohen's descriptive guidelines [65], effect sizes were regarded as small when f2 was between 0.02 and 0.15, medium for f2 between 0.15 and 0.35 and large when f2 was ≥0.35. In case of significant main effects for personality dimensions and parenting practices, interaction terms were computed. In order to investigate both linear and curvilinear relationships, we entered linear and quadratic interaction terms in the final block.


   Results
Top
 Abstract
 Introduction
 Methods
 Results
Discussion
 Conflict of interest statement
 Acknowledgements
 References
 
Univariate analyses
Mean and median intake of soft drink was, respectively, 479.9 ml (SD = 499.5) and 342.9 ml per day (Table I). Forty-three percent indicated to consume at least one can of soft drink on every day of the week. Those who consumed more soft drink had a more negative intention and attitude towards limited soft drink consumption. Additionally, they perceived less behavioural control and parenting practices regarding soft drink consumption.


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  		Table I. Mean scores, standard deviations and bivariate correlations for study variables (n = 208)

 
Multivariate analyses
Normality analyses revealed that soft drink consumption, subjective norm and attitude moderately differed from normality and were subsequently transformed using square root transformation [61]. After transformation, acceptable normal distributions were obtained. Furthermore, regression diagnostics revealed no cases with an undue influence on regression estimates with Cook's distance ranging from 0.000 to 0.071.

Regarding the mediation analyses, the first step showed that parenting practices were significantly associated with subjective norm (ß = 0.21, P = 0.003). Additionally, the personality dimension Agreeableness was associated with subjective norm (ß = –12, P = 0.084). In the second step, parenting practices (ß = –0.22, P = 0.002) and Agreeableness (ß = 0.13, P = 0.054) were associated with soft drink consumption. The third step showed that attitude (ß = 0.32, P < 0.001), subjective norm (ß = 0.22, P = 0.001) and PBC (ß = 0.16, P = 0.015) were significantly associated with intention, while in the fourth step, intention was significantly associated with soft drink consumption (ß = –0.20, P = 0.003). In the final step, all variables were simultaneously regressed on soft drink consumption (Table II). When controlling for the influences of parenting practices, personality and social-cognitive variables, intention was no longer significantly associated with soft drink consumption. Moreover, the effects of parenting practices and Agreeableness were largely unmediated by social-cognitive variables. Those who perceived more parenting practices towards limited soft drink consumption, were less agreeable, had a more positive attitude and those who perceived less subjective norm towards limited soft drink consumption drank less soft drink. The final model explained 14% of variance in adolescent soft drink consumption (f2 = 0.16), indicating a medium effect size.


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  		Table II. Standardized regression coefficients, P values and explained variance from regression analysis for soft drink consumption (n = 208)

 
Moderator analyses
Since the final step in the multivariate analyses showed that parenting practices and Agreeableness had significant main effects on adolescent soft drink consumption, we computed linear and quadratic interaction terms with those factors in order to investigate the potential moderating role of this personality dimension. We followed the recommendation of Aiken and West [66] to mean-centre the constituent interaction variables in order to minimize problems with multicollinearity.

First, the linear interaction was added in the second step to the final model of the regression model. Second, the quadratic interaction term was entered in the third step. Table II shows that the quadratic interaction term was statistically significant, indicating a curvilinear relationship regarding the moderating effect of personality on the practices–behaviour relationship. We therefore performed stratified analyses based on validated cut-off points for Agreeableness [59]: those with scores ≤32 were classified as low (n = 73), those with scores between 32 and 36 were classified as medium (n = 59) and those with scores ≥36 were classified as high (n = 76). The effect of parenting practices on soft drink consumption was most pronounced (ß = –0.28, P = 0.041) in adolescents with moderate levels of Agreeableness. The effect of parenting practices for adolescents with either low or high levels of Agreeableness was smaller and non-significant (ß = –0.17, P = 0.145 and ß = –0.18, P = 0.129, respectively).


   Discussion
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
Conflict of interest statement
 Acknowledgements
 References
 
In the present study, we investigated the association between parenting practices and TPB variables with adolescent soft drink consumption. Results showed that adolescents who perceived more strict practices towards soft drink intake consumed less soft drink. Notably, this effect was largely unmediated by social cognitions and intention. Moreover, results indicated that the effect of strict parenting practices was moderated by the adolescent personality dimension Agreeableness, with strict practices being most effective in adolescents with moderate levels of Agreeableness.

Adolescents with low levels of Agreeableness appear less willing to obey parental practices, reflecting Darling and Steinberg's theoretical model [39]. This model postulates that adolescent's ‘willingness to be socialized’, a concept that bears resemblance to the personality dimension Agreeableness, is a moderator of the relationship between parenting practices and adolescent outcomes. In contrast to the linear relationship theorized by Darling and Steinberg, our results are not consistent with a linear relationship but may indicate a curvilinear relationship. For adolescents with high levels of Agreeableness, this may reflect compliance (as opposed to internalization) in the home setting, but opposite behaviour in a different setting. Indeed, Fisher and Birch found that parental restriction to palatable foods can actually promote the intake of these foods in an unrestricted situation, for instance, when parents are absent [44].

