There is strong epidemiological evidence of a protective role of fruit and vegetables in the prevention of cancerReference Van Duyn and Pivonka1, 2, coronary heart diseaseReference Ness and Powles3 and becoming overweightReference Ello-Martin, Ledikwe and Rolls4. According to the National Health and Nutrition Examination Survey 1999–2002, 16% (>9 million) of all US children and teens were overweight5. Overweight and obesity substantially increase the risk of illness from high blood pressure, high cholesterol, heart disease and stroke6, type 2 diabetesReference Fagot-Campagna, Burrows and Williamson7 and some cancersReference Greenwald, Kramer and Weed8.
Children’s dietary intake behaviour is important because it may track into adolescence and adulthoodReference Resnicow, Smith, Baranowski, Baranowski, Vaughan and Davis9–Reference Singer, Moore, Garrahie and Ellison11. Persons of all agesReference Crane, Hubbard and Lewis12, all ethnic subgroups (non-Hispanic white, non-Hispanic black, Hispanic, and other)Reference Krebs-Smith, Cook, Subar, Cleveland, Friday and Kahle13, Reference Te Velde, Wind, van Lenthe, Klepp and Brug14 and in various countriesReference Yngve, Wolf, Poortvliet, Elmadfa, Brug and Ehrenblad15–Reference Agudo, Slimani, Ocké, Naska, Miller and Kroke17 eat fewer than the recommended number of servings of fruit, juice and vegetables (FJV). Factors identified as contributing to low FJV consumption in the past have included low home availability and accessibilityReference Baranowski, Domel and Gould18, Reference Rasmussen, Krølner, Klepp, Lytle, Brug and Bere19, low preference and low self-efficacyReference Rasmussen, Krølner, Klepp, Lytle, Brug and Bere19, Reference Roos, Lahelma, Virtanen, Prättälä and Pietinen20.
FJV preferences predicted FJV intake in both child and adult populationsReference Haire-Joshu, Kreuter, Holt and Steger-May21–Reference Bere and Klepp32. FJV self-efficacy – the ability to select, prepare and eat FJV – has been associated with consumption in some studiesReference Cullen, Bartholomew, Parcel and Koehly23, Reference Granner, Sargent, Calderon, Hussey, Evans and Watkins25, Reference Vereecken, Van Damme and Maes28, Reference Wind, de Bourdeaudhuij, te Velde, Sandvik, Due and Klepp31, Reference Martens, van Assema and Brug33, Reference Sandvik, De Bourdeaudhuij, Due, Brug, Wind and Bere34 but not in othersReference Domel, Baranowski, Thompson, Davis, Leonard and Baranowski24, Reference Resnicow, Davis-Hearn, Smith, Baranowski, Lin and Baranowski27. This might indicate that previously reported associations between self-efficacy and FJV intake may be influenced by other factors. Home FJV availability has been associated with consumption among childrenReference Reynolds, Baranowski, Bishop, Farris, Binkley and Nicklas29, Reference Cullen, Baranowski, Owens, Marsh, Rittenberry and de Moor30, Reference Bere and Klepp32, Reference Sandvik, De Bourdeaudhuij, Due, Brug, Wind and Bere34–Reference Kratt, Reynolds and Shewchuk36.
Interactions among predictors of FJV consumption have been investigated. The interaction between preference and home availability predicted FJV consumptionReference Cullen, Baranowski, Owens, Marsh, Rittenberry and de Moor30. An interaction between self-efficacy and availability also predicted FJV consumptionReference Reynolds, Baranowski, Bishop, Farris, Binkley and Nicklas29, Reference Kratt, Reynolds and Shewchuk36. A recent study investigated whether psychosocial determinants of FJV intake differed between normal and overweight boysReference De Bourdeaudhuij, Yngve, Te Velde, Klepp, Rasmussen and Thorsdottir37. FJV consumption among overweight children may differ from that of children with normal weight-for-age as a result of lower FJV preference, home availability or efficacy. The primary objective of the present study was to assess whether FJV preferences, home FJV availability and FJV self-efficacy were associated with FJV consumption. The secondary objective was to examine possible interactions among FJV preference, home availability, self-efficacy and body mass index (BMI) on consumption, as a new contribution to the existing literature.
