Insulin resistance is an important underlying condition and a critical link between adiposity and increased chronic disease risk, given its association with type 2 diabetes and the metabolic syndrome in both children and adults( Reference Lee 1 , Reference Seidell 2 ). There is a high prevalence of insulin resistance in obese adolescents( Reference Lee 1 ); however, it is also important to investigate the possible role of dietary behaviours in relation to insulin resistance in healthy children and adolescents as well( Reference Ambrosini, Huang and Mori 3 ). Identification and modification of potentially unfavourable dietary behaviours might be an easy-to-follow method for improving diet-related health problems.
One dietary behaviour that is usually investigated among children and adolescents is breakfast consumption. Skipping breakfast has been reported to be an index of erratic dietary patterns during the day which consequently can adversely affect cardiometabolic risk factors( Reference Farshchi, Taylor and Macdonald 4 ). Irregular breakfast intake has been associated with increased waist circumference and obesity in children( Reference Lehto, Ray and Lahti-Koski 5 ). Additionally, skipping breakfast in both childhood and adulthood is associated with larger waist circumference and higher fasting insulin levels( Reference Smith, Gall and McNaughton 6 ), while in female adolescent girls skipping breakfast is also related to an increased prevalence of insulin resistance( Reference Sese, Jimenez-Pavon and Gilbert 7 ).
Furthermore, dietary patterns have been shown to be useful for assessing relationships between diet and chronic diseases( Reference Hu 8 ). Lately they are widely used due to the understanding that nutrients are rarely eaten in isolation and that nutrient-only investigations underestimate the possible interactions between nutrients or between single foods and other dietary components( Reference Jacques and Tucker 9 ). Food is usually consumed as meals consisting of a variety of foods and, therefore, simple consideration of nutrient intake ignores the interaction of various food combinations. Dietary patterns represent the types and amounts of foods consumed and focus on the entire diet rather than just a single food or nutrient( Reference Jacques and Tucker 9 ). Recent data have shown that dietary patterns characterized by high intakes of low-fat dairy products, fruit, whole grains, poultry, fish and vegetables and low intake of alcohol have an inverse relationship with insulin resistance defined by use of the homeostasis model assessment of insulin resistance index (HOMA-IR) in adults( Reference Akter, Nanri and Yi 10 , Reference Anderson, Harris and Tylavsky 11 ). Likewise, a ‘healthy’ dietary pattern, rich in whole grains, vegetables and fruits and low in processed meats, high-fat dairy products and sugar, was inversely correlated with insulin resistance in children and adolescents( Reference Ambrosini, Huang and Mori 3 , Reference Kynde, Johnsen and Wedderkopp 12 ).
Identification of dietary patterns and behaviours, such as breakfast consumption and snacking choices, associated with insulin resistance could be useful in future interventions for improving lifestyle( Reference Owen, Pettman and Haas 13 ). Although insulin resistance has been linked to breakfast skipping and both have been implicated in chronic diseases( Reference Smith, Gall and McNaughton 6 , Reference Sese, Jimenez-Pavon and Gilbert 7 ), there is no study investigating dietary patterns and breakfast consumption habits in relation to insulin resistance, particularly among children and adolescents.
Therefore, the aim of the present study was to investigate the possible relationship of certain dietary patterns comprising the consumption of specific food groups/items and breakfast consumption habits with insulin resistance in children.
Experimental methods
Sampling
The Healthy Growth Study was a large-scale, cross-sectional epidemiological study conducted from May 2007 to June 2009. The study was approved by the Greek Ministry of National Education and the Ethics Committee of Harokopio University of Athens and it was in accordance with the ethical standards of the Declaration of Helsinki. The study participants were schoolchildren aged 9–13 years attending primary schools within the counties of Attica, Aitoloakarnania, Thessaloniki and Iraklio. The sampling of schools was random, multistage and stratified by parents’ educational level and total population of students attending schools within municipalities of these counties, as described in more detail elsewhere( Reference Moschonis, Tanagra and Vandorou 14 ). The sampling procedures took into consideration the urban–rural distribution of the population in the four regions, sex and the parental educational level, which is one of the most important indices of socio-economic status (SES). The study population was representative of the 9–13-year-old schoolchildren living in the four counties under study. None the less, these counties are scattered throughout the Greek territory, covering the northern (i.e. Thessaloniki), central (i.e. Attica), western (i.e. Aitoloakarnania) and southern (i.e. Iraklio–Crete) parts of Greece. This, combined with the random, multistage and stratified sampling procedures followed to recruit our sample, is indicative of the representativeness of our population. Written informed consent was obtained from all subjects. Parents who agreed to the participation of their children in the study had to sign the consent form and provide their contact details. Signed parental consent forms were collected for 2656 out of 4145 children (response rate of 64·1 %).
