Background
Overweight and obesity has been called a global epidemic by the World Health Organization [
1]. The prevalence of overweight and obesity is especially dramatic in economically developed countries [
2] and not only in adults but also in children and adolescents. In Germany for instance, 17% of adolescents aged 14 to 17 years are overweight and nearly 9% are obese [
3]. Similarly, in the United States, 18% of adolescents aged 12 to 19 years were obese in 2007/2008 [
4]. In accordance with the literature [
5‐
9], the term overweight includes obesity in this review.
Several health conditions and disorders have been attributed to being overweight in children and adolescents [
10]. For instance, overweight children and adolescents are more likely to suffer from cardiovascular, metabolic, pulmonary, skeletal or psychosocial disorders [
11]. Even if these conditions or disorders are not manifested during childhood, being overweight in childhood increases the risk of illness in adulthood [
10]. Hence, it is critical to identify risk factors for overweight in children and adolescents and to address overweight during childhood and adolescence.
Being overweight may originate from many different factors ranging from environmental influences to genetic variations [
12]. The heritability of predisposition for a high body mass index (BMI) or body fat content is between 25 and 40% [
13], which suggests that other factors such as environmental factors may also play a critical role. According to Bouchard et al. [
13], both the family environment and genetic predisposition influence the development of body fat content and distribution. Other important factors include lifestyle factors such as physical activity (PA), nonsmoking, high-quality diet, sedentary activities and normal weight [
14]. Lifestyle factors are also important in the description of the obesogenic environment that is based on the four pillars family, sport and leisure time, eating behavior and social education [
15].
Several epidemiological and intervention studies [
16,
17] have identified the role of physical activity and physical fitness for overweight in children and adolescents, and hence we focused on the role of sport during leisure time. Previous reviews [
18‐
20] provided an overview of studies on the relationship either between physical activity and overweight or between fitness and overweight in children or adolescents. Despite of the influence of physical activity and fitness similarly on health outcomes including overweight, to date results of studies on the interaction between all three parameters have not been synthesized although these parameters cannot be considered independently [
21]. In addition, most reviews omitted studies on adolescents and young adults or did not include longitudinal studies.
The purpose of this systematic review was to provide an overview of cross-sectional and longitudinal studies published in or after 2000 on physical activity, fitness, and overweight in adolescents, and to identify mediator and moderator effects in the interrelationship among these three parameters particularly considering gender differences because of the significant differences in these parameters between boys and girls [
22].
Definitions
Physical activity comprises all modes of movement caused by muscle activity resulting in increased energy expenditure [
19,
23].
Physical fitness consists of the three components muscle strength, endurance and motor ability, and is a prerequisite for completing daily activities without fatigue and for participating in leisure time activities [
24].
Overweight and obesity are defined as abnormally high fat content that may impair health and as high bodyweight (exceeding the standard measure) caused by an increased fat consumption [
11].
Discussion
The purpose of this systematic review was to provide an overview of cross-sectional and longitudinal studies published in or after 2000 on physical activity, physical fitness and overweight in adolescents, and to identify mediator and moderator effects in the interrelationship among these three parameters particularly considering gender differences. Objectivity of self-reported physical activity has been questioned because of potential over- or underestimation [
32] and thus should be considered with caution. However, because only few studies examined the interaction between physical activity, physical fitness and overweight, we combined results of objectively and subjectively assessed physical activity.
To the best of our knowledge, this article is the first review on the interrelationship between physical activity, physical fitness and overweight, and hence our results cannot be related to the literature or to other study populations. Synthesizing the interaction between all three parameters was difficult because only four studies specifically investigated this interaction. While the literature reported inconsistent results, all studies showed an interaction between these parameters. Several studies [
6,
9,
26,
32] confirmed that physical activity and physical fitness are equally important for health [
21]. In the following the results will be discussed with reviews analyzing only the relationship between two parameters, because no comparable reviews (reviews analyzing the interaction) were found.
The different strengths of the correlations between the three parameters may be at least in part attributed to the different measurements of physical activity. For instance, two studies [
6,
26] assessed physical activity via questionnaire, one via accelerometer [
32] and one via activity monitor and questionnaire [
9], and the collection period of objectively measured physical activity ranged from three [
9] to six [
32] days. In addition, the two studies that measured physical activity subjectively omitted reporting details on their measurement instruments. Further, Ortega et al. [
26] measured physical activity outside of school for only four days. While Fogelholm [
6] measured the activity during leisure-time in and outside of sports clubs, they only reported frequency and duration and not intensity or setting of physical activity. Hence, reliable and valid questionnaires assessing frequency, duration, intensity and setting of the different physical activities are still needed [
35] especially because, for instance, intensity is an important aspect in overweight prevention [
36]. Interestingly, studies that used unspecific measurement instruments for physical activity reported weak or no relationships between physical activity and overweight [
9,
26,
29]. The poor quality of physical activity measurement instruments may also explain the stronger influence of cardiorespiratory fitness than that of physical activity on overweight. The main limitation of subjective measurement instruments is potential over- and underestimation of physical activity [
32]. In comparison, objective measurement instruments for physical activity can only capture specific activities and require a high effort by the participants. For instance, subjects have to regularly wear the accelerometers or pedometers for extended periods of time and on different days.
