The CVD risk score was significantly higher among males and females who did not meet versus those who did meet the CRF standards (Fig. 1). Logistic regression analysis showed that males who met the CRF standards (≥42.0 mL·kg−1·min−1) had a significantly increased odds ratio (OR) of having a low CVD risk when compared with males with CRF levels below this value (12-15 yr: OR = 5.17, 95% CI = 2.44-10.95; 16-19 yr: OR = 3.78, 95% CI= 2.06-6.92; all P < 0.001). Females aged 12-15 yr who met the CRF standard (12 yr old: ≥37.0 mL·kg−1·min−1; 13 yr old: ≥36.0 mL·kg−1·min−1; 14 yr old: ≥35.0 mL·kg−1·min−1) did not have a statistically significant higher OR of having a low CVD risk than those with CRF levels below these values (OR= 1.56, 95% CI = 0.72-3.39; P = 0.254). Females aged 16-19 yr who met the CRF standard (≥35.0 mL·kg−1·min−1) did not have a statistically significant higher OR of having a low CVD risk than those with CRF levels below these values OR of having a low CVD risk than those with CRF levels below this value (OR = 2.00, 95% CI = 0.76-5.23; P = 0.158).
ROC analysis showed a significant discriminating accuracy of CRF for identifying low versus high CVD risk score in males but not in females (Fig. 2). In males, the optimal pair of true-positive and false-positive rates was 58% and 13% in those aged 12-15 yr and was 82% and 46% in males aged 16-19 yr. The respective CRF values at these points were 44.1 and 40.3 mL·kg−1·min−1 in the younger and the older groups, respectively. In females aged 12-15 yr, the optimal pair of true-positive and false-positive rates was 64% and 44%, respectively, and 84% and 66% in females aged 16-19 yr. The respective CRF values at these points were 36.0 and 35.5 mL·kg−1·min−1 in the younger and the older groups, respectively.
The results of the present study suggest that adolescents who meet the CRF standards established by FITNESSGRAM have a significantly lower CVD risk score compared with those who do not meet the standards. The results did not change when the analyses were performed using CRF cutoffs calculated by ROC curve analyses. These findings reinforce the clinical validity of the FITNESSGRAM criterion-referenced CRF standards for adolescentsin relation to their overall cardiovascular risk profile, measured by a composite score including blood pressure, total body fat, insulin resistance, TG, and the TC/HDLc ratio. From a public health point of view, these findings emphasize the importance of avoiding low CRF states in youth for purposes of early CVD prevention. Clinical screening and monitoring as well as epidemiologic surveillance of youth who fail to meet FITNESSGRAM CRF standards can help identify youth at increased risk of cardiometabolic diseases who could benefit from intervention programs.
Several studies have shown that children and adolescents with higher levels of CRF also have a more favorable cardiovascular profile compared with their unfit counterparts (32). Data from the Swedish and Estonian part of the European Youth Heart Study (EYHS) showed that children aged 9-10 yr who met the FITNESSGRAM standards were three times more likely to have a lower CVD risk score when compared with those who did not meet the standards (39). In the present study, males aged 12-15 yr who met the FITNESSGRAM standards were 5.17 times more likely to have a low CVD risk when compared with those who did not meet the standards. Similarly, males aged 16-19 yr who met the standards were 3.78 times more likely to have a low CVD risk when compared with those who did not meet the standards. For females, the results from the present study are less clear, similar to a study in Spanish adolescents (29). These sex differences may be because females might be protected from the deleterious consequences of having a low CRF by the role of estrogens (14). This is consistent with other studies on young people (20,23,31). Furthermore, rapid and dynamic changes in various metabolic systems, including hormonal regulation, changes in body fat content and body fat distribution, and transient changes in insulin resistance, are known to occur during growth and puberty (12). Thus, sex differences in pubertal development also may play a role in explaining these results. The results of the present study indicate that adolescents who fail to meet established criterion-referenced CRF standards are more likely to already express high CVD risk profiles and that the strength of the low CRF-high CVD risk association is greater in males than in females.
In addition, although physicians have ready access to the criterion measures of CVD (e.g., fasting blood samples, blood pressure), some of these procedures are invasive, expensive, and not suitable for CVD risk screening among large populations of children and adolescents. The information collected in schools as part of the FITNESSGRAM program is not diagnostic but could be used as part of surveillance and/or screening systems to help detect youth at high CVD risk and as such has potentially large clinical and public health implications. Unfortunately, despite the important information provided by a simple assessment of CRF, such data rarely become available or are not used by public health and clinical practitioners for purposes of health promotion or disease prevention.
