MetS in adolescents and the prevalence of its components
At present, there are no unified criteria for diagnosing MetS in adolescents, which makes it difficult to compare between studies in the literature. The prevalence of MetS differs across studies, which could be associated with differences in diagnostic criteria, age (especially IR in adolescents), gender, regions, and races . In this study, the prevalence of MetS as measured using the modified NCEP ATP III and IDF criteria was 2.3% and 1.2%, respectively. These values were lower than those summarized in literature [21–25]. The prevalence of MetS measured using this study’s criteria was 4.3% and there were significant differences observed between males and females. In particular, MetS was more prevalent in males than females, which is in line with an American study .
Differences in the percentage of abnormal measures of MetS components are a result of different diagnostic criteria. According to this study’s criteria, 10.7% of high school students had a slightly large WC, 10.8% had high TG, and 21.2% had low HDL-C. The study population had a higher percentage of individuals with abnormal measures for these three components than for the other two components (elevated BP and elevated FG).
The criteria developed in this study identified a significantly higher percentage of subjects with low HDL-C (21.2%), compared to that seen using the modified NCEP ATP III (4.2%) and the IDF (5.1%). A possible explanation could be that a unified criterion (< 40 mg/dL) was used in the modified NCEP ATP III (in which subjects were 12 to 19 years old) and the IDF (in which subjects were below 16 years old). Moreover, gender differences were not adjusted for in the cutoff values, which resulted in a low prevalence among females. This has also been observed in another study .
Differences were observed between the results from the modified NCEP ATP III and IDF in the percentage of individuals with abnormal measures for TG (9.7% vs. 2.9%) and BP (18.6% vs. 15.7%). This is because the modified NCEP ATP III criteria define elevated BP as ≧ 90th percentile after adjusting for age, gender, and height, while high TG are defined as ≧ 110 mg/dL after adjusting for age , whereas the IDF criteria define elevated BP in adults as ≧ 130/85 mmHg and high TG as ≧ 150 mg/dL.
Compared with the modified NCEP ATP III criteria adjusted by Ford et al., the percentage of subjects identified as having a large WC in this study(10.6%) was similar to that in a Korean population (9.7%) , but lower than that in an American population (19.1%) ; the percentage of subjects with elevated BP in this study(18.6%) was also similar to that in a Korean population (20.4%), but higher than that in an American population (6.9%). These differences could due to differences in the populations, which indicates the importance of establishing a large database on the WC and BP of people in different regions. The percentage of subjects with high TG in this study(9.7%) was lower than that in studies conducted in Korea (21.2%) and the USA (25.6%); the percentage of subjects with low HDL-C in this study(4.2%) was lower than that in Korea (11.6%) and the USA (19.3%); and the percentage of subjects with elevated FG was 2.9%, which is significantly lower than that in the USA (14%) and Korea (11.4%) [21, 22]. The components of MetS in adolescents must be adjusted for regional differences. This highlights the importance of the results of this study.
Moreover, according to the criteria of the modified NCEP ATP III, the IDF, and this study, about 34.4%, 28.3%, and 44.6% of adolescents, respectively, had at least one MetS component. The fewer of these components that are present during childhood, the lower the cardiovascular risk in the future . Some researchers  have emphasized that the effects of metabolic risk factor clustering are more important than diagnosing MetS in children. Based on these arguments, it is not only crucial to detect MetS in adolescents, but those present with MetS components despite not yet reaching the diagnostic criteria should receive attention as well, to provide prompt intervention and prevention [7, 28].
The predictive power of MetS components
According to the results of this study, WC has the highest predictive power, sensitivity, and specificity, regardless of gender. This is in agreement with other studies which have suggested that WC is a good indicator for predicting MetS during adolescence [25, 29, 30] and adulthood . A study on American adolescents between 12 and 19 years of age revealed that abdominal obesity was closely associated with MetS and other MetS components . Another study on 15-year old Greek teenagers showed that a WC at the 75th percentile or higher is closely related to the phenotypes of MetS ; while a study on Chinese adolescents between 11 and 16 years old indicated that WC has the best predictive power toward MetS .