Social desirability has been defined as a tendency to overestimate desirable traits and underestimate undesirable ones, when using self-report measuresReference Dadds, Perrin and Yule38. A significant negative association was found between reported sweetened beverage preference and social desirability among a sample (n = 95) of 8–10-year-old African American girls, suggesting that social desirability biased participant response; and social desirability was also a confounder between BMI and self-reported levels of energy intakeReference Klesges, Haddock and Eck39. It is therefore important to control for this possible self-report bias.
This study assessed the BMI, psychosocial and demographic influences on the amount of FJV intake reported by 11–14-year-old males.
Methods
Study population
Participants were a convenience sample of 11–14-year-old Boy Scouts residing in Houston, Texas, USA or surrounding communities, recruited to participate in an achievement badge programme. The eight-week intervention was implemented at two separate periods of time, spring 2003 and fall 2003. The total baseline number of participants recruited (n = 473) was randomised by troop into either the Fit for LifeReference Jago, Baranowski, Baranowski, Thompson, Cullen and Watson40 or 5 A Day Achievement41 Badge Intervention. This study was approved by the Institutional Review Board, and all participants provided parental consent and assent.
Measures
Demographic variables
Demographic variables included race/ethnicity, age and family education. All variables were self-reported by either a parent or guardian. Race/ethnicity of the participant was selected as one of the following: ‘black or African American’, ‘white’, ‘American Indian or Alaska Native’, ‘Asian or Pacific Islander’, ‘Hispanic or Latino’, and ‘Other (please specify)’. Family education was selected as one of the following: ‘6th grade or less’, ‘8th grade or less’, ‘attended some high school’, ‘high school graduate or GED’ (General Education Development), ‘technical school’, ‘some college’, ‘college graduate’, and ‘postgraduate study’.
Psychosocial variables
The Social-Cognitive TheoryReference Bandura42 served as the theoretical framework in selecting psychosocial determinants for inclusion in this study. FJV preferences were assessed using a previously validated questionnaire with a mean test–retest reliability of 0.73 (fruit, P < 0.001) and 0.71 (vegetables, P < 0.001)Reference Domel, Baranowski, Davis, Leonard, Riley and Baranowski43. Response categories included: ‘I do not like this’, ‘I like this a little’ and ‘I like this a lot’, and were coded as 1, 2 or 3, respectively, and summed to achieve preference scores for fruit and juices (FJ) and vegetables (V). FJ and V self-efficacy were assessed using a previously validated 21-item questionnaire with a test–retest reliability ranging from 0.35 to 0.67Reference Domel, Baranowski, Thompson, Davis, Leonard and Baranowski24. Home FJ and V availability were assessed using a previously validated 48-item questionnaire that reported correlations between actual and reported availability of fruits (r = 0.56, P < 0.001), juices (r = 0.52, P < 0.001) and vegetables (r = 0.44, P < 0.001)Reference Marsh, Cullen and Baranowski44. Social desirability was measured through the administration of the ‘lie scale’ from the revised Manifest Anxiety Scale to control for socially desirable responsesReference Reynolds and Paget45.
Anthropometry
Height (cm) was measured to the nearest 0.1 cm using a stadiometer (Perspective Enterprises). Body weight (kg) was measured to the nearest 0.1 kg using a calibrated scale (SECA, model 770 or 882). BMI (kg m−2) and BMI percentile were calculated for all participants using age- and gender-specific percentiles from the 2000 growth charts of the Centers for Disease Control and Prevention46. BMI percentile was categorised into one of the following categories: normal (BMI < 85 percentile), at risk (85 percentile < BMI < 95 percentile), or overweight (BMI > 95 percentile). BMI percentiles were developed for the US population and are the most common indicator to assess the size and growth patterns of American children because body size changes with age and differs according to gender.