Previous studies on personality and diet have generally found positive health effects of Agreeableness [51, 53, 55]. Our results indicate, however, that more agreeable adolescents are engaged in unhealthy behaviour, i.e. consumed more soft drink. Since individual-level factors are thought to interact with environmental factors [67], individual-level factors may thus be health beneficial in some environments (for instance, the home environment), but these same individual-level factors may be health detrimental in others. Nowadays, soft drink vending machines are widely available at schools [68] and in those environments, peer influences may be stronger than parental influences. Since peer influences have been found to be positively associated with adolescent soft drink consumption [69] and because Agreeableness is linked to prosocial motives, those high on Agreeableness may be more likely to comply with those peer influences and, consequently, have higher soft drink consumption. This may be even more pronounced, since parents tend to view the issue of soft drink vending machines as a matter of personal choice for their children [70]. An additional explanation might be related to marketing: soft drink companies increasingly use adolescent-targeted advertisements and marketing through the use of prototypes. According to Gibbon's Prototype/Willingness Model [71] of adolescent health-risk behaviour, prototypes influence health-related behaviours through the process of social comparison [72]. Adolescents with high level of Agreeableness may be more sensitive to social comparison and therefore more inclined to live up to expectations raised by prototype-based advertisements and marketing. Indeed, a recent study found social comparisons to be of particular interest in the relation between media images and body dissatisfaction [73], while Rivis and Sheeran [74] found prototype similarity to be positively associated with health behaviour.

Individual-level factors in health education research are generally limited to social cognitions, most likely because these are modifiable through interventions [75] and assumed to be under conscious control [76, 77]. According to the TPB, the influence of personality factors is mediated by social cognitions. Hence, one may argue that those with high levels of Agreeableness are, for instance, more likely to perceive more subjective norm and, consequently, intend and act on these subjective norms. However, the present study and other recent studies [27, 29] indicate that these personality influences on health behaviour are largely unmediated by social cognitions and thus pose a theoretical threat to the TPB. It has been argued that social cognitions lack temporal stability in measurement [27, 78], which Ajzen [79] acknowledged to be a limitation of the TPB. Intentions have been found to be unstable even over a 2-day duration [80]. Despite these findings, social cognitions are still the predominant individual-level factors in health education research. However, the inclusion of more stable and global individual dispositions, such as personality, may allow us to gain a better understanding of the determinants of current health behaviour, but may also prove helpful in predicting future health behaviour. Importantly, our results indicate that adolescent personality influences may provide an explanation for the mixed effects of parenting practices on adolescent health behaviour.

Regarding social cognitions, attitude and subjective norm were significantly associated with soft drink consumption. For subjective norm, counter-theoretical results were found: adolescents who perceived stronger subjective norm to limit their soft drink consumption actually consumed more soft drink. This may, however, reflect the cross-sectional nature of our study: those already consuming higher amounts of soft drink may, as a result, perceive stronger subjective norm to limit their intake. Additionally, both intention and PBC did not add significantly to the explanation of adolescent soft drink consumption when parenting practices were taken into account. While intention was significantly associated with soft drink consumption in the univariate analysis, this was no longer the case when parenting practices, personality dimensions and social-cognitive variables were controlled for. This finding may underline the central role parents can play in adolescents' health-related behaviours, such as soft drink consumption. The present study indicated that a one unit increase on the parenting practices scale would result in a decrease of 39 ml soft drink consumption per day. Based on 37 kcal 100 ml–1 soft drink consumption [81], such a decreased consumption may, if energy intake and expenditure from other sources remain constant, result in a 0.83-kg weight loss in adolescents over a 1-year period [82].

A few limitations of our present study need commenting. First, respondents in our study were not representative for the total Dutch adolescent population, with a larger proportion of females than the Dutch adolescent population at large. Additionally, response rate was <20% [53] and since females were more likely to respond, caution is needed to generalize findings. Also, soft drink consumption has been found to be associated with socio-economic status [83] and since no information on socio-economic status was available, we could not control for such factors. Second, our model was able to explain only 14% variance in soft drink consumption. Recent TPB reviews [84, 85] show that the TPB is capable of accounting for some 30–40% of the variance in self-reported behaviour. Moreover, these reviews indicate that intention is the most important variable in the TPB, accounting for two-thirds of the total explained variance in health behaviour. Since adolescent soft drink consumption appears to be a behaviour not largely determined by intention, explanatory power of our model may have been hampered. Third, although our results indicate that the mixed effects of parenting practices on adolescent health behaviour might be attributed to adolescent personality influences, other studies have suggested that additional social-level factors may play a role. For example, a recent study [86] suggested that these mixed results might also be attributed to the contextual influence of a general parenting style. Additionally, peer influences have been found to be positively associated with adolescent soft drink consumption [69]. Thus, further research is warranted to understand how individual-level factors interact with other social-environmental factors and subsequently influence health behaviour. Finally, we assessed social cognitions with only a few items. From both a theoretical and practical perspective, this may seem plausible and preferable [87]. Nevertheless, internal consistency values are highly dependent upon the number of items in a scale [88] and the modest psychometric properties for attitude and subjective norm may have limited their mediating capacities.

Environmental influences are thought to play an important role in the current obesity epidemic [14, 89]. Although the TPB postulates that these influences are mediated by PBC and intention, the present study and other recent studies [29, 30] indicate that the TPB is inadequate to fully account for such influences. In addition, TPB-based interventions are not always successful in inducing behavioural change [90], which may be a result of the direct influence of social and physical environmental factors on health behaviour. Moreover, the apparent lack of intentional action in adolescent soft drink consumption necessitates focusing on environments instead of on cognitions in health behaviour interventions [86]. Indeed, (social) ecological models look promising for obesity prevention [25]. Consequently, interventions aimed at preventing weight gain are likely to be more effective if the direct influences of the physical and social environments are taken into account [16].


   Conflict of interest statement
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 Conflict of interest statement
Acknowledgements
 References
 
None declared.


   Acknowledgements
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 Conflict of interest statement
 Acknowledgements
References
 
This study is part of the NRG-project funded by the Netherlands Heart Foundation (2000Z002/2000T201). The authors would like to thank the GPs of the Corporation of Family Practices in Eindhoven (SGE) for their cooperation.


   References
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 Conflict of interest statement
 Acknowledgements
 References
 
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Received on September 2, 2005; accepted on May 30, 2006


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