*2.5. Obesity or Cardiorespiratory Fitness—What Does Really Matter?*

Cardiorespiratory fitness (CRF), also known as cardiorespiratory endurance, cardiovascular fitness, aerobic capacity, or aerobic fitness, refers to the "capacity of the circulatory and respiratory systems to supply oxygen to skeletal muscle mitochondria for energy production during PA" [105,106]. This is only 1 of 4 distinct health-related fitness components (CRF, muscular fitness, flexibility and body composition). Although often confused, PA and CRF are related but distinct concepts. "PA is voluntary movement produced by skeletal muscles that results in energy expenditure" [106], while "Exercise refers to a subset of PA in which the goal is to improve performance, health, or both" [106]. While CRF is having the capacity to perform or not perform a certain type of PA, PA is an action or behavior.

Over the last few years, CRF has acquired special scientific interest in the evaluation of youth´s health because it has been shown that it is a predictor of various indicators such as cardiometabolic health [107,108], premature CVD [109], academic achievement [110], and mental wellness [111]. In a cohort of overweight, obese, and control participants, Redon et al. concluded that CRF was inversely related with fasting insulin and the HOMA index, which is considered as a fingerprint for future metabolic disease [112]. In a systematic review and meta-analysis, low CRF in children and adolescents was notably associated with the development of metabolic syndrome [113].

CRF is able to be measured objectively and it can be tracked over time and compared over different populations [114]. Even though the cardiopulmonary exercise test (CPETs) is considered the gold standard method to evaluate CRF, it is not easily implemented [105]. There are also questionnaires designed for examining CRF in youth, but they are only recommended for epidemiological studies and not for estimating CRF in individuals [115]. As a result, outdoor or field procedures have been conveniently developed, among them, the 20m shuttle run test and the Cooper test [116,117] which are the most common. Additionally, there are some suitable tests for use in office settings such as the 6-Minute Walk Test and the Step Test. The Step Test could be an alternative to CPETs in order to estimate office-CRF, because it is easy to administer in limited indoor spaces and requires minimal equipment and training to be implemented [118].

CRF in youth is affected by non-modifiable factors as genetics, age, sex, race/ethnicity and prematurity and by modifiable factors as habitual PA and training, sedentary time, diet, social-economic-environmental factors and obesity. Many obese children and adolescents meet these modifiable factors, and it is shown that youth with obesity have lower CRF than their normal-weight peers [119]. Nevertheless, there is evidence that improvement in CRF in obese children and adolescents increases CV health, even among those who do not improve their body composition [120]. Moreover, overweight children and adolescents with a high fitness level (fat-but-fit subjects) have a healthier CV profile than their overweight, low fit peers and a similar profile to their normal-weight low-fit peers [121]. This suggests that high fitness levels may compensate the negative consequences attributed to body fat.

Unfortunately, in a large epidemiological study conducted in the USA, it was found that just 1 in 5 obese youth has healthy CRF [122]. Therefore, physical exercise programs aimed at improving CRF in this group of patients can be of enormous health interest. Among these programs, those that include high-intensity interval training have demonstrated an increased impact on youth's CRF [123,124].

Considering the high prognostic power of CRF, the American Heart Association proposes to measure it as a vital sign, as is done with the assessment of other risk factors such as BP, tobacco use, alcohol consumption, blood glucose or blood lipid levels [125]. Another factor to be aware of is that overweight/obese youths may have some limitations in performing moderate and vigorous PA. In this context, personalized interventions should be designed according to the subject's objective and up-to-date scientific knowledge. Therefore, the measurement of CRF in obese children and adolescents is not only of prognostic importance, but also allows for the personalization of the treatment according to the physical condition of each individual.