Dietary intake
Dietary intake data were obtained using the 24 h recall technique, for two consecutive weekdays and one weekend day. Children were asked to report, as detailed as possible, the type and amount of foods and beverages consumed during the previous day. Interviews with children were conducted by experienced dietitians who had undergone central and rigorous training in order to conduct the 24 h recalls in a standardized way so as to avoid inter-observer variations. At the end of each interview, trained dietitians reviewed the collected data with the respondent in order to clarify entries, servings and possible forgotten foods. Food intake data were analysed using the Nutritionist V diet analysis software version 2·1 (First Databank, San Bruno, CA, USA), which was extensively amended to include traditional Greek recipes( Reference Trichopoulou 15 ). Furthermore, the database was updated with nutritional information of processed foods provided by independent research institutes, food companies and fast-food chains.
The food-grouping scheme was designed for all foods or entries (core and recipe) appearing in Nutritionist V. In total, forty-seven food groups were initially established, based on similar source characteristics and nutrient contents. Composite food items, such as recipes, were analysed and assigned to food groups according to primary ingredients. A similar methodology for the extraction of food groups was previously reported in studies with a smaller sample size, but with only one 24 h recall available( Reference Nicklas, Webber and Srinivasan 16 ). Examples of foods included in the food groups were documented previously( Reference Nicklas, Farris and Johnson 17 ).
Breakfast
Using data from the 24 h recalls, the portions of each food group were distributed to meals during the day (i.e. breakfast, morning snack, lunch, afternoon snack, dinner, before-bed snack). We considered as breakfast the first things the participant ate and/or drank within 2 h after getting up in the morning. This could be at home, on the way to school or just before entering school. During the weekend, breakfast was considered as anything the participant drank and/or ate before 11.00 hours. Participants were characterized as ‘breakfast eaters’ based on whether they had breakfast on all three typical days as recorded from the 24 h recalls. On the other hand, participants who did not have any portion of food and/or drink at breakfast on at least one out of three days were characterized as ‘breakfast skippers’.
Anthropometry and physical examination
Participants underwent a physical examination by two trained members of the research team. The protocol and equipment used were the same in all schools. Weight was measured to the nearest 10 g using a Seca digital scale (Seca Alpha, Model 770, Hamburg, Germany). Students were weighed without shoes in the minimum clothing possible. Height was measured to the nearest 0·1 cm using a commercial stadiometer (Leicester Height Measure, Invicta Plastics, Oadby, UK) with the participant standing barefoot, keeping shoulders in a relaxed position, arms hanging freely and head in the Frankfort horizontal plane. The cut-off points of the International Obesity Task Force( Reference Cole, Bellizzi and Flegal 18 , Reference Cole, Flegal and Nicholls 19 ) were used to categorize participants as ‘underweight’, ‘normal weight’, ‘overweight’ or ‘obese’. Waist circumference was measured to the nearest 0·1 cm with the use of a non-elastic tape (Hoechstmass, Germany) with the student standing, at the end of a gentle expiration. Furthermore, one well-trained and experienced female paediatrician in each prefecture determined pubertal maturation (Tanner stage) after thorough visual inspection of breast development in girls and genital development in boys( Reference Tanner 20 ).
Biochemical indices
Blood samples were obtained between 08.30 and 10.30 hours after a 12 h overnight fast. Part of the blood was collected in test-tubes with no added anticoagulant, where it was allowed to clot for approximately 2 h, as this was designated for serum separation. Clotted blood was centrifuged at 3000 rpm for 15 min and the collected serum was divided into aliquots and stored at −80°C.
