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Aerobic fitness is considered to be an important health marker. In adults, strong independent associations have been reported between aerobic fitness and cardiovascular disease mortality1 and risk status.2 In children and adolescents, weak-to-moderate associations have been reported between aerobic fitness and cardiovascular disease risk status,3 with some studies (but not all) showing that these relationships persist even after controlling for fatness.4 Furthermore, aerobic fitness and cardiovascular disease risk factors track moderately well from childhood and adolescence into adulthood.5 From a public health perspective, this evidence highlights the need for aerobic fitness testing to be included as part of health screening and monitoring systems, but it also begs the question—is there a threshold level of aerobic fitness associated with low cardiovascular disease risk in children and adolescents? At present, there are no agreed threshold levels for aerobic fitness. However, by using a large pooled dataset of children and adolescents from four European countries, the linked study by Adegboye and colleagues (see pp 00)6 proposes age and sex-specific threshold levels for aerobic fitness associated with a clustering of cardiometabolic risk factors.
That study examined threshold levels for aerobic fitness using cross-sectional data on 4500 randomly selected 9 and 15-year olds from Denmark, Estonia, Norway and Portugal, who were tested as part of the European Youth Heart Study. Aerobic fitness, operationalised as mass-specific peak oxygen consumption (V̇O2peak), was estimated using a maximal cycle ergometer test. Cardiometabolic risk was operationalised as a sex and age-specific composite risk score, calculated as the mean of five standardised risk factors—systolic blood pressure, triglycerides, total cholesterol/high-density lipoprotein ratio, homeostatic model assessment of insulin resistance and the sum of four skinfolds. A composite risk score of greater than 1 SD defined high cardiometabolic risk. Receiver operating characteristic curve analyses identified that the ‘optimal’ threshold levels for aerobic fitness that best discriminated between low and high cardiometabolic risk were: 43.6 ml/kg per minute (9-year-old boys); 46.0 ml/kg per minute (15-year-old boys); 37.4 ml/kg per minute (9-year-old girls) and 33.0 ml/kg per minute (15-year-old girls).
Despite differences in methodological approaches used to identify threshold levels and differences in outcome measures and sampling characteristics (eg, age, country), these threshold levels are similar to those previously proposed in the literature.7,–,9 Given these results, it seems that the identification of children and adolescents who fail to meet these threshold levels for aerobic fitness may help detect those at increased cardiometabolic risk. However, longitudinal studies are required to determine whether children with low aerobic fitness (ie, below the threshold levels) have a higher incidence of cardiometabolic diseases later in life compared with children with high aerobic fitness (ie, above the threshold levels).
These results raise a number of questions, two of which will be considered here, and have implications for future research. First, how useful is aerobic fitness diagnostically? Whereas directly measured V̇O2peak is generally accepted as the ‘gold standard’ measure of aerobic fitness in children and adolescents, it is complicated and time-consuming to measure, requiring sophisticated laboratory procedures and is therefore not practical for mass testing. Even though the maximal exercise test protocol described by Adegboye et al6 can be performed in the clinical setting with minimal equipment, it too is impractical for mass testing. On the other hand, field tests of aerobic fitness (eg, distance runs) would be diagnostically more useful, given that they are simple, cheap, reliable and reasonably valid alternatives, and are widely administered to school children. However, field tests are affected by factors other than aerobic fitness (at best ∼50–60% of the variance in them is explained by the variance in V̇O2peak), such as anaerobic capacity, fat mass and psychosocial factors.10 Furthermore, it is important to remember that V̇O2peak can be estimated in different ways, and special care must be taken when comparing V̇O2peak values with threshold values that have been estimated using different test protocols and different prediction equations. Future studies are required to compare the accuracy of predicting cardiometabolic risk between different types of aerobic fitness tests.
Second, in terms of interventions, how amenable is children's aerobic fitness to change? Whereas aerobic fitness is influenced by several factors (eg, genetics, age, sex, fat mass, etc), it is mainly determined by habitual physical activity.11 Children's V̇O2peak typically improves by 5–6% in response to aerobic training (with the magnitude of change dependent on initial fitness level),12 which is equivalent to a change in V̇2peak of approximately 0.3 SD using pooled data from Adegboye et al.6 This suggests that children and adolescents with low aerobic fitness will typically experience small improvements in aerobic fitness in response to an aerobic activity intervention, which may not be enough to increase their fitness above the threshold level for low cardiometabolic risk. Future studies are needed to determine whether increased aerobic activity will improve children's cardiometabolic health.
It is also important to remember that these threshold levels for aerobic fitness relate only to cardiometabolic health and not to other components of health such as skeletal health and mental health. Because fitness components (eg, aerobic fitness, muscular fitness) relate in different ways to different health outcomes, future studies should examine the accuracy of different fitness components for identifying children and adolescents ‘at risk’ of skeletal and mental health outcomes. Until then, the study by Adegboye et al6 provides good evidence that aerobic fitness testing should be included in paediatric health screening and monitoring systems.
Competing interests None.
Provenance and peer review Not commissioned; externally peer reviewed.