**1. Introduction**

Noncommunicable diseases, cardiovascular diseases, cancer, respiratory diseases, obesity, and diabetes are the main causes of death worldwide [1]. These deaths are caused, in large part, by poor diet, physical inactivity, and the consumption of alcohol and tobacco [2,3], thus establishing lifestyle as a good predictor of health and morbidity and mortality [4,5].

Childhood and adolescence are important periods of life, since many physiological and psychological transformations take place at these ages. Similarly, lifestyle and healthy or unhealthy behaviours are established during these years, which may influence adult behaviour and health status [6,7]. In children and young people, the prevalence of overweight and obesity has increased in recent years [8], causing premature deaths and increasing the risk of cardiovascular and metabolic diseases [9]. Childhood and adolescent obesity are a transcendental challenge for public health, both in its magnitude and in its consequences [10]. Lifestyle intervention is the most common treatment strategy in children and adolescents with obesity. Although there are studies with significant effects of lifestyle treatment in children and adolescents with obesity [11], additional research is required to draw conclusions about this type of population [12,13].

Among the habits that lead to a healthy lifestyle are physical activity (PA) and eating healthily [14]. Defined as 'any bodily movement produced by skeletal muscles that results in energy expenditure' [15], PA is a vital part of a healthy lifestyle and has been extensively documented and associated with health benefits in children and adolescents. Some of the benefits include reductions in blood cholesterol, hypertension, metabolic syndrome, obesity, and associated health problems such as diabetes mellitus type 2, cardiovascular diseases, or bone health problems in this population [16–22]. Moreover, physical fitness, mainly cardiorespiratory fitness, muscular fitness, and motor ability, have been shown to be powerful markers of health in young people [23]. Several PA tracking studies have been analysing activity patterns in childhood and adolescence and the risk of maintaining sedentary behaviours [24]. A low level of PA is associated with metabolic risk factors in young people that can also persist until adulthood [25].

Food intake in adolescence is a significant predictor of intake in adulthood [26]. In the context of overall dietary patterns, the MD has been accepted as one of the healthiest dietary patterns in the world [27], showing significant protection concerning mortality and morbidity when there is high adherence to it [28–31]. The MD has shown health benefits by reducing cardiovascular diseases, type 2 diabetes, certain types of cancer, and some neurodegenerative diseases [27,28]. Studies focusing on the influence of the MD in children and adolescents have increased in recent years [32]. In general, adherence to the MD has been shown to be associated with physical benefits and high levels of health-related quality of life, reducing the different factors associated with obesity, among others [33,34].

There is growing evidence that health behaviours are grouped. For example, the combination of regular PA and healthy eating habits helps to maintain and improve health and physical and mental well-being [35]. During youth, healthy eating combined with regular PA increases the likelihood of a healthy pattern of consistent physical maturation [36]. In addition, there are independent and combined associations between physical fitness, physical activity, and adherence to the MD with quality of life related to health in children, adolescents, and adults [33,37–40], with significant improvements in joint interventions [41–43].

However, despite all the benefits mentioned above, the current data show unhealthy patterns of eating and PA during the transition from childhood to adolescence [44], substantially contributing to the global burden of morbidity, mortality, and disability [45], and increasing the prevalence of overweight and obesity at those ages [46]. As mentioned before, obesity and hypertension, among others, have been largely attributed to unhealthy diets and a decrease in PA [47]. Thus, PA and nutrition are shown as fundamental pillars in the prevention and control of obesity [48,49].

The aim of the present study is to analyse the independent associations between health-related physical fitness components, body composition, and adherence to the MD of adolescents from Reykjavik (Iceland).