Fasting glucose (GF) was determined using a commercially available enzymatic colorimetric assay (Roche Diagnostics SA, Vasilia, Switzerland). Fasting insulin (IF) was determined using a chemiluminescence immunoassay (Kyowa Medex Ltd, Minami-Ishiki, Japan) for Siemens Diagnostics USA. Hyperinsulinaemia was considered when children had blood insulin values of >20 μU/ml( Reference Kratz, Ferraro and Sluss 21 ). Insulin resistance was assessed through homeostasis model assessment( Reference Matthews, Hosker and Rudenski 22 ), calculated using GF and IF, as follows: HOMA-IR=[IF (μU/ml) × GF (mmol/l)]/22·5, which is a valid tool for assessing insulin resistance in children and adolescents( Reference Conwell, Trost and Brown 23 ). As no widely acceptable definition has been proposed for insulin resistance, we used a threshold for HOMA-IR (HOMA-IR >3·16)( Reference Conwell, Trost and Brown 23 ) which is validated in children and adolescents.
Socio-economic, perinatal and other information collected from the parents
Data on the socio-economic background of the families were collected from the parents (most preferably from the mother) during scheduled face-to-face interviews at school. For those parents not able to attend the meetings (approximately 5 % of the total sample), data were collected via telephone interviews. The socio-economic data included parental educational level (years of education); home square metres (m2); number of family members living at home; number of cars owned by the family; and home ownership. All aforementioned data were grouped, scored and combined for the development of an index of SES (SES index) as indicated in Table 1. The total score of the SES index was obtained by summing the scores for each index component. The values of the total SES index score ranged between 0 and 13, with higher values indicating higher SES of the family. The self-reported weight and height of the mother and the father were also recorded. Paediatric records were used to obtain data about children's birth weight.
Scoring of the index of socio-economic status (SES index)
Physical activity assessment via step count
To assess step count as an objective estimate of physical activity, study participants were provided with and instructed to wear a waist-mounted pedometer (Yamax SW-200 Digiwalker, Tokyo, Japan) for one week, i.e. from Monday to Sunday. The pedometer was positioned according to the manufacturer's instructions on the right of the waistband, vertically aligned with the patella. Children were instructed to wear the pedometer from the time they woke up in the morning until the time they went to bed at night (except when taking a shower, bathing or swimming) and to record their total number of daily steps displayed by the pedometer in a diary template before bedtime.
Statistical analysis
Dietary component derivation
Principal components analysis (PCA) was used to identify dietary patterns. The Kaiser–Meyer–Olkin (KMO) criterion was applied and it was equal to 0·52. The orthogonal rotation (varimax option) was used to derive optimal non-correlated components (dietary patterns). To decide the number of components to retain, the Kaiser criterion (eigenvalues > 1) was used. Factor loadings represent the correlations of each food or food group with the dietary pattern score. Higher absolute values of factor loadings indicate that the food or food group predictor contributes most to the construction of this particular component. The dietary components (patterns) were named according to the factor loadings of those foods or food groups correlated most with the component (factor loadings > |0·4|). When a behaviour had a factor loading value >|0·4| in more than one component, then this food or food group was used to name the component for which it had the higher factor loading value.
Descriptive and other statistical analyses
Categorical variables were presented as frequencies. Associations between categorical variables were examined by using the χ 2 test and the two-sample z test for proportions whenever appropriate. Multiple linear regression analysis was used to evaluate the associations between insulin resistance (HOMA-IR > 3·16), breakfast consumption habits and dietary patterns derived from PCA, which were treated as continuous variables. More specifically, three different models were applied: model 1 was unadjusted, model 2 was adjusted for sex and Tanner stage, and model 3 was adjusted for sex, Tanner stage, waist circumference, mean parental BMI, SES index score and birth weight. The results from the linear regression models are presented as standardized beta coefficients and the level of significance was defined at P < 0·05. Breakfast consumption was used as a categorical variable with 0 representing breakfast skippers and 1 representing breakfast eaters. Moreover, participants’ dietary pattern scores were categorized into tertiles so that, for each dietary component, tertile 3 consisted of persons whose dietary intake most adhered to that particular pattern. Based on the statistically significant associations provided by the linear regression models, logistic regression analyses were performed to evaluate the association between the tertiles of each dietary component and the probability of insulin resistance. The results of logistic regression models were presented as odds ratios and 95 % confidence intervals. Data were analysed using the statistical software package SPSS version 13·0.
