An increase in childhood obesity and related complications have been one of the greatest causes for concern worldwide for the past 30 years [1,2]. Insulin resistance in obese children could play a pivotal role as a ‘driving force’ behind the association of metabolic syndrome with other conditions, such as dyslipidemia and hypertension [3,4]. Childhood obesity is an important cardiovascular risk factor and is associated with increased rates of premature death [5]. Abdominal obesity has also been reported as a risk factor for cardiovascular and metabolic disease in younger children [6-8]. Thus, the issue of prevention and control of obesity in younger individuals is increasing in importance.

Body mass index (BMI) and waist circumference (WC) have been used as traditional diagnostic tools to measure obesity and central adiposity. Both BMI and WC are considered useful parameters to identify cardiometabolic risks. However, there are some limitations to these currently used measures of adiposity. BMI and WC are strongly linked with ethnicity, age, and sex in children and adolescents. Age- and sexspecific reference tables are required to assess an individuals’ status of adiposity [9]. Although BMI is usually well related to body fat, BMI cannot fully reflect body composition and sometimes cannot distinguish fat tissues from muscle mass, because BMI contains information for height and weight only [10]. WC is a commonly used index to diagnose abdominal obesity, defined as 90th percentile or more by age and sex. WC is height-dependent, which might lead overor underestimation of abdominal obesity according to the height of children. In recent years, the waist-height ratio (WHR), which divides the WC by height, is also being used as an alternative index for the assessment of abdominal obesity. WHR has the advantage of being both a simple and convenient measurement. It has been reported the WHR is more accurate than BMI and WC in identifying cardiometabolic risk in adults [11]. In a prior systematic review, studies from 14 different countries including Caucasian, Asian, and Central American subjects showed that a WHR of ≥0.50 covers all cardiometabolic outcomes in both men and women [12]. In children and adolescents, there are reports that WHR is more suitable than WC and BMI for indicating the percent body fat in the United States [13] and some studies demonstrate that WHR and BMI produce similar results in identifying cardiometabolic risk worldwide [14-17]. Some studies using WHR have been reported in Korea. The results of these studies showed that WHR is a similar or better index for screening abdominal obesity with or without overweight or obesity, when compared with BMI. WHR is helpful in determining children at higher cardiometabolic risk [18-21]. Some of these studies presented the cutoff values obtained [19,22].

Although previous studies of WHR have been published, the distribution of WHR by sex and age in Korean children and adolescents has not been examined. In addition, there is a lack of analysis as to whether a WHR value of 0.5 is an appropriate cutoff of abdominal obesity and cardiometabolic risk in the pediatric population. Thus, the aims of this study were (1) to identify the secular changes of WC and WHR, (2) to compare WC with the prior Korean reference, (3) to confirm the distribution of mean WHR values by age and sex, and (4) to determine if WHR cutoff value of 0.5 is an appropriate predicting factor of cardiometabolic risk in childhood, using nationally representative data in children and adolescents.

This is the first study evaluating change over time in WC and WHR in Korean children and adolescents. Mean of WC and WHR in Korean children and adolescents have been maintained without significant change between 4 waves of KNHANES over 10 years, in contrast to the previously reported trend of increasing obesity and metabolic syndrome in Korea [25]. Mean WC was lower than in the 2007 Korean National Growth Charts. Mean WHR was similar between boys and girls aged 13 years or more. Based on this, the obtained cutoff value to determine abdominal obesity was 0.48, independent of sex, which was lower than 0.5.

As interest in WC and WHR has increased, a search for reference index measurement representing the populations of specific countries has been conducted worldwide. Fig. 2 compares the mean of previously published worldwide WHR data with the data generated in this study. According to a Norwegian study [26], 6- to 8-year-old girls have longer mean WC than the Korean girls presented in this paper and the mean WC of girls aged 9 years or more were shorter in the Norwegian study. This implies that Norwegian girls have similar or higher mean WHR in age 6–8 year and lower mean WHR in age after 9 years. Compared with Norwegian boys, mean WC and WHR was greater in Korean boys at all ages except for 7 and, 16–18 years. Data from a Greek study of 12- to 17-year-old adolescents [27] showed that mean WC is longer in both sexes aged 12–14 years than in Korean adolescents and similar or shorter in Greek adolescents aged 15–18 years. Mean WHR is higher in Greek boys aged under 14 years, similar in those aged 15, 16, 18 years, and shorter in those aged 17 years. The mean WHR of Greek girls is higher in 12 years old, similar in 13–14 years old, and shorter in 15–18 years old than Korean girls. Mean WC and WHR were greater in Portuguese adolescents aged 15–18 years than in Korean adolescents [28]. Data from Thailand shows 6–12 years old girls have longer mean WC than Korean and Thai boys have similar or shorter WC except at 7 years old. Mean WHR of Thai adolescents were higher than Korean adolescents in all ages [29]. In the report for Hong Kong Chinese children aged 6–18 years, mean WC and WHR were much lower than values shown than Korean children [30]. Except for Portugal, in many countries, the WHR fluctuation does not appear to be largely significant after 13 years for men and after 11 years for women. The WHR pattern of the adolescent period seems to be similar, although there may be national and ethnic differences.