Fruit, 100% juice and vegetable consumption
FJV consumption was assessed using a food-frequency questionnaire previously validated with four 24-hour dietary recall interviews, with a mean test–retest reliability of 0.54 (P < 0.01) (fruit, 0.71 (P < 0.0001); juice, 0.42 (P < 0.05); vegetables, 0.53 (P < 0.01)) and a mean validity of 0.77 (P < 0.001) (fruit, 0.74 (P < 0.001); vegetables, 0.41 (P < 0.01))Reference Cullen, Baranowski, Baranowski, Hebert and de Moor47. Participants reported consumption of 18 specific brands or types of drinks in addition to four types of 100% fruit juices. All the scores were divided by seven to produce average servings of FJV consumed daily. Intake was computed for each variable (fruit and juice, vegetables).
Data analysis
As previous studies have shown that different associations are detected for the prediction of fruit and juices than for vegetablesReference Cullen, Baranowski, Owens, Marsh, Rittenberry and de Moor30, separate models were run in which either FJ consumption or V consumption was the dependent variable. Descriptive statistics were used to describe key variables and a Pearson correlation matrix was generated to assess correlations between psychosocial variables. Stepwise linear models that controlled for the clustering of the Boy Scouts by troops were performed using the PROC MIXED procedure in SAS version 8.0 (SAS Institute). The models were built in three stages: (1) demographics (and social desirability); (2) BMI and psychosocial variables; and (3) interaction terms including BMI and psychosocial variables. Non-significant psychosocial variables and interaction terms were removed from the multiple regression models in a backward deletion process (P < 0.10) until all variables remaining were significant (P < 0.10). Troop-related intra-class correlations were calculated using the formula developed by SingerReference Singer48. Level 1 (within-unit variance) and level 2 (between-unit variance) R 2 values were estimated with the formulas developed by Snijders and BoskerReference Snijders and Bosker49.
Results
Participants were on average 12.8 (standard deviation (SD) 1.1) years of age and predominantly white (73%) (see Table 1). The mean BMI was 21.3 (SD 4.5) kg m−2, and 32.6% of the subjects were in the BMI-for-age percentile categories of at risk or overweight. This sample had a high family education, with close to 70% living in a home with a college graduate.
Table 1 Frequency of demographic characteristics (n = 473)
BMI – body mass index; HS – high school.
The mean reported daily consumption of total FJV was 5.9 (SD 4.8) servings (FJ servings, 3.2 (SD 2.9); V servings, 2.6 (SD 2.2)). Mean daily consumption of FJV and the psychosocial variables did not differ by ethnic group, overweight status or family education. Psychosocial determinants of FJV consumption were found to be significantly inter-correlated (Table 2).
Table 2 Inter-correlations between psychosocial variables of fruit and juice (FJ) and vegetable (V) consumption
Data are presented as Pearson correlation (P-value, two-tailed).
After inclusion of all demographics and social desirability, ethnicity (white vs. non-white) was the only variable that significantly predicted FJ consumption. There were no significant first-level predictors of vegetable consumption. When main effect psychosocial terms were added to the model, preferences were significantly associated with daily consumption of FJ (P < 0.001) and V (P < 0.001); self-efficacy was significantly associated with daily V (P < 0.001), but not FJ consumption; home availability was a significant predictor of FJ (P < 0.001) and V (P < 0.001). Social desirability was not significantly associated with FJ or V consumption (see Table 3). When the interactions were added to the models in stage 3 only the preferences by home availability interaction was associated with daily FJ (P = 0.04) and V (P = 0.01) consumption. The interaction of self-efficacy by home availability was marginally associated with V (P = 0.08), but not FJ consumption. Excluding potatoes (sweet potatoes, white potatoes, potato salad, French fries) from vegetable intake resulted in similar findings (results not shown). No significant interactions were found between BMI and the psychosocial variables.
Table 3 Three-stage hierarchical regression analysis of fruit and juice (FJ) and vegetable (V) consumption
IV – independent variable; DV – dependent variable; E – estimate; SE – standard error; Pr – probability of significance, R2L1 – R 2 level 1; R2L2 – R 2 level 2; BMI – body mass index; GED – General Education Development; HS – high school.