To determine the optimal cutoff point for WC, Cook et al. (2003)  took into account the differences between adolescents and adults, and defined abdominal obesity as at the 90th percentile or higher; in 2004, de Ferranti et al. adopted a value at the 75th percentile or higher as their standard; afterward, in 2007, the IDF study (in which subjects were below 16 years of age)  and numerous studies [21, 22, 26] adopted a WC at the 90th percentile or higher as their standards. A Chinese study on children and adolescents between 7 and 18 years of age found that a WC at the 75th percentile and the 90th percentile was the optimal cutoff point for predicting the risk of cardiovascular risk . The optimal cutoff point for WC specified in this study was 86.8 cm for males and 76.25 cm for females, which was around the 90th percentile and similar to that of the previous studies.
In this study, TG level also had adequate predictive power, with an optimal cutoff point of 108 mg/dL for males and 104.05 mg/dL for females, similar to that of the modified NCEP ATP III criteria (< 110 mg/dL). Even though the sensitivity and specificity of HDL-C were slightly inadequate, gender differences were also observed for HDL-C in this study. These results highlight that definitions should be specific for gender. According to the criteria of the modified NCEP ATP III and the IDF, male subjects in this study had a higher prevalence of high TG, low HDL-C, elevated BP, and high FG than their female counterparts. After adjusting the criteria of this study, the prevalence of low HDL-C significantly increased for both genders, in which females (25.5%) had a higher prevalence than males (18.3%).
SBP, DBP, and FG had weaker predictive powers. In particular, FG had the weakest predictive power, which was also observed in other studies with adolescents  and adult  subjects. Furthermore, according to the IDF criteria, the prevalence of elevated BP was significantly higher in males (22.3%) than females (6.1%). This could be caused by the use of unified criteria for adults ( ≧ 130/85 mmHg) that neglects gender differences in BP.
In this study, the cutoff points for MetS components were redefined based on the results of first year high school students in Taipei. This indicates that it is necessary to take into account regional differences when determining definition criteria.
The predictive power of BMI
Based on the results of this study, BMI also had good predictive power on MetS in adolescents, after that of WC and TG. The optimal cutoff point for males was between the 80th to 85th percentile (25.6 kg/m2) for males and approximately the 90th percentile (24.65 kg/m2) for females.
The WC and BMI of adolescents are good predictive indicators of cardiovascular risk factors . A longitudinal study highlighted the close association between BMI and many other cardiometabolic risk factors, while changes in WC mainly have a stronger correlation with FG . A study on children and adolescents between 8 and 19 years of age  revealed that a high BMI has strong predictive power for cardiometabolic risk factors. In addition, the sensitivity of BMI is higher among obese adolescents while its specificity is higher among overweight adolescents .
The definitions of overweight and obesity are currently based on a person’s BMI, and their criteria differ for adolescents [37–39]. In 2007, the WHO defined overweight as having a BMI between the 85th and 95th percentiles, while obesity is defined as having a BMI greater than the 95th percentile ; in 2012, the International Obesity Task Force (IOTF) deduced the cutoff points for BMI in adolescents and adults after mathematical adjustments based on the definition of overweight and obesity in adults I [38, 39]. One study  used the three aforementioned criteria (WHO; Conde and Monteiro; IOTF) to analyze Brazilian adolescents between 12 and 20 years old; it revealed that the IOTF criteria had the best predictive power for MetS (AUC = 0.75–0.89), with a sensitivity ranging from 59.4–84.2% and a specificity ranging from 88.2% to 93.6%. In contrast, according to this study’s adjusted definition of MetS, BMI had a better predictive power (AUC = 0.915–0.926) and sensitivity (0.817–0.9) for the high school students. The differences between these results and those of the aforementioned studies could be due to the differences in the diagnostic criteria, age, region, and race in adolescents .
The cross-sectional research design of this study hindered observations of the causal relationships in the data. Moreover, the subjects were adolescents from an urban region of northern Taiwan, which limited the extrapolation of results to adolescents in rural locations as well as those with special circumstances. However, the results of this study are still valuable and can serve as a reference for defining MetS in Taipei adolescents. This study analyzed the differences in the discriminatory power of relevant risk factors as well as their optimal cutoff points, which could provide markers for early interventions in the future. Subsequent research could include the potential confounders of MetS, such as the influence of puberty and temporary IR during adolescence ; as well as taking into account more biochemical markers, and participating in cross-regional and cross-cultural cohort studies. A combination of these approaches would make the understanding of MetS in Taiwanese adolescents more comprehensive.