A detailed description of the methods used to calculate F&V portion sizes can be found in Chapter 2 (Section 2.4.2).
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Confounding variables included the individual level factors of ethnicity, sex of the child, height, average physical activity energy expenditure (kJ/kg/day), and the English Indices of Multiple Deprivation (IMD) 2010 score of the household. Detailed descriptions of the sources of these variables can be found in Chapter 2 (Section 2.5).
4.3.6 Statistical methods
Statistical analysis was performed using STATA 13 (StataCorp LP, Texas, US). For the composite HFE score, a standard 5% two-sided significance level was used. Eight factors of the HFE were also considered individually; therefore, their
significance was assessed using a more conservative significance level of 1% to account for the type one error associated with multiple testing. Continuous descriptive characteristics were summarised using mean and standard deviation where normally distributed and median and inter-quartile range otherwise.
Categorical variables were summarised as the number and percentage of respondents.
Multilevel logistic regression models were developed to investigate associations between both the composite HFE score, and its individual elements, with the odds of overweight/obesity. Similarly, multilevel linear regression models were developed to investigate the associations between both the composite HFE score, and its
individual elements, with portions of F&V consumed. Model one only adjusted for school as a random effect to account for the clustered nature of the sample. Model two added ethnicity, sex of the child, IMD score (as a proxy for socioeconomic
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status), height, and physical activity energy expenditure as fixed effects. In instances where there were three categories being compared (i.e. frequency of breakfast consumption, snacking without permission, frequency of sitting together at a table for the evening meal, frequency of eating out, and frequency of fast food), separate multi-level linear regressions, using the HFE element as a continuous variable and adjusting for the Model 2 confounders, were conducted to assess the trend across the groups (p-for-trend).
4.3.7 Subgroup and sensitivity analysis
Subgroup analysis for boys and girls was conducted, using the further adjusted model (Model 2), excluding sex of the child as a covariate. Age was not considered as a modifier in this analysis due to the narrow age range of the study sample.
All analyses were repeated for F&V intake using only those children deemed
plausible reporters via the Goldberg methods (n = 789) to assess the potential effect of misreporting of dietary intake on the outcomes.
Additionally, analysis of total sample models was repeated on an imputed dataset to assess the impact of missing data on the outcomes of the further adjusted models. Imputation was conducted allowing for clustering of the data within the procedure. The following items were included in the imputation processes: weight status at baseline, height at baseline, physical activity expenditure at baseline, WAVES study trial arm, ethnicity of child (White, South Asian, Black African-Caribbean and
Mixed/Other ethnicities), deprivation score of household (IMD 2010), sex of the child, school free school meal entitlement proportion, school level ethnic mix (White, South Asian, Black African-Caribbean and Mixed/Other ethnicities). Ten sets of estimated
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parameters were then pooled and the mixed effect regression models repeated using the imputed data. Generation of imputed datasets was conducted in REALCOM- Impute (152) and analysis conducted was in STATA 13.
4.4 Results
4.4.1 Sample description
Of the 1467 children participating in the WAVES study, there were 944 (64.3%) whose parent/carer (subsequently referred to as parents) had completed the relevant questionnaire (Figure 11). Compared to children for whom a questionnaire was not returned, responders were more likely to be ranked in the least deprived quintiles (IMD quintiles 3-5; 31% of children who returned questionnaires vs. 15% of children who did not return a questionnaire) and to be from a White ethnicity (51% of children who returned a questionnaire vs. 31% of children who did not return a questionnaire). There was no clear difference in the average energy consumed or average physical activity energy expenditure (kJ/kg/day) in the children of returners, compared to non- returners. Of the parental questionnaires returned, 18 did not have a corresponding child BMIz available for analysis and 81 did not have a complete dietary record for the 24-hour period (Figure 11).
Table 15 highlights key demographic differences between the children by weight status. Parents of overweight/obese boys were significantly more likely to return a questionnaire than parents of overweight/obese girls. Black and mixed/other ethnicity children who returned a questionnaire, were more likely to be overweight/obese than White children who returned a questionnaire. Additionally, those in the least deprived quintile of deprivation (quintile 5) were less likely to be overweight or obese than
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those in the most deprived quintile of deprivation (quintile 1) of those who returned a questionnaire. Between 3-20% of the sample did not answer specific questions in relation to the HFE. For questionnaires that were returned individual question response rates ranged from 81% (screen time) to 97% (snacking without permission).
10
4
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Table 15: Chapter 4 sample description, by weight status
Not overweight/obese
Overweight Obese p- value (n = 740) (n = 81) (n = 105) Age of the child (years; mean (SD); N=926) b 6.3 (0.3) 6.3 (0.3
)
6.3 (0.3) 0.956 Sex of the child (n (%); N=926) c
Male 364 (77.3) 47 (10. 0) 60 (12.7) - Female 376 (82.6) 34 (7.5 ) 45 (9.9) 0.019 Ethnicity (n (%); N = 924) c White 385 (82.4) 43 (9.2 ) 39 (8.4) - South Asian 208 (81.3) 22 (8.6 ) 26 (10.2) 0.522 Black 35 (64.8) 3 (5.6 ) 16 (29.6) <0.00 1 Other/mixed ethnicity 111 (75.5) 13 (8.8 ) 23 (15.7) 0.039 Index of Multiple Deprivation 2010 (n (%); N = 914) c
Quintile 1 (more deprived) 359 (76.6) 43 (9.2 ) 67 (14.3) - Quintile 2 138 (83.1) 15 (9.0 ) 13 (7.8) 0.059 Quintile 3 87 (84.5) 6 (5.8 ) 10 (9.7) 0.091 Quintile 4 71 (79.8) 7 (7.9 ) 11 (12.4) 0.472 Quintile 5 (less deprived) 75 (86.2) 9 (10.
3)
3 (3.5) 0.012 Median energy intake (kcal; median (IQR);
N = 855) b 1651.9 (507.1) 1688.9 (55 1.1) 1757.2 (517.6 ) 0.051 Average physical activity energy
expenditure (kJ/kg/day; mean (SD); N=802)
b
95.4 (25.1) 92.2 (20. 4)
94.9 (21.5) 0.574 HFE score (mean (SD); N = 678) b 4.7 (2.4) 4.4 (2.1
)
5.2 (2.8) 0.202
a
Based on the UK 1990 growth reference data (UK90) and the population cut-off of the 85th (overweight) and 95th (obese) centile
b
p-values generated using mixed effect linear regression models, fitting weight status as a continuous variable, and school attended as a random effect
c
p-values generated using multinomial logistic regression models, fitting weight status as a continuous variable, and using robust standard errors to account for clustering
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4.4.2 Associations between the composite score and individual elements of