Health profile with body mass index and physical fitness in Swedish adolescents: a cross-sectional study

Introduction

Physical inactivity and poor physical fitness in children and adolescents are growing public health problems globally [1]. Physical fitness has been defined as “a set of attributes one has or achieves that relates to the ability to perform physical activity”. There are various components that contribute to one’s physical fitness. The health-related components include body composition (frequently expressed as body mass index, kg/m²), aerobic fitness (also known as cardiorespiratory fitness), muscular strength and endurance and flexibility [2]. Although physical activity and physical fitness are often used interchangeably, they are not the same. However, both provide beneficial health outcomes and they are related. Gender, age, genetics and health status are all determinants of physical fitness. However, the main controllable and modifiable determinant is physical activity habits and/or pattern [3], [4]. Cardiorespiratory fitness is commonly used to measure habitual physical activity. It is defined as the ability of the circulatory and respiratory system to supply oxygen to the working muscles during sustained exercise. Cardiorespiratory fitness, which is generally quantified as maximal oxygen uptake (VO₂ max) is a reliable and low-cost measure that not only measures habitual physical activity, but it is also a valuable indicator of health [5]. Muscular strength is also an essential part of physical fitness which is when a muscle or a muscle group, in one single contraction, generates the maximum force [6]. Muscular strength has been shown to provide health benefits, not only for adults but also for youths. Evidence suggests that strength training may aid in weight loss and weight control through which the resting metabolic rate increases as a result thereof. Strength training may also improve bone mineralization, which in turn can prevent injuries from sports. Although peak bone mass is mainly influenced by genetics, increased bone mineral density resulting from strength training is also necessary for young girls who are at increased risk of developing osteopenia and/or osteoporosis [7]. One’s flexibility, which is also included as a component in physical fitness, may be enhanced through stretching exercises. It is believed that flexibility or hyper flexibility may increase the risk of injury while performing sports [8], [9]. In other words, moderate flexibility is preferable.

Physical power, Mental harmony and Social capacity (FMS) student profile is a tool designed by the Swedish Institute Physical Mental Social AB for adolescents between the ages of 12 and 19 years old [10]. FMS student profile assesses teenagers’ physical, mental and social health. The purpose of developing an assessment method as such was to enable individuals to take part of their own information associated with the person’s lifestyle, health and well-being [10].

Hitherto, no evidence has been found on how Body mass index (BMI) affects physical fitness. A recent study with field test, for Latin-American adolescents, showed worse cardiorespiratory fitness in the obese group than their healthy counterparts, independent of anthropometric parameters [11]. Another study by Reid et al. [12] reported that tests of lower body power, body mass and fat percentage provide limited information concerning physical activity. Furthermore, a study by Xu et al. [13] showed a nonlinear relationship between weight status and physical fitness, and adolescents who were classified as underweight or obese had poorer physical fitness than their normal-weight peers.

Engaging in habitual physical activity, including cardio, strength and flexibility training, early on in adolescence is therefore important for achieving a good physical fitness, which may even possibly halt the increasing trends in overweight and obesity among adolescents worldwide.

Therefore, the aim of the present study was to investigate the correlation between BMI and three physical fitness components, including overall fitness, according to the Swedish FMS profile. Another aim was to investigate test-retest reliability of the FMS questionnaire and fitness tests.

Materials and methods

Results

Discussion

The main finding in the present cross-sectional study of a weak inverse association between BMI and fitness was found amongst all the females and males. The negative inverse association was more strongly correlated when normal weight and overweight/obese girls, and boys were separated. Lowest fitness scores were observed amongst boys and girls with the highest BMI (overweight/obese). The correlation between BMI and fitness was predominantly noticeable in VO₂ max and overall fitness, especially amongst the boys. These findings support the notion that normal weight adolescents are more likely to have higher physical fitness than overweight/obese individuals, which is also in agreement with previous studies [23], [24], [25], [26], [27], [28]. However, these studies [23], [24], [25], [26], [27], [28] only examined cardiorespiratory fitness (VO₂ max) and its association with BMI, and they did not take into account strength or flexibility, which are both important components of overall physical fitness. Braitwaite et al. reported that physical activity is not associated with lower BMI among children and adolescents. They stated that physical activity and BMI may be best undertaken in conjunction with other variables in the energy expenditure [29]. Nevertheless, Chen et al., Zar et al. and Joshi et al. did look at fitness tests measuring strength and flexibility [23], [24], [28]. These studies also found that normal weight individuals scored higher in, not only fitness test measuring cardiorespiratory fitness but also in strength and flexibility tests. However, overweight participants either performed as good as their normal weight counterparts or not far from it in the strength tests. Therefore, although in our study the participants with lower BMIs were more likely to achieve a higher overall fitness score compared to those with higher BMIs, it should still be noted that a proportion of overweight/obese subjects did score a three or four in overall fitness. Similarly, a few of the normal weight participants scored one or two in overall fitness. In other words, an individual categorised as overweight/obese according to BMI can still be physically fit due to the person’s muscle mass. Conversely, a person categorised as normal weight can have poor physical fitness as a result of low muscle mass. Thus, BMI as a tool is not always optimal. Nevertheless, it is not completely correct to draw such conclusions that they seem to be a representative group just by looking at height. If data on the students’ parents’ educational level, ethnicity or socioeconomic status were available, it could have given a better picture of whether or not they are a representative sample.

