Participants

The present study includes children from the Identification and Prevention of Dietary- and Lifestyle-Induced Health Effects in Children and Infants study (IDEFICS). IDEFICS is a prospective cohort study with an embedded intervention, including eight European countries (Belgium, Cyprus, Estonia, Germany, Hungary, Italy, Spain and Sweden). The general purpose of IDEFICS is to understand how to prevent overweight in children while considering its multifactorial etiology. Ethics approval was obtained from review boards responsible in each country. Parents provided written informed consent, and children gave oral consent for examinations and sample collection. Further information about the IDEFICS study can be obtained from previous publications [13, 14].

Originally, 16,228 children aged 2 to 9 years participated at IDEFICS baseline (September 2007 to June 2008). The baseline survey was followed by a community intervention in half of the sample and then a 2 years follow-up examination was conducted in 9920 children from the original baseline cohort. The prospective design required that the same instruments and examinations were employed at both time points. To assure quality and comparability across research centers a translation/back-translation for each local language was performed, together with a re-administration to a sub-sample for assessing reliability [13]. Data on diet and indicators of well-being were obtained by parental proxy reporting using questionnaires. Only children with complete data on diet and indicators of well-being from both time points were included, hence the final sample consisted of 7675 children, 51% males (Fig. 1).

Fig. 1 Flowchart on participants included in the present study Full size image

The healthy dietary adherence score (HDAS)

An a priori diet score, the Healthy Dietary Adherence Score (HDAS), was calculated from a 43-item food frequency questionnaire (FFQ). At baseline and follow-up parents (or other caregivers) were asked to report the usual consumption frequency in a typical week during the preceding 4 weeks for all meals consumed at home or in the presence of the parents excluding e.g. foods served at school: In the last month, how many times did your child eat or drink the following food items? Possible options for answering were: never/less than once a week, 1–3 times a week, 4–6 times a week, 1 time per day, 2 times per day, 3 times per day, 4 or more times per day, I have no idea. Children for whom more than 21 (50%) of the FFQ items were missing were excluded. Based on this definition the rate of complete FFQ (less than 50% of food items missing) were 93% at both baseline and follow-up. In the remaining sample of children the missing food items as well as the answer ‘I have no idea’ were treated as not consumed when creating composite scores. This is a common practice in nutrition surveys because food items are often left blank if not consumed [15,16,17]. A pilot study found the FFQ to be reproducible with mean Kappa coefficients ranging from 0.41 to 0.60 and Spearman’s correlation higher than 0.5 for 81% of the food items [18]. Further, a validation study against repeated 24-h dietary recall found that under 12% of the food groups were classified in the wrong quartile of intake [19].

The HDAS was developed to reflect the guidelines established by Waijers et al. [20, 21]. Specifically, the HDAS aimed to capture adherence to healthy dietary guidelines common for all eight countries participating in the IDEFICS study. Moreover, the design of the HDAS allows for a standardization of number of foods and beverages reported, and consumption frequency, in order to avoid misclassification of children into low or high adherence just because they consume all types of food frequently. The guidelines included: limit the intake of refined sugars, reduce fat intake, especially of saturated fat, choose whole meal when possible, consume 400–500 g of fruits & vegetables per day and fish 2–3 times per week. Hence, the HDAS contains five components: sugar, fat, whole meal, fruits & vegetables, and fish. Each component has a minimum score of 0 and a maximum score of 10, summed to a maximum score of 50, where the highest score indicates the highest possible adherence to the dietary guidelines. Both the HDAS and its components were dichotomized into “lower adherence” and “higher adherence” at the group median with the median included in the higher adherence group. A more detailed description of the HDAS can be found in Additional file 1.

Indicators of psychosocial well-being

Four indicators of psychosocial well-being (referred to as well-being) were examined at baseline and follow-up, namely self-esteem, parent relations, emotional and peer problems. Self-esteem and parent relations were calculated from responses to the validated Kinder Lebensqualität Fragebogen (KINDL®) [22,23,24]. The IDEFICS study included a version of the KINDL® developed for parent response on behalf of children and adolescents between 7 and 17 years of age. The self-esteem score included: During the last week my child… (1) had fun and laughed a lot, (2) didn’t feel much like doing anything, (3) felt alone, and (4) felt scared or unsure of him/herself. The parent relations score included: During the last week my child… (1) got on well with us as parents, (2) felt fine at home, (3) we quarreled at home, and (4) felt that I was bossing him/her around. The items were scored from 1 (never) to 4 (often or always) with reversals according to the wording of the question, summed to total scores and transformed to percentage scores ranging from 0 to 100%. The total scores where then dichotomized into ‘poor’ or ‘good’ using sex- and age-specific cut-off scores from the KINDL® manual [22]. However, self-esteem was later re-categorized into ‘lower’ and ‘better’ scores at the group median since a majority of the children (98% at baseline and 97% at follow-up) reported ‘good’ scores suggesting that our population experienced higher self-esteem compared to the reference population of corresponding sex- and age-groups.