*Backwards deleted least significant psychosocial main effects and psychosocial interactions.
The interaction of preference by home availability with FJ consumption appears in Fig. 1. For each level of home FJ availability, FJ consumption was proportionally higher for those with higher vs. lower FJ preference. The two preference lines intersected at a level of approximately 1 on FJ availability and FJ consumption. Exactly the same pattern was obtained for V preference×home V availability and V self-efficacy × home V availability terms.
Fig. 1 Mean daily servings of fruit and juice (FJ) by availability according to high and low preference
Discussion
The preference by home availability interaction was associated with FJ and V consumption (Table 2). The full models accounted for 30% or more of the variance in intake, which is better than found in previous analyses that did not include these interactionsReference Haire-Joshu, Kreuter, Holt and Steger-May21–Reference Cullen, Baranowski, Owens, Marsh, Rittenberry and de Moor30, Reference Hearn, Baranowski, Baranowski, Doyle, Smith and Lin35, Reference Kratt, Reynolds and Shewchuk36. Preferences and home availability were the highest correlates of FJ and V consumption in previous studies that reported preferenceReference Cullen, Eagan, Baranowski, Owens and de Moor22–Reference Granner, Sargent, Calderon, Hussey, Evans and Watkins25, Reference Resnicow, Davis-Hearn, Smith, Baranowski, Lin and Baranowski27–Reference Cullen, Baranowski, Owens, Marsh, Rittenberry and de Moor30 and/or availabilityReference Reynolds, Baranowski, Bishop, Farris, Binkley and Nicklas29, Reference Cullen, Baranowski, Owens, Marsh, Rittenberry and de Moor30, Reference Hearn, Baranowski, Baranowski, Doyle, Smith and Lin35, Reference Kratt, Reynolds and Shewchuk36. Previously an interaction was detected between preference and availability, and these results support those findingsReference Cullen, Baranowski, Owens, Marsh, Rittenberry and de Moor30. Dichotomising preference (low and high) at the mean and plotting the fitted regression lines for the association between FJ consumption and home availability indicated that those with higher preference had proportionally higher consumption of FJ for every level of home availability (Fig. 1). These findings suggest that increasing preference and availability will likely have more than an additive impact on increasing FJ and V consumption. Future research should determine whether the interaction of preference and availability holds true for all ages, females, and more ethnically diverse populations.
When the preference by home availability interaction term was in the FJ model, the main effect terms were no longer significant. This suggests the importance of the interactive nature of the relationship and may explain low predictive variance for the main effects alone in previous researchReference Haire-Joshu, Kreuter, Holt and Steger-May21, Reference Cullen, Eagan, Baranowski, Owens and de Moor22, Reference Domel, Baranowski, Thompson, Davis, Leonard and Baranowski24–Reference Cullen, Baranowski, Owens, Marsh, Rittenberry and de Moor30, Reference Hearn, Baranowski, Baranowski, Doyle, Smith and Lin35, Reference Kratt, Reynolds and Shewchuk36, Reference Cullen, Baranowski, Nwachokor, Baranowski, Hajek and Lones50.
No significant interactions were observed between BMI and the psychosocial variables as was seen in one previous studyReference De Bourdeaudhuij, Yngve, Te Velde, Klepp, Rasmussen and Thorsdottir37 for V consumption and home availability. This may be attributable to cultural, age or measurement (self-reported vs. measured height and weight) differences, and suggests that more research is needed.
In previous research V self-efficacy was associated with V consumption, but low R 2 values were obtained (0.02–0.17)Reference Cullen, Bartholomew, Parcel and Koehly23, Reference Granner, Sargent, Calderon, Hussey, Evans and Watkins25, Reference Young, Fors and Hayes51. The self-efficacy by home availability interaction was marginally associated with V consumption, which confirms previous findingsReference Reynolds, Baranowski, Bishop, Farris, Binkley and Nicklas29, Reference Kratt, Reynolds and Shewchuk36, and the main effect term for self-efficacy was no longer significant. This may explain inconsistencies in the self-efficacy to behaviour relationship in the past.