Results
Full socio-economic, demographic, parental BMI, perinatal, clinical, biochemical and dietary data were collected for 1912 children (50·1 % females and 49·9 % males). Regarding the descriptive characteristics of the study sample, Table 2 illustrates that the total prevalence of overweight and obesity was 30·8 % and 11·7 %, respectively, with a significantly higher prevalence of obesity in boys than in girls (14·2 % v. 9·2 %, P < 0·05). Furthermore, Table 2 shows that the prevalence of insulin resistance (i.e. HOMA-IR > 3·16) was significantly higher in girls than in boys (33·6 % v. 23·4 %, P < 0·05).
Anthropometric, clinical and biochemical characteristics of the study population and prevalence of central obesity, insulin resistance and hyperinsulinaemia: representative sample of 1912 Greek schoolchildren aged 9–13 years, Healthy Growth Study, May 2007–June 2009
HOMA-IR, homeostasis model assessment of insulin resistance index; WC, waist circumference.
*P < 0·05 for sex differences based on Student's t test.
†P < 0·05 for sex differences based on the χ 2 test and the two-sample z test for proportions for multiple comparisons, where appropriate.
Table 3 summarizes the loadings of the factors retained from the PCA. The value of the KMO criterion indicates that the dietary variables entered in the analysis were strongly inter-correlated and that PCA could be correctly used for assessing ‘healthy’ or ‘unhealthy’ dietary patterns. The PCA indicated five dietary components explaining 60·7 % of the total variance with regard to the examined variables. These components were characterized as follows: fried potato, red meat and sugared beverages (component 1); processed meats and cheese (component 2); margarine, sweets and savoury snacks (component 3); legumes and fruits (component 4); and higher egg and lower fish consumption (component 5).
Factor loadings for the five dietary patterns derived from principal components analysis regarding certain food groups in the study population: representative sample of 1912 Greek schoolchildren aged 9–13 years, Healthy Growth Study, May 2007–June 2009
Variable with the highest factor loading (>|0·4|) within the component is shown in bold.
*Beef, pork, lamb, goat meat and meat balls.
†Cola-type soda, orange and sprite soda.
‡Salami, pariza, turkey ham, pork ham, sausages, bacon and also in croissants and in savoury pies.
§Hard and semi-hard cheese (gouda/edam, etc.), soft cheese (‘anthotyro’, a type of cheese similar to ricotta) and also in pies (e.g. cheese pie).
||Sweets high in sugar and low in fat (candies, lollipops, jellies, traditional fruit in heavy syrup) and salty snacks such as chips, cheese puffs and popcorn (not home-made).
¶Boiled or poached eggs; consumption of boiled or steamed fish.
Table 4 presents the associations derived from the three different models of the multiple linear regression analysis between HOMA-IR (dependent variable), the dietary components and regular breakfast consumption. Dietary component 3 was positively associated with HOMA-IR (β = 0·15, P < 0·001; model 1) and even after adjusting for several potential confounding factors in models 2 and 3, this association remained statistically significant (β = 0·17, P < 0·001 and β = 0·08, P < 0·001, respectively). Dietary component 5 was also found to be positively associated with HOMA-IR in model 1 (β = 0·09, P = 0·02) but lost its significance after adjusting for several confounders (models 2 and 3). Regular breakfast consumption was inversely associated with HOMA-IR in model 1 and model 2, but lost its significance in model 3 for each dietary component (Table 4).
Associations of different dietary patterns and breakfast consumption with HOMA-IR in the study population: representative sample of 1912 Greek schoolchildren aged 9–13 years, Healthy Growth Study, May 2007–June 2009
HOMA-IR, homeostasis model assessment of insulin resistance index; β, standardized beta coefficient; WC, waist circumference; SES, socio-economic status.
Significant results are shown in bold font.
Model 1 = adjusted for breakfast consumption. Model 2 = adjusted for breakfast consumption, sex and Tanner stage. Model 3 = adjusted for breakfast consumption, sex, Tanner stage, WC, parental BMI, SES index, birth weight and physical activity.