The data on the relationship between physical activity and overweight are inconsistent. Specifically, the different levels of physical activity (measured by objective measurement methods) showed different relationships to overweight. In addition, the effect of gender on the relationship between physical activity and overweight was inconsistent. While Deforche et al. [
31], Haerens et al. [
8] and Gonzales-Suarez et al. [
28] reported no gender effect on the relationship between overweight and physical activity, other studies [
26,
29] revealed that gender affected the relationship between overweight and physical activity but that this association depended on the anthropometric measurement method used to measure overweight. Similar to our observations in adolescents, Must et al. [
19] found inconsistent results in children with a higher tendency to an inverse relationship between physical activity level and overweight in
cross-sectional studies and differences in the relationship between physical activity and overweight between boys and girls emphasizing the inconsistent state of research not only in adolescents but also in children. The previously discussed large number and poor quality of methods for measuring physical activity might explain this observation. In addition, these results show that capturing physical activity in youth is difficult. In a review of cross-sectional studies that analyzed self-reported and objectively measured physical activity in overweight children and adolescents, Winkler et al. [
36] reported inconsistent results and that the intensity of physical activity played a critical role independent of age and gender. In addition, Winkler et al. [
36] reported that physical activity was related to overweight in two
longitudinal studies, which contradicts the results of the two longitudinal studies [
5,
33] included in our review that found no relationships between physical activity and overweight. In contrast, Must et al. [
19] reviewed longitudinal studies and reported comparable results to our findings in longitudinal studies. Similarly to adolescents, the results in children are inconsistent and low physical activity level was not related to changes in BMI [
5,
19,
33]. However, according to Must et al. [
19], most cross-sectional and longitudinal studies showed no relationships between physical inactivity and overweight in adolescents and inconsistent gender specific results.
All studies included in our review observed inverse relationships between physical fitness and overweight. Because of the different measurement instruments for cardiorespiratory fitness used in these studies (shuttle run: [
5‐
7,
26,
28‐
31,
33], maximal treadmill test: [
25], maximal cycle test: [
9], cooper test: [
8], PWC 170: [
32], 2,000/1,500 m: [
27]), final comparisons are difficult. Adolescents with lower cardiorespiratory fitness were more likely to be overweight or obese than those with high cardiorespiratory fitness [
5‐
9,
25,
26,
28‐
32]. However, gender influenced the relationship between overweight and cardiorespiratory fitness. These results are in agreement with the results of other studies including those reported by Ostojic et al. [
37]. Similar results were observed for motor fitness and overweight. The measurement instruments were also inconsistent in motor fitness (Eurofit [
29,
31]: two studies; unknown [
6,
28]: two studies). While motor fitness in overweight and obese adolescents was lower than that in normal weight adolescents [
6,
28,
29,
31,
33], the influence of gender on the relationship between motor fitness and overweight was heterogeneous.
Interestingly, some studies [
6,
25,
27,
31‐
33] included weight as independent parameter in their statistical models while other studies [
5,
7‐
9,
26,
28‐
30] used weight as dependent parameter. This observation illustrates that the causality between physical activity and overweight and between physical fitness and overweight is still unclear. For instance, Metcalf et al. [
38] suggested that overweight influences level of physical activity but not vice versa. Similar data for the causal relationship between physical fitness and overweight are not available. Hence, future longitudinal studies are warranted to tease out this causal relationship. Furthermore, additional longitudinal analyses are necessary to determine the interrelationship (mediator or moderator effect) between physical activity, physical fitness and overweight which has important implications for public health policy making and developing optimal obesity prevention or treatment programs.
Limitations
Because of the small number of studies the results were not categorized based on objective or subjective physical activity measurement. In addition, studies on metabolic syndrome or cardiovascular diseases were not included (even if physical activity, physical fitness and overweight measures were used), and only studies with the primary goal of analyzing the relationship between the three parameters were included.
Competing interests
The authors declare no competing interests.
Authors’ contribution
AR performed the literature search and drafted the manuscript. FM revised the manuscript and supported the process of writing the manuscript. AW gave the final approval of the version to be published. All authors read and approved the final manuscript.