Participants selected for this cross-sectional study were enrolled in the UP&DOWN study.16 In brief, the UP&DOWN study was a 3-year longitudinal study designed to assess the impact of physical activity and sedentary behaviours over time on health indicators as well as to identify the psychoenvironmental and genetic determinants of physical activity in a Spanish sample of children and adolescents. Data from the present study were collected from September 2011 to June 2012. Children and adolescents were recruited from schools in Cádiz and Madrid, respectively. A total convenience sample of 2225 participants aged 6 to 18 years participated in the UP&DOWN study although the present study includes 2198 participants (1060 girls) with complete data at baseline on anthropometric and body composition indices. According to the database of the Spanish Institute of National Statistics, our sample size represented the 50% (n=1179) and 5% (n=1019) of the total population size of school children and adolescents, respectively, with a 3% percentage of error for both sample sizes. Blood sampling was randomly performed in one-fourth of the recruited children and adolescents (514; 244 girls).

Parents and school supervisors were informed by letter about the nature and purpose of the study, and written informed consent was provided. The study protocols were approved by the Ethics Committee of the Hospital Puerta de Hierro (Madrid, Spain), the Bioethics Committee of the National Research Council (Madrid, Spain) and the Committee for Research Involving Human Subjects at University of Cádiz (Cádiz, Spain).

Anthropometric and body composition indices and cardiovascular markers are sex dependent; therefore, boys and girls were analysed separately. Descriptive data are shown as mean and SD unless otherwise indicated. The Student's t-test was used to test differences in sociodemographic and clinical characteristics by sex, except for the Tanner stage, which was analysed by χ² test. Pearson’s correlation coefficients were obtained to analyse the association between NC and the other anthropometric and body composition indices (BMI, WC, WHtR, BFP and FMI) and the association between all anthropometric and body composition indices and single CVD risk factors. To test the association of single and cluster CVD risk factors with anthropometric and body composition indices, adjusted multiple linear regression analyses were performed. Single and clustered CVD risk factors were entered as dependent variables and anthropometric and body composition indices as independent variables in separate models.

To predict specific NC cut-off values for overweight and obesity, a linear regression analysis following a stepwise selection procedure for the derivation of prediction equation models including NC as dependent variable and BMI, WC and BFP as independent variables in separate models were performed. In the prediction equations, Cole’s BMI,21 Katzmarzyk’s WC25 and McCarthy’s BFP26cut-off values for overweight/obesity to obtain the corresponding NC cut-off values were used. These are optimal threshold values for predicting high-risk groups to be used in the white population worldwide. All models were controlled for age and Tanner stage. The statistical analyses were performed using the Statistical Package for the Social Sciences V.22.0 (IBM SPSS Statistics for Windows), and the statistical significance was set at p<0.05.

BMI is the most common anthropometric index used to determine overweight and obesity not only in children and adolescents but also in adults. BMI has become a very popular screening tool for overweight and obesity due to its simplicity and ease. However, it is becoming increasingly clear that it is not a good proxy for regional adiposity.27 Regional deposition of fat is a better predictor of some obesity-related complications, such as metabolic disorders and CVD risk factors.28 In this regard, WC seems to be a better anthropometric index in children and adolescents.29 On the other hand, WHtR30 and FMI31 have been proposed as a marker of adiposity in children and adolescents, however, these indices have been built in analogy with BMI.

Recently, NC has been proposed as a surrogate marker of regional obesity in children and adolescents.7 8 These studies reported strong correlations between NC and BMI and WC (r>0.7). Concurring with these studies, NC was correlated with a larger number of anthropometric and body composition indices (ie, BMI, WC, WHtR, BFP and FMI). More specifically, NC was highly correlated with WC and BMI and in both sexes, which confirms its use as a screening tool for identifying overweight/obese children and adolescents. NC is a simple, quick and low-cost method that requires less effort from both the examiner and the examinee than other anthropometric and body composition methods. NC has shown very good inter-rater and intrarater reliability32 and unlike WC, NC is not influenced by the timing of measurement (eg, preprandial and postprandial period; measurement at the end of a gentle expiration as suggested for WC). Additionally, this technique does not require the patient to remove his/her clothing to obtain a more accurate measurement. Finally, the measurement of NC may be more socially acceptable and convenient for overweight and obese children, thus making this measurement more tolerable for them.