Breakfast consumption = eating v. skipping. Dietary pattern 1 = higher consumption of fried potatoes, red meat and sugared beverages. Dietary pattern 2 = higher consumption of processed meat and cheese. Dietary pattern 3 = higher consumption of margarine and higher consumption of sweets and savoury snacks. Dietary pattern 4 = higher fruit and legumes consumption. Dietary pattern 5 = higher egg consumption and lower fish consumption.
The logistic regression analysis summarized in Table 5 showed that the third tertile of dietary component 3 was significantly associated with the incidence of insulin resistance. Specifically, children who showed greater adherence to dietary component 3 (highest tertile) were 2·51 (95 % CI 1·30, 4·90) times more likely to have insulin resistance (based on HOMA-IR >3·16) compared with children in the first tertile, after controlling for several potential confounding factors.
Logistic regression analysis examining the correlation between tertiles of dietary pattern 3 and the likelihood of insulin resistance in the study population: representative sample of 1912 Greek schoolchildren aged 9–13 years, Healthy Growth Study, May 2007–June 2009
HOMA-IR, homeostasis model assessment of insulin resistance index; WC, waist circumference; SES, socio-economic status.
Significant results are shown in bold font.
Dietary pattern 3 = higher consumption of margarine and higher consumption of sweets and savoury snacks.
Model 1 = adjusted for breakfast consumption. Model 2 = adjusted for breakfast consumption, sex and Tanner stage. Model 3 = adjusted for breakfast consumption, sex, Tanner stage, WC, parental BMI, SES index, birth weight and physical activity.
Discussion
The prevalence of insulin resistance, obesity and diabetes in children and adolescents is constantly increasing( Reference van Vliet, Heymans and von Rosenstiel 24 ), with overweight and obese children being more insulin resistant than normal-weight children. It is evident that these conditions increase the risk of developing the metabolic syndrome, CVD and diabetes in adulthood( Reference Magnussen, Koskinen and Chen 25 ). Therefore it is essential to identify factors related to diet and lifestyle in childhood as an attempt to modify the risk of unfavourable health outcomes later in life( Reference Halfon, Verhoef and Kuo 26 ).
In terms of anthropometric indices, concerning the prevalence of overweight and obesity shown in Table 2, our study results are in accordance with proportional results from previous studies in both European( Reference Brug, van Stralen and Te Velde 27 ) and Greek children( Reference Manios, Yiannakouris and Papoutsakis 28 – Reference Magkos, Manios and Christakis 30 ), introducing obesity levels of about 5–14 % and overweight levels of about 20–31 %.
The PCA conducted in the present study revealed predominantly five dietary components. Out of these, only component 3 comprising higher consumption of margarine, sweets (i.e. candies, lollipops, jellies, traditional fruit in heavy syrup) and savoury snacks (i.e. chips, cheese puffs and not home-made popcorn) was significantly associated with HOMA-IR after adjustment for several potential confounders. This dietary behaviour is very common in children and adolescents and has been previously described in studies concerning the food preferences of children and adolescents, which showed that nearly 40 % of total energy consumed by 2–18-year-olds was ‘empty calories’ coming from desserts, added sugars, whole milk and pizza( Reference Reedy and Krebs-Smith 31 ).
Studies concerning dietary fat intake and insulin resistance have shown inconsistent results, mainly due to several study flaws that limit the validity of their findings( Reference Galgani, Uauy and Aguirre 32 ). The main outcome coming from correlations between the quality of fat intake and insulin resistance is either that there is no significant association or that there is an inverse association between saturated fat consumption and insulin sensitivity( Reference Levy-Marchal, Arslanian and Cutfield 33 ). It seems that diets rich in SFA reduce insulin sensitivity in healthy adults( Reference Vessby, Uusitupa and Hermansen 34 ), which is in line with our findings showing that a dietary behaviour that included foods rich in saturated fat (chips and cheese puffs) is significantly associated with insulin resistance. However, these previous studies were conducted mainly in healthy or diabetic adults and only one relevant study in children has shown that increased total fat intake, regardless of its quality, is inversely associated with insulin sensitivity in black but not in white children( Reference Weigensberg, Ball and Shaibi 35 ).
