Moderate levels of intensity and duration of physical activity have been shown to have a stress-reducing effect, but an additional increase of either the duration or the intensity will not have further beneficial effects. For children and adolescents, it is assumed that physical activity is associated with good mental health, especially in relation to self-esteem, self-efficacy, greater perceived physical competence, greater perceived health and well-being, but there is almost no evidence that the amount of physical activity is related to better social and moral development or to psychological variables such as body image, academic functioning, social skills, anxiety, hostility and aggression.

An important issue that must be considered with regard to the relationship between physical activity and mental health is that it is difficult to distinguish between cause and effect. Physical activity can have a positive effect on self-esteem or perceived physical competence, but on the other hand, children with higher self-esteem and/or perceived physical competence will be more likely to participate in sports activities.

For young children, not much research is performed regarding the relationship between physical activity and mental health. However, the few studies performed also show for this age group that self-esteem is increased by an increase in physical activity.

No real dose–response relationship or threshold value could be determined. Surprisingly, in contrast to elderly individuals in whom relatively high levels of physical activity can postpone the natural cognitive decline, in children and adolescents, physical activity does not seem to be related to cognitive (or academic) performance.

Diabetes mellitus develops when there is inadequate production of insulin by the pancreas, or inadequate utilization of insulin by the cells. Clinically, two major forms are distinguished: type 1 diabetes, also known as juvenile onset diabetes, and type 2 diabetes, also known as adult onset diabetes.

Although about 90% of all diabetes patients suffer from type 2 diabetes, type 1 diabetes is more common among children and adolescents and presents an important health problem in youth. However, the prevalence of type-2 diabetes among youngsters has increased enormously over the last 10 years. This increase has been mainly linked to the ‘obesity epidemic’ among youngsters.

In adults physical activity has many desirable effects for people with diabetes, particularly those with type 2 diabetes. Glycaemic control is improved, possibly due to the insulin like effect of muscle contractions on translocating glucose from the plasma into the cell. Exercise leads to an increase in muscle mass and therefore to lower blood glucose levels, assisting in better glycaemic and blood sugar control. The latter can reduce insulin resistance. Some researchers believe that physical activity can have an effect on glycaemic control in children with both type-1 and type-2 diabetes, but in other studies this has not been confirmed.

The outcome variables most commonly used in studies relating physical activity to insulin metabolism disorders are glucose and insulin concentrations of blood serum. When reviewing the literature there are not many studies investigating the relationship between physical activity and glucose and insulin concentrations in children and adolescents.

Regarding blood glucose, the few studies carried out have not shown any influence of physical activity in children and adolescents. More consistent results are found in the more limited number of studies on obese children and adolescents. In these studies, a positive (i.e. healthy) effect of physical activity was found on parameters related to insulin metabolism.

However, the mechanisms responsible for the decrease in blood pressure with physical activity have yet to be determined. A reduced cardiac output is mentioned as a reason for the fact that activity lowers blood pressure, although this cardiac output reducing effect of physical activity is not found in all studies. If there is no influence on cardiac output, then the blood pressure decreasing effect may be caused by a reduction in peripheral vascular resistance, which may be due to a reduction of sympathetic nervous system activity.

In addition, the relation between physical activity and blood pressure can be caused by the anxiety-reducing effect of physical activity. It is questionable, however, if this mechanism is also present in children and adolescents.

It appears that essential hypertension may begin early in life and that detection and treatment of possible blood pressure abnormalities at young ages is important. There is, however, no direct evidence that elevated blood pressure in children is related to CVD later in life. There is also not much evidence that physical activity has beneficial effects on blood pressure in children and adolescents. There are many cross-sectional studies investigating this relationship, but again the best evidence comes from longitudinal studies and well-controlled intervention studies.

In the CATCH study, a 30-month multidisciplinary intervention in order to improve physical activity among more than 4000 children and adolescents did not have any effect on blood pressure (Webber et al 1996). It is argued that, as in adults, the possible lowering effect of physical activity on blood pressure only holds for children and adolescents with hypertension and not for young people with normal blood pressure values.

This implies that this effect is difficult to observe in population studies in children with low incidence of hypertension. It should also be kept in mind that the effect of reducing blood pressure in hypertensive children and adolescents is probably only true for high intensity aerobic type physical activity and not for normal (or habitual) physical activity.

It is assumed that LDL may act directly or indirectly to cause endothelial damage, with subsequent proliferation of arterial smooth muscle cells resulting in an accumulation of lipids and a progression to atherosclerotic plaque formation. HDL, on the other hand, is assumed to be protective against CVD; HDL seems to be responsible for carrying cholesterol from peripheral tissue, including the arterial walls, back to the liver where it is metabolized and excreted.

Besides HDL and LDL, very low-density lipoprotein cholesterol (VLDL) and plasma triglycerides (TG) also need to be considered. Although the atherogenic effects of VLDL and TG are not firmly established, both are assumed to be risk factors for CVD. It is further assumed that during exercise, fatty acids are freed from their storage sites to be burned for energy production.

Several studies suggest that human growth hormone may be responsible for this increased fatty acid mobilization. Growth hormone levels increase sharply with exercise and remain elevated for up to several hours in the recovery period. Other research has suggested that, with exercise, the adipose tissue is more sensitive to either the sympathetic nervous system or to rises in circulating catecholamines. Either situation would increase lipid mobilization.

The fact that children who were very active during adolescence show the largest decrease in physical activity between adolescence and adulthood. This unfavourable change in physical activity was found to be related to unfavourable HDL values at adult age. This finding, although preliminary, suggests that it is important to maintain high levels of physical activity throughout life.

However in the beginning of the 1990s, Cale & Almond (1992) reviewed 15 studies conducted on British children and reported that children seldom participate in activity at a level that would have a cardiovascular training effect or a health benefit.

On the other hand, in the same period, Sallis examined nine studies and concluded that the average child is sufficiently active to meet the adult recommendations for conditioning activities, with the exception of the average girl in mid to late adolescence (Sallis 1993). It has further been noted that young children are highly and spontaneously active and that children are generally fitter and more active than adults and most of them are active enough to receive important health benefits from their activity.

In the United Kingdom, around 70% of boys and 61% of girls, aged between 2 and 15 years, meet the recommended level of 1 hour of physical activity each day (including sport and organized exercise, active play, walking, gardening or housework). For girls, however, participation in physical activity declines after about 11 years of age, so that by age 15 years, only 50% undertake an hour of physical activity each day (Department of Health 2003). On the other hand, from longitudinal studies there is also evidence that for both boys and girls during especially the adolescent period, there is a (huge) decrease in physical activity levels, which continues into the adult period.

Another way of looking at the prevalence of physical inactivity is to look at the prevalence of sedentary behaviours such as television viewing, computer use or video game playing. Survey data from the USA, for instance, show that up to a quarter of American children aged between 8 and 16 years watch more than 4 hours of television each day (American Academy of Pediatrics 2003). However, the amount of time spent watching television and playing video games is not inversely correlated to the amount of time spent in physical activity.

There is some evidence that the level of physical activity of children and adolescents is lower than that of similarly aged children a few years ago. This is mainly based on the finding that the caloric intake today is lower than the caloric intake in previous generations, and yet the previous generations were less fat. This can only be caused by a decrease in physical activity. This secular trend observed for physical activity levels of children and adolescents suggests the importance of this age period for interventions aimed at an improvement (or maintenance) of physical activity levels.