The high proportion of adolescents who failed to meet the CRF standards for a Healthy Fitness Zone in this study is of concern. Approximately one third of adolescents aged 12-19 yr in the United States fail to meet the required levels of CRF. These data are comparable with other population-based studies performed among Swedish (∼40%), Estonian (∼43%), Portuguese (∼30%), and Spanish (∼20%) children and adolescents (30,31,38). Longitudinal studies show that CRF tracks moderately from youth to adulthood (27,46) and that CRF levels during adolescence predict adult total and central body fatness (15), blood pressure (4), blood lipids (22), prevalence of metabolic syndrome (16), and large-artery stiffness (5). Collectively, these data indicate that adolescents with low levels of CRF represent an important public health concern because they already express increased levels of CVD risk factors and seem to be at a higher risk for the development of cardiometabolic morbidity and mortality (25).
CRF is influenced by several factors, including body fatness, age, sex, health status, and genetics, yet its principal modifiable determinant is habitual physical activity (6). There is a positive association between objectively measured physical activity and CRF in children and adolescents (19,40). Recent findings from observational studies also have shown an inverse association between physical activity measured by accelerometry and both individual and clustered CVD risk factors in youth (41), which highlight the pivotal role of physical activity in both increasing CRF and improving CVD profile. Intervention studies indicate that it is possible to improve CRF levels in youth by ∼10% through aerobic training (36). Moreover, aerobic exercise interventions have been shown to decrease visceral fat, insulin resistance, and TG levels among overweight youth (18,24). Carefully controlled randomized clinical trials are needed to clarify the dose-response relationship between physical activity and CVD risk in youth. Interventions to improve physical activity participation among youth who fail to meet CRF standards are warranted as part of programs aimed at the early prevention of CVD risk.
In addition to physical activity, body fatness influences both CVD risk and CRF, and it might confound the association between the two. Previous studies have shown that CRF is linked to CVD risk in youth even after accounting for the role of fatness (27). In the present study, we did not attempt to assess the potential confounding effect of fatness but focused on assessing the association between CRF categories and an overall measure of CVD, which necessarily includes an index of fatness. In real life, CRF tests are measures of physical work performance, which includes movement of all the body mass, including any excess fat. However, the interpretation of a CRF variable expressed as milliliters per kilogram per minute might be different if the focus is excess adiposity instead of performance.
Limitations of the present study include its cross-sectional nature, the limited subsample of adolescents with complete CVD and CRF data, and the use of a submaximal test to extrapolate CRF levels. However, the CRF estimation protocol applied in NHANES correlates well with direct oxygen uptake measurement (21,48), and it is adequate for large-scale epidemiological and surveillance studies. The positive predictive value of the CRF test can be affected by "false-positives," which would be influenced by measurement error or, potentially, by lack of motivation or pacing if field tests are used. Also, it might be argued that the reliability of V˙O2max estimates based on field test, as currently administered in school settings, will be lower if compared with estimates derived from laboratory-based tests that are performed under standardized and controlled conditions such as those used in the present study. A field validation is needed to evaluate if the accuracy with which high CVD risk can be predicted differs between field tests and laboratory-based submaximal tests. Such a study will have public health relevance because field tests are commonly used for assessing and monitoring V˙O2max levels in the community. Finally, the results of this study are based on a sample with slightly lower (6%) body mass index and higher (1%) CRF than the overall sample of adolescents examined in NHANES, which could result in an underestimation of the association between low CRF and high CVD risk.
The inclusion of a relatively large number of participants, and their racial and ethnic diversity, and the use of a robust outcome variable encompassing the overall CVD risk of every individual are notable strengths of this study.
Felipe Lobelo was supported by the American College of Sports Medicine's Paffenbarger-Blair Fund for Epidemiological Research on Physical Activity. Jonatan R. Ruiz was supported by the Spanish Ministry of Education (EX-2007-1124) and the European Union in the framework of the Public Health Programme (ALPHA project, Ref:2006120). The authors thank Gaye Groover Christmus, M.P.H., for editing the manuscript. The results of the present study do not constitute endorsement by the American College of Sports Medicine.
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