Fatness, Fitness, and Cardiometabolic Risk Factors among Sixth-Grade Youth

Data from the 1999–2002 US National Health and Nutrition Examination Survey indicated that less than one-third of 12- to 19-yr-old adolescents in the United States met fitness recommendations (28). Low aerobic fitness levels have been associated with increased risk for cardiovascular diseases (21) and diabetes mellitus among adults (23). In children and adolescents, low aerobic fitness has also been associated with cardiometabolic (5) risk factors (large waist circumference, high blood pressure, and high insulin, glucose, and lipid levels) (15). Data from 9- and 15-yr-old youths participating in the European Youth Heart Study indicated that low aerobic fitness was associated with increased waist circumference, insulin, glucose, and lipid concentrations and a clustered score that included all of the cardiometabolic risk factors (13). Interestingly, the researchers also found that these associations were independent of both physical activity (the behavior that influences fitness) and fatness (13). Extensive research has shown that the risk of cardiovascular disease and diabetes among overweight adults is ameliorated if the adult has a high level of fitness (22,32). For example, data from the Cooper Clinic in Dallas showed that unfit, lean men had double the risk of cardiovascular disease and all-cause mortality than lean men who were fit (22). Moreover, unfit lean men had a higher risk of cardiovascular disease and all-cause mortality than fit but overweight men (22). These findings suggest that fitness offers a degree of protection against cardiovascular disease and mortality even if the individual is overweight. Similar findings have been reported for women (32). Although there is a lack of evidence about the associations among childhood fitness, adiposity, and mortality, the prevalence of cardiometabolic risk factors has been shown to increase with higher levels of obesity among youth (20,31). Higher levels of obesity among children and adolescents have also been associated with low levels of aerobic fitness (16). Comparable associations among fitness, adiposity, and cardiometabolic risk factors findings have also been reported among a relatively small (296 boys and 188 girls) sample of 8- to 18-yr-old US youths (12). It is not clear if associations are evident for all cardiometabolic risk factors among adolescents. Understanding the importance of fitness, obesity status, and the prevalence of cardiometabolic risk factors among adolescents will provide key information to aid the design of strategies to reduce the prevalence of cardiometabolic risk factors among adolescents, thereby reducing disease risk. Boys have higher levels of aerobic fitness than girls (28), with some evidence of ethnic differences in fitness levels (16). Maturation differences in both fitness (28) and cardio-metabolic risk factors have also been reported (39). Thus, in examining the associations among fitness, obesity, and cardiometabolic risk factors, it is important to account for these biological and sociodemographic factors. It is also important to recognize that the majority of previous studies that have examined associations between aerobic fitness and cardio-metabolic risk factors among youth have reported fitness in relation to body mass (mL O2·kg–1·min–1 or W·kg–1) (4,30), which precludes the necessary examination of the independent effects of fitness and fatness. In light of the evidence presented above, the aim of this article was to examine the extent to which fitness and obesity status are independently associated with cardiometabolic risk factors among a large, ethnically diverse sample of sixth-grade US adolescents.