The prevalence of MetS among children of different ethnicities and backgrounds has been reported in few studies, and the multiple definitions of MetS make it difficult to directly compare population prevalence. Researchers using data from a nationally representative sample of approximately 1,700 adolescents found MetS prevalence to be 13% among 12- to 19-year-old adolescent Mexican Americans, 11% among non-Hispanic whites, and 2.5% among non-Hispanic blacks (4
). In our study, using the same MetS criteria, overall prevalence of MetSglucose
was 17% (9% of boys, 23% of girls) among a sample of 9- to 13-year-old African American children recruited from inner-city Oakland, California. This finding was lower than the 31% prevalence reported for 12- to 19-year-old adolescents with a BMI in the 85th percentile or higher (4
), a difference that may be attributable to the lower age of children in our sample. Our prevalence of 22% for girls was somewhat higher than the 18% prevalence for a sample of predominantly African American, mostly obese, adolescent girls aged 13 to 15 reported by others who used the same MetS criteria (11
). The prevalence among girls in our sample was double the prevalence among boys, a finding that is consistent with the sex differences we observed in body fatness (13
). Using National Health and Nutrition Examination Survey (NHANES) III data for adolescents aged 12 to 19 — a sample that is more representative of the American civilian population — others have reported a higher overall prevalence among boys than girls (3
). A follow-up study with a larger sample size will be needed to confirm the sex differences we observed for younger African American overweight and obese children.
The prevalence of MetSglucose
was twice as high among obese as among overweight children in our sample (19% and 10%, respectively). In the obese group, 10% of boys and 25% of girls met the criteria for MetSglucose
whereas in the overweight group, 7% of boys and 14% of girls met the MetSglucose
criteria. Our findings are consistent with analyses of the NHANES III data set for young people aged 12 to 19 years, in which the prevalence of MetS was reported to increase with BMI category (3
). Thus, our results are similar to previous data yet provide additional information that describes the prevalence of MetS among overweight and obese African American children and suggest the need for additional assessments to further compare boys and girls.
Although fasting glucose concentration has been included by others as a MetS component, our results suggest that insulin resistance may be more reliably used to assess MetS in African American children. In our study, only 1 participant had a fasting blood glucose concentration that exceeded the cut point of 110 mg/dL for MetS. Thus, although highly specific (100%), its use alone would have resulted in a large number (94%) of false negatives and very low sensitivity (6%). Other studies have suggested that, for African American children, insulin resistance is a strong predictor of type 2 diabetes (19
), and insulin resistance has always been included previously as a MetS component (20
). In our sample, fasting blood glucose and insulin concentrations were not significantly correlated. This is not surprising because hyperinsulinemia is known to developmentally precede the hyperglycemic phase. Both fasting insulin concentrations and HOMA-IR have been shown to be highly correlated with more invasive, exacting, and labor-intensive measures of insulin sensitivity in obese children and adolescents (21
). Also, in our sample, fasting glucose concentrations, dichotomized for MetS assessment, were poorly correlated with the other 4 dichotomized components, whereas dichotomized HOMA-IR was significantly correlated. Finally, internal consistency among the MetS components was lower when MetSglucose
was included than when MetSHOMA-IR
was included. The high levels of specificity (83%) and sensitivity (88%) observed when using the HOMA-IR cutoff of 2.5 as a MetS component suggests that, for African American children, insulin sensitivity should be used instead of glucose concentration to assess children for MetS. This conclusion is further supported by our comparison of HOMA-IR versus glucose when assessed at the same percentile for our sample (ie, the 57th percentile); efforts to identify a glucose concentration that outperformed HOMA-IR as a component were not successful.
Others have attempted to establish the best cutoff value for the HOMA-IR index as a predictor of MetS in children and adolescents. One group concluded that HOMA-IR values "close to 3" seem to be adequate (22
), whereas a second group recommended that a cutoff for HOMA-IR of 2.5 be used for obese prepubertal children (18
). We chose to use a cutoff of 2.5 for our African American participants for comparison purposes, although Cronbach α was somewhat higher using HOMA-IR 2.4 than 2.5. Our results suggest the necessity of replacing the glucose component with HOMA-IR for MetS diagnosis in this population; the MetS prevalence of 38% in the current sample, determined using HOMA-IR in place of glucose as a component, suggests that this population of children is seriously in need of intervention. A follow-up study is warranted to evaluate MetS prevalence in a larger and more diverse sample of African American children. The optimal HOMA-IR cutoff could also be confirmed in this larger sample.
Limitations of this study include restriction to low-income, inner-city African American children and exclusion of children with a BMI less than the 85th percentile when matched for age and sex. These limitations preclude comparisons among children of different races, ages, and socioeconomic backgrounds, and comparisons with lower BMI children. This is a cross-sectional analysis of data, precluding a cause-and-effect relationship.
In conclusion, among African American boys and girls living in inner-city Oakland, we found that MetS prevalence was 2 to 3 times higher for girls than for boys, even when separated according to the CDC-defined BMI categories, and was twice as high using HOMA-IR (38%) in place of glucose (17%) as a MetS component. Our data suggest that insulin resistance should be used as a MetS component in place of fasting blood glucose, because insulin resistance was more highly correlated with other MetS components, provided fewer false negatives and false positives, and was more sensitive for identifying MetS in this high-risk pediatric population.