According to the results of this study, WC is lower overall than the reference data obtained from the 2007 Korean National Growth Charts, based on measurements form 1997 and 2005, when obesity in children and adolescents had already increased. The WC and WHR were not included on the 2017 Korean National Growth Chart which was established by using the data before the increase of obesity in consideration of the increase trend of obesity [31]. Therefore, considering the data showed that both mean WC and WHR have remained similar over 10 years, it may be necessary to make a new WC and WHR reference based on the 2017 Korean National Growth Charts.

The differences of WHR between boys and girls before 13 years of age exist through all 4 waves of KNHANES showing a peak at 10 years of age. Similar differences between sexes are seen in the WHR reference made using the previous 2007 growth chart [22]. These differences between sexes could be found in some other countries. In Hong Kong Chinese children, the differences between sexes were similar to those of Korean children, with a peak difference seen at 10–12 years of age [30]. Norwegian adolescents have similar differences to Korean children, and the peak difference is observed at 11 and 16 years of age [26]. WHR from Greek boys aged 12–17 years was higher than from girls [27]. In the study of Portuguese children and adolescents, 15–18 year old girls had higher WHR than boys [28]. WHR data from United Sates and Thailand did not show specific patterns or differences [29,32].

There are 2 opposing positions on the use of a universal cutoff of 0.5 for WHR. In a study by McCarthy and Ashwell, a WHR of 0.5 was proposed (‘Keep your WC to less than half your height’) as a unique cut-off to predict cardiovascular risk in individual of all ages including children [33]. Whereas, Inokuchi et al. [34] recommend age and sex specific reference values and preclude using a universal cutoff of 0.5. In previous studies in Korea, Gil et al. [22] suggested that the optimal WHR for determination of obesity was 0.51 in 6–18 year old boys and 0.48 in girls of the same age. In the study by Choi et al. [19], the optimal WHR cutoff for metabolic syndrome was 0.44 in 10- to 19-year-old boys and 0.43 in girls, which is less than the value of 0.5. These cutoffs are generated taking into account the ethnic and sex-specific features in order to improve accuracy. These studies are partially in line with Ashwell’s proposal to use a universal cutoff. We have also made efforts to find an appropriate cutoff based on the KNHANES data. In this study, the optimal WHR cutoff for abdominal obesity was 0.48 in Korean children and adolescents aged 13–18 years. The other cutoff values for cardiometabolic risk were not useful for practical clinical applications. All cutoff values obtained were lower than 0.5. Comparing with the cutoff value of 0.48 for abdominal obesity, 0.5 as candidate of universal value showed higher specificity in order to reduce false negative results. This data suggests that if WHR of a subject was 0.5 or higher, it would be more likely that the individual has abdominal obesity. However, it is necessary to detect obesity early and managed it efficiently. This is of particular importance, as abdominal obesity is strongly associated with many metabolic abnormalities, therefore, active interventions are required even at a lower WHR cutoff than 0.5.

There are several limitations on this study. First, this was a crosssectional study. Thus, it cannot provide enough evidence of predictive values of WHR for cardiometabolic risk. To compensate for these shortcomings, 4 KNHANES waves over a decade were collected and presented, but due to existing time intervals between waves, these data cannot fully explain the trends seen in children and adolescents during 2005–2015. Future prospective cohort studies are needed for more rigorous evaluations. Second, because the feature of differences between sexes on distribution of WHR, we only could not generate a reference for children between 6 to 18 years old. Additionally, to create optimal WC cutoff values for cardiometabolic risk, different reference values must be applied, depending on sex and age. Therefore, the numbers of subjects for each sex and age should be sufficient, but, present study did not provide enough numbers, and analysis was not conducted. Thus, larger-scale cohort studies should be performed. Finally, onset of puberty that could be associated with obesity and other cardiometabolic disease was not reflected in this study.

In conclusion, WC and WHR in Korean children and adolescents have shown no secular change over the past 10 years. The mean WC values of both sexes from the present study were lower than those from the 2007 Korean National Growth Charts. WHR was higher in boys than in girls but, there is no difference in adolescents over 13 years old. Considering the distribution of WHR by age and sex, the value of 0.48 as an optimal cutoff could be applied for Korean adolescents over 13 years old and be helpful to diagnose abdominal obesity. With regard to the lack of secular change in WC, it might be necessary to make a new WC and WHR reference based on 2017 Korean National Growth Charts.