Additionally, it was shown previously that sugar intake in the form of sugar-sweetened beverages was associated with insulin resistance in adolescents( Reference Kynde, Johnsen and Wedderkopp 12 , Reference Harrington 36 ). Data from children and adolescents showed that total sugar intake is associated with insulin resistance in girls, but not boys( Reference Kynde, Johnsen and Wedderkopp 12 , Reference Harrington 36 ). It is already known that the short absorption time that follows the consumption of sugar may impair blood glucose control, which may result in hyperinsulinaemia and peripheral insulin resistance( Reference Harrington 36 ). Our results are in accordance with the above-mentioned findings as the dietary behaviour which was related to insulin resistance included candies, lollipops, jellies and traditional fruit in heavy syrup, all foods with a high glycaemic index.
Regarding breakfast, it seems that many adolescents in Western countries skip breakfast with a prevalence reaching even 34 %( Reference Rampersaud, Pereira and Girard 37 ). Skipping breakfast has been associated with overweight and obesity, as well as increased fasting insulin and insulin resistance in children and adolescents( Reference Farshchi, Taylor and Macdonald 4 , Reference Smith, Gall and McNaughton 6 , Reference Sese, Jimenez-Pavon and Gilbert 7 , Reference Moschonis, Kalliora and Kostarelli 38 ). In our study there was a strong inverse association between breakfast eating and insulin resistance even after adjustment for gender and Tanner stage, which is in accordance with the aforementioned studies. However, this association lost its significance when parental BMI, SES index, waist circumference and birth weight were entered in the multivariate analysis, as additional covariates. It is already evident that parental weight status, children's birth weight and most socio-economic data constituting the SES index, like parental education level, are associated with childhood obesity( Reference Moschonis, Tanagra and Vandorou 14 , Reference Birbilis, Moschonis and Mougios 39 ). Additionally, childhood obesity assessed through waist circumference and other anthropometric indices is associated with insulin resistance and specifically HOMA-IR( Reference Manios, Kourlaba and Kafatos 40 , Reference Manios, Moschonis and Kourlaba 41 ). Therefore, according to our data, breakfast consumption is inversely associated with insulin resistance. However this association is confounded by indices related to childhood obesity.
The current study has both strengths and limitations. The Healthy Growth Study was a large-scale epidemiological study conducted in a representative sample of children from four prefectures within the wider region of Greece. Furthermore, the adjustment for specific confounders resulted in the extraction of more accurate results. However, as the study has a cross-sectional design, it is not suitable for interpreting causal relationships. Additionally, use of parental BMI coming from self-reported height and weight as a confounder could be another limitation. Finally, it should be noted that use of self-reported 24 h recalls is a subjective measure of dietary intake.
Conclusion
A dietary pattern comprising increased consumption of margarine, sweets (candies, lollipops, jellies) and savoury snacks (chips, cheese puffs) was positively associated with insulin resistance in children aged 9–13 years, while breakfast consumption was inversely associated with HOMA-IR, but lost its significance after adjustment for indices related to childhood obesity. This finding could be taken into consideration when developing public health prevention programmes in order to reduce cardiometabolic risk factors and future unfavourable health outcomes in adulthood. However, more studies are needed in order to explore the association of dietary patterns with insulin resistance in children.
Acknowledgements
Sources of funding: This research has been co-financed by the European Union (European Social Fund – ESF) and Greek national funds through the Operational Programme ‘Education and Lifelong Learning’ of the National Strategic Reference Framework (NSRF) – Research Funding Programme ‘Heracleitus II. Investing in knowledge society through the European Social Fund’. Neither the European Union nor the Greek national funding had any role in the design, analysis or writing of this article. Conflicts of interest: There is no conflict of interest for any of the authors. Authors’ contributions: K.K., analysis and/or interpretation of data, drafting/revision of the manuscript, approval of the final version of the manuscript. G.M., conception and design of the study, collection, analysis and/or interpretation of data, drafting/revision of the manuscript, approval of the final version of the manuscript. A.-A.B., analysis and/or interpretation of data, drafting/revision of the manuscript. C.L., interpretation of data, drafting/revision of the manuscript, approval of the final version of the manuscript. G.P.C., interpretation of data, drafting/revision of the manuscript, approval of the final version of the manuscript. Y.M., conception and design of the study, interpretation of data, drafting/revision of the manuscript, approval of the final version of the manuscript.