Emotional and peer problems were calculated from Goodmann et al.’s validated Strengths and Difficulties Questionnaire (SDQ) [25,26,27] developed for children ages 4 to 16 years. The IDEFICS study used the informant-rated version which has been found to correlate well with the child-rated version [27, 28]. The peer problem score included: To what extent do the following characterizations apply to your child? (1) rather solitary, tends to play alone, (2) has at least one good friend, (3) generally liked by other children (4) picked on or bullied by other children, and (5) gets on better with adults than with other children. The emotional problem score included: To what extent do the following characterizations apply to your child? (1) often complains of headaches, stomach-aches or sickness, (2) many worries or often seems worried, (3) often unhappy, depressed or tearful, (4) nervous in new situations, easily loses confidence, and (5) many fears, easily scared. Items were scored from 0 ‘not true’ to 2 ‘certainly true’ and summed to total scores ranging from 0 to 10 where a high value indicated more difficulties or life struggles. In accordance with the SDQ manual [28] the emotional and peer problem scores were divided into: ‘inconspicuous’, ‘borderline’ and ‘abnormal’. Thereafter, a dichotomized variable was created, as previously done by Hunsberger et al. [12], consisting of poor well-being (including both ‘borderline’ and ‘abnormal’ groups) versus the remaining children with no detectable (‘inconspicuous’) poor wellbeing.

Covariates

Measured anthropometrics were collected at both baseline and follow-up. Weight was measured to the nearest 0.1 kg with a Tanita BC 420 SMA scale and height was measured to the nearest 0.1 cm by a SECA 225 Stadiometer. Examinations were conducted in the morning, with the children fasting and in light clothing. Body Mass Index (BMI) and age-and sex specific BMI z-scores and cut-points for children and adolescents developed by the International Obesity Task Force (IOTF) [29] were calculated and used to categorize children as normal weight (including thin) or overweight (including obese).

Data on parental education and income was collected from the parental questionnaire. The education level is based on the International Standard Classification of Education (ISCED) for cross-country comparability and was used to determine the highest level of either parents’ education [30]. Levels 1–3 represent upper secondary education (classified as lower education level) and levels 4–6 represent post-secondary education (classified as higher education level). Country-specific income levels were assigned with reference to the average net equivalence income, considering the median income and poverty line. Levels 1–5 represent lower income level and levels 6–9 represent higher income level.

Statistics

Descriptive characteristics are presented as mean, standard deviation, minimum and maximum for continuous variables (age, BMI z-score, indicators of well-being and the HDAS scores), and number and percentage for categorical variables (sex of the child, weight status, SEP, and categories of well-being, HDAS and its components). Due to the hierarchical structure of the data Generalized Linear Mixed Models (GENLINMIXED) were used to analyze the prospective association between higher adherence to the HDAS and its components at baseline and indicators of well-being 2 years later. Random intercepts for country were included to consider the clustered study design. The model was adjusted for age, sex, BMI z-score, baseline well-being, and highest parental education and income. To investigate directionality, associations between well-being at baseline and adherence to the HDAS and its components 2 years later were analyzed using the same procedure, now adjusting for baseline diet factors.

A sensitivity analysis was performed to further explore the chronology of associations between the HDAS and well-being. In these analyses, group medians were used as cut-off for the indicators of well-being (as both exposure and outcome) to estimate standardized effect sizes in both directions. The relationship with parents was dichotomized into ‘lower’ and ‘higher’ scores (median included in the higher group), with a higher score indicating higher well-being. Furthermore, emotional and peer problems were dichotomized into ‘lower’ and ‘higher’ scores (median included in the lower group), and here a lower score indicated higher well-being. As previously described, self-esteem was already dichotomized based on group median. An additional sensitivity analysis using quartiles of adherence to the HDAS was performed in order to further explore the potential dose-response relationship between baseline diet and children’s subsequent well-being.

Next, stratified analyses were performed to investigate if the association between diet and well-being differed between children with overweight compared to children with normal weight. Finally, a drop-out analysis was conducted to compare children included in the present study with those who only participated in the IDEFICS baseline measurements. Student t-test was used for continuous variables (age, BMI z-score, the HDAS) and Pearson’s χ2-test to compare categorical variables (sex of the child, parental education, parental income, weight status, indicators of well-being). All analyses were performed with IBM SPSS Statistics Version 20. The significance level was set to 0.05.