Mean consumption of FJ and V did not differ by demographic variables (age, ethnicity, family education). Past studies have found lower consumption of FJV among less educated populationsReference Johnson, Guthrie, Smiciklas-Wright and Wang52–Reference Serra-Majem, Ribas, Pérez-Rodrigo, García-Closas, Peña-Quintana and Aranceta54 and HispanicsReference Krebs-Smith, Cook, Subar, Cleveland, Friday and Kahle13. The lack of such a difference in this sample may be at least partially attributed to the homogeneity of the convenience sample of Boy Scout troops selected. However, Boy Scout troops have been shown to be an effective channel to reach large numbers of boysReference Cullen, Baranowski, Nwachokor, Baranowski, Hajek and Lones50, Reference Baranowski, Baranowski, Cullen, deMoor, Rittenberry and Hebert55. Scouting membership is nearly 6 million in the USA and more than 20 million internationally in 155 countries56, thus generalisation of these findings to Scouts alone would be substantial. Social desirability was not found to be significantly associated with FJ or V consumption. This differs from a past studyReference Klesges, Haddock and Eck39, possibly due to the difference in age and gender of the current study population.
Findings indicate that influences on consumption of FJ and V vary. FJ and V preferences and availability were significant predictors for FJ and V consumption. Self-efficacy was found to be a marginally significant predictor of V only. Interventions attempting to increase FJV consumption among boys similar to this population should strongly focus on increasing FJV preference and FJV availability. The significance of the interaction implies that increasing prevalence and availability should have a substantially higher impact on increasing FJ and V consumption than increasing either of the psychosocial variables alone.
Strengths/limitations of the research
The strengths of this study include the large number of participants, the use of previously validated measures and the examination of FJ and V consumption in relation to both psychosocial and demographic determinants.
Limitations of this study include the cross-sectional nature of the data that prevents the inference of causal relationships. While participants were asked about 100% fruit juice and fruit drinks with added sugar separately, it is possible boys had difficulty in discriminating between the two. The convenience sample of Boy Scout troops selected to participate in this intervention may have been homogeneous. Recruitment required participants to have access to a computer and an email address, which may have introduced bias to groups not having a personal computer and may have contributed to the high percentage of the study population from households with a high amount of education. The self-reported nature of the data is likely to have substantial error.
Conclusion
Daily consumption of fruit and juice (FJ) and vegetables (V) were significantly associated with the corresponding preference by home availability interaction. Vegetable consumption was also associated with the self-efficacy by home availability interaction. Understanding these interactions and child consumption of FJV should help in the design of interventions tailored to reduce BMI in specific groups.
Acknowledgements
Sources of funding: This research was supported by a grant from the American Cancer Society (ACS TURSG-01) and a contract from the National Cancer Institute (NCI 263-MQ-31958). This work is also a publication of the US Department of Agriculture (USDA)/Agricultural Research Service (ARS) Children’s Nutrition Research Center, Department of Pediatrics, Baylor College of Medicine and Texas Children’s Hospital, Houston, Texas. This project has been funded in part by federal funds from the USDA/ARS under cooperative agreement 58-6250-6001. The contents of this publication do not necessarily reflect the views or polices of the USDA or NCI, nor does mention of trade names, commercial products or organisations imply endorsement by the US Government.
Conflict of interest declaration: All funding sources have been reported and the authors have no conflicts of interest.
Authorship responsibilities: All named authors have made a substantial contribution to the published manuscript as follows: M.S.G. – design and conduct of research, statistical analysis, interpretation of data, preparation and approval of manuscript, R.J. – Fit for Life intervention management, research study design, interpretation of data, preparation and approval of manuscript; T.B. – Fit for Life intervention development and management, research study design, interpretation of data, preparation and approval of manuscript; J.C.B. – Fit for Life intervention development and management, approval of manuscript; P.M.D. – design and conduct of research, interpretation of data, preparation of manuscript.
