ACTIVITY AND SEDENTARY TIME WITH BODY COMPOSITION AND

CARDIORESPIRATORY FITNESS IN MID-CHILDHOOD ... 113  7.1 Abstract ... 113  7.2 Key points ... 114  7.3 Introduction ... 115  7.4 Subjects and methods ... 116  7.4.1 Sample ... 116  7.4.2 Assessment of body size and composition ... 116 

7.4.3 Assessment of cardiorespiratory fitness ... 117  7.4.4 Assessment of physical activity and sedentary time ... 117  7.4.5 Other assessments ... 118  7.4.6 Statistics ... 119  7.5 Results ... 120  7.5.1 Sample description ... 120  7.5.2 Cumulative intensity analyses ... 125  7.5.3 Categorical analyses and isotemporal substitution models ... 128  7.6 Discussion ... 130  7.7 Conclusion ... 133  7.8 Notes ... 133  7.8.1 Acknowledgments ... 133  7.8.2 Authors contributions ... 134  7.8.3 Compliance with ethical standards ... 134  7.8.4 Ethical approval ... 134  7.8.5 Informed consent ... 134  8  GENERAL DISCUSSION ... 135  8.1 Summary ... 135  8.1.1 Study design and population ... 135  8.1.2 Cross-sectional findings ... 138  8.1.3 Longitudinal findings ... 139  8.2 Strengths and limitations of the study ... 139  8.3 Perspectives ... 140  9  CONCLUSIONS ... 143  REFERENCES ... 144 

ABBREVIATIONS

BMI Body mass index

CRF Cardiorespiratory fitness

DXA Dual-energy X-ray absorptiometry

EMT Electronic media time

FFM Fat-free mass

FFMI Fat-free mass index

FMI Fat mass index

HDL High-density lipoprotein

HOMA Homeostatic Model Assessment

IDF International Diabetes Federation

LDL Low-density lipoprotein

LPA Light physical activity MET Metabolic equivalent of task

MetS Metabolic syndrome

MPA Moderate physical activity

MVPA Moderate-to-vigorous physical activity

NAFLD Non-alcoholic fatty liver disease

NMF Neuromuscular fitness

PA Physical activity

PAEE Physical activity energy expenditure

SB Sedentary behavior

ST Sedentary time

TFMI Trunk fat mass index

TG Triglycerides

VPA Vigorous physical activity

WHO World Health Organization

1 INTRODUCTION

Childhood obesity has reached epidemic proportions on a global scale and has emerged as one of the leading public health problems since it increases the risk of chronic cardiometabolic diseases (1–3). The prevalence of overweight and obesity has steadily increased in recent decades among children of all ages and young people also in Finland. Over 15% of Finnish pre-school age girls and 7.5% of boys of the same age are overweight or obese (4,5). In the United States and many other high-income countries, the growth rate of childhood obesity has become decelerated during the past few years and the prevalence of obesity has

plateaued, but nonetheless the number of children with overweight and obesity has remained alarmingly high (6–9).

Children and adolescents with overweight and obesity are prone to develop many chronic diseases or disorders later in life such as metabolic syndrome (MetS), non-alcoholic fatty liver disease, type 2 diabetes (T2D), and cardiovascular diseases (10–13). Independent factors that increase the risk of these metabolic and cardiovascular diseases are called cardiometabolic risk factors. These risk factors include the accumulation of excessive fat tissue, especially in the central body area, insulin resistance, impaired glucose tolerance, dyslipidemia, and elevated blood pressure (Figure 1). The simultaneous occurrence of cardiometabolic risk factors in an individual can be described with the term MetS (14). It is noteworthy that childhood obesity also causes adverse metabolic changes and clustering of independent cardiometabolic risk factors that meet the characteristics of MetS.

There have been numerous diagnostic criteria proposed for children's MetS (15–

19) but there was no consensus on the preferred criteria until 2007 (20). There are also dissenting opinions about whether the diagnosis of MetS is in general

necessary among children because of their drastic changes in body size and composition with age and development. Therefore, the International Diabetes Federation (IDF) recommends that the MetS should not be diagnosed in children younger than 10 years (20).

There is a high probability that overweight and cardiometabolic risk factors in childhood and adolescence will remain in adulthood, and therefore effective interventions addressing these risk factors should be implemented as early as possible (21–23). The most important reasons for an individual becoming overweight, obesity and other cardiometabolic risk factors are an unhealthy diet

and physical inactivity, in other words, too much unhealthy food, too little physical activity (PA), and excessive time spent doing sedentary activities (Figure 1).

The role of PA in preventing and treating obesity and other cardiometabolic risk factors has been increasingly clarified in the past few years (23). PA has been identified as one of the major protective factors for cardiometabolic diseases, and physical activity interventions have been found to be effective in reducing the cardiometabolic risk (24). In addition, PA can improve the overall health and life quality of people of all ages and can be used in the primary and secondary prevention not only of cardiometabolic diseases but also of many other chronic diseases and conditions (25).

According to the Finnish PA recommendations, children and adolescents aged 7-18 years should have at least 60 minutes of moderate-to-vigorous PA (MVPA) daily (26). The international recommendations are essentially parallel and, likewise the World Health Organization (WHO) recommends 60 minutes of MVPA daily for children and adolescents (27), but there are also some country-specific differences in PA recommendations. Worryingly, it has been estimated that only about every third child meets the current recommendations for PA. In addition, there is a lack of evidence on the required volume, intensity, and type of PA needed to achieve the optimal beneficial effect on cardiometabolic risk factors among children.

The objectives of this doctoral thesis were to investigate both cross-sectional and prospective associations of PA, sedentary time (ST), cardiorespiratory fitness (CRF), and neuromuscular fitness (NMF) with adiposity and other cardiometabolic risk factors among Finnish primary-school children. This doctoral thesis provides novel information on these associations in a population sample of children 6–8 years of age who participated in the baseline examinations of the Physical Activity and Nutrition in Children (PANIC) study; they were followed-up for two years and underwent comprehensive and objective assessments of PA, ST, CRF, NMF, cardiometabolic risk factors, and possible confounding factors.

Figure 1. Causes and consequences of cardiometabolic risk factors and their clustering.