There is evidence to support that NC is a screening tool for identifying CVD in adults in worldwide.33 However, little is known about the association between NC and CVD risk factors in children and adolescents. In the present study, NC, as well as all other anthropometric and body composition indices, was associated with cardiometabolic syndrome risk factors. Androutsos et al 10 and Kurtoglu et al 9 also observed that increases in NC were associated with cardiometabolic syndrome risk factors in Greek and Turkish normal or overweight/obese children and adolescents. Moreover, Guo et al 34 showed that NC could predict prehypertension in Chinese normal weight children and adolescents, but not in those overweight and obese. It is widely accepted that obesity is associated with metabolic disorders and CVD risk factors both in youth and adulthood.35 Free fatty acid concentration is related with the development of CVD risk factors. It has been demonstrated that upper body subcutaneous fat is responsible for a much larger proportion of systemic free fatty acid release than visceral fat, particularly in obese individuals.36 Moreover, The Framingham Heart Study showed that NC was associated with CVD risk factors even after adjustment for visceral adipose tissue and BMI, suggesting that upper body subcutaneous fat may be a unique fat depot conferring additional cardiovascular risk above and beyond central body fat.37 These previous results are in line with this study findings, suggesting that thicker NC is associated with greater risk of CVD.

Inflammatory markers have been proposed as new emerging CVD risk factors.38 Indeed, persistent low-grade inflammation in children, especially in obese, may increase the risk of metabolic and cardiovascular events in later life and play a role in the pathogenesis of atherosclerosis.39 Thus, inflammatory markers have a potential interest for paediatric CVD risk factor control and future preventive strategies. Overall, obesity in children and adolescents is associated with increased CRP, C-3, C-4 and leptin levels and decreased adiponectin levels,39–41 which concurs with the results of the present study. To the best of our knowledge, no studies have been published analysing the associations between NC and inflammatory markers in children and adolescents. In addition, data with WHtR, BFP and FMI are sparse. In the present study, similar associations were found among all anthropometric and body composition indices and inflammatory markers. There is controversy in the relationship between some inflammatory markers (ie, plasma IL-6, TNF-α and visfatin) and obesity, with some studies reporting positive40 42 or no associations40 41 with obesity. In the present study, IL-6 and visfatin were associated with WHtR and FMI in both sexes and only with NC in boys. However, no associations among all anthropometric and body composition indices and TNF-α. Differences among studies might be due, as Jaleel et al 40 suggested, to differences in degrees of maturation, sex distribution and ethnic specificity. Further studies are needed to clarify this association.

A clustered CVD risk factors reflects cardiovascular health better than single CVD risk factors and might, to some extent, compensate for the day-to-day fluctuations in the single risk factors.43 Even if none of the participants suffer from clinical disease, clustered risk is certainly an undesirable condition, and it has been shown to track into young adulthood.44 The results of the current study showed that NC was associated with clustered CVD risk factors in boys and girls.

Previous studies found a strong association between some anthropometric and body composition indices and clustered CVD risk factors.11 14 43 However, to the best of our knowledge this is the first study in examining the association between NC and a cluster of CVD risk factors and also the first study in including a large number of inflammatory markers (ie, CRP, C-3, C-4, IL-6, leptin and adiponectin) as part of the cluster, as recently suggested, due to its relation to clustering of CVD risk factors at early ages.38 In addition, VO_2max was also included in the cluster as performed in previous studies43 as it is considered a CVD risk factor in children and adolescents. Andersen et al 43 previously followed this strategy and it is noteworthy that their findings were consistent with those of the present study.

Results from the present study showed that NC could be used as a screening tool to identify children and adolescents at CVD risk. Therefore, providing with sex and age-specific cut-off values to classify children and adolescents as normal weight or overweight/obese is of relevance from a public health and clinical perspective. The criteria used to classify children and adolescents as normal weight or overweight/obese were based on BMI,21 WC25 and BFP26 widely accepted cut-off values in children and adolescents. The explained variances of the full models were high (86% for boys and 80% for girls with BMI, 88% for boys and 83% for girls with WC and 80% for boys and 73% for girls with BFP), suggesting that NC cut-off values can be established according to BMI, WC and BFP values. The results show NC cut-offs that increase with age and are higher in boys than girls. In addition, cut-offs for the same age are very similar independently of the criteria used to classify children and adolescents as normal weight or overweight/obese. It is known that body fat distribution and its metabolic effects are different in men and women with sex hormones playing a role in this difference. However, a definite cause has not been established.45 These results are consistent with those reported in other study based only in BMI cut-off values.7 8 To note is that NC cut-off values from the current study were slightly lower than those of previous studies,7 8 which may be due to ethnic differences. To our knowledge, this is the first study that establishes NC cut-offs for overweight/obesity according to BMI, WC and BFP values in the same sample.