The test-retest study revealed that the FMS questionnaire, which measures physical, social and mental health in adolescents, is a reliable tool to use. Almost perfect test-retest reliability was found for strength, flexibility and balance. The reliability for the parameters concerning lifestyle showed moderate to perfect agreement. Questions regarding psychological health and somatic pain as well as school environment and relationship showed substantial to almost perfect test-retest reliability. Nevertheless, stress, perceived health, diet, physical activity and drugs had low ICC values between the two test trials. When measuring a person’s perceived health or stress, it is expected to obtain moderate to good test-retest reliability. This is due to the fact that one’s perceived health and stress are likely to change within short time periods [30], even within a week interval as in this case. Obtaining higher ICC values, which would be indicative of perfect agreement, would instead mean that the actual question being tested is not catching these changes. Measuring diet, physical activity and drugs is however usually not due to intra-individual variability and day-to-day changes. The obtained substantial to moderate agreement is most probably attributable to under-reporting regarding diet and drugs and over-reporting in physical activity levels [31].

Measuring the validity of a questionnaire is difficult and it has several dimensions. However, in this study, the fitness tests used to measure cardiorespiratory fitness, i.e. VO₂ max, have been validated elsewhere [15], [16], [17], [18], [19] against a criterion instrument such as a treadmill. All of the tests used in this study have shown to have a correlation factor ranging from 0.76 to 0.90, indicating to be valid tests to measure cardiorespiratory fitness.

Being overweight and/or obese can lead to other diseases such as, for example, the metabolic syndrome (hypertension, hyperglycaemia, dyslipidaemia) [32], [33], [34]. All of these illnesses are in turn risk factors for cardiovascular disease (CVD) such as heart disease. The developing atherosclerotic process of heart disease starts early on in childhood and adolescence, and continues into adulthood if one or many of the risk factors are present [33], [34], [35]. Similarly, it has been proposed that low physical fitness is associated with the clustering of these CVD risk factors in Swedish adolescent males later in life [34]. Therefore, being overweight and/or obese along with having low physical fitness increases the risk of developing CVD later in life [35].

Besides the physical consequences of being overweight and obese, there are psychosocial concerns as well. Adolescents who are overweight or obese often suffer from having low self-esteem, social isolation and having increased risk of developing depression. In addition, when dealing with such a group as adolescents, it is important to not only try to solve the overweight and/or obese problem by increased physical activity and healthy eating, but it is also necessary to take into account the actual problem that may be the underlying cause of why an individual has become overweight or obese. In many cases, traumas of some kind, bullying or not thriving in school are examples of some underlying bases, which must be addressed as well [36].

By participating in the present study, the students have been given motivational talks with either an instructed nurse or PE teacher. The aim of this was to arouse the student’s own thoughts of his/her own lifestyle behaviour and give advice on how to change it. The nurse/PE teacher has also followed some principles during the talk. These include supporting self-reliance, showing empathy, avoiding arguments and having a collaborative approach as well as showing to be an equal partner instead of patronising. The students have also been taught how to correctly perform strength and flexibility exercises by experienced PE teachers. Furthermore, participating in the FMS study does not really include any risks, except if the participants would injure themselves while carrying out any of the fitness tests; however, they had been informed about that prior to agreeing to participate.

Having a student profile, such as the one conducted by FMS, which assesses physical, social and mental health among adolescents is very important. This can help schools who decide to use the student profile to acquire an overview of the health among the students. Subsequently, the school can work on improving the health by implementing certain measures and interventions.

The strengths of this study were that the anthropometric measurements and the fitness tests were conducted by experienced nurses and PE teachers. All the equipment and the fitness tests were standardised. Additionally, the large sample size was also a strength in our study. Moreover, the fact that a test-retest study was included is also unique in the present study.

Although BMI is a useful tool to use in large populations, particularly among children and adolescents, solely using it alone to assert an association with physical fitness was a limitation. Instead, measurements of body fat and muscle mass (which were measured in our study, but only in a small number of individuals) ought to be used together with BMI. Also, physical activity pattern was measured in this study; however, it was only done in a very small proportion of the study population, which is a drawback. Physical fitness depends on genetics and other non-modifiable factors (age, gender, etc.). However, the foremost controllable factor is physical activity [3], [4]. Since data on PA habits were not complete in our study, its relative contribution cannot be determined. In addition, whether or not there is a correlation between the measured VO₂ max and the amount of PA reported in the questionnaire cannot be asserted either. The fact that a number of students were excluded since data on weight and height were not available is also another limitation that could have influenced the results. Lastly, the cross-sectional design of this study is also a limitation, given that it can only show an association and not causality. Therefore, the results ought to be considered with caution. However, it is rational to believe that adolescents with higher BMI values will also score lower in fitness tests.

Conclusions

Physical fitness and BMI were weakly negatively associated. Overweight and obese individuals scored lower in the fitness tests compared to their normal weight counterparts. The test-retest study revealed that the FMS questionnaire and fitness tests are reliable tools.

In order to combat the increasing prevalence of overweight and obesity among adolescents, the promotion of physical activity in school settings has to be improved. Walking to and from school, increasing physical education time, and food and nutritional education are some suggestions.