We have shown that the association of TV viewing with clustered metabolic-risk factors appears to be mediated by adiposity. In contrast, PA was significantly and inversely associated with most individual metabolic-risk factors and with clustered metabolic risk, independently of TV viewing, adiposity, and other confounders. Our results appeared not to be modified by gender or age group and suggest the existence of separate associations between both TV viewing and PA, and individual and clustered metabolic-risk factors.
There are several limitations that should be considered when interpreting the findings from this study. It is difficult to infer a causal relationship, and it is impossible to determine its direction from cross-sectional data. Furthermore, only randomization within a trial can deal with issues of unmeasured confounding. Although we controlled for several potential confounders such as gender, age group, study location, sexual maturity, smoking status, birth weight, and parental SES, we cannot be certain that other unmeasured confounders, such as total energy intake, genetic variation, and other socio-cultural factors could not explain our observations.
TV viewing was measured by self-reporting, and the respondents were asked about TV viewing before and after school on an average weekday. It is difficult to assess the validity of these self-reports. However, the observed associations between TV viewing and individual risk (i.e., adiposity) and clustered metabolic risk are unlikely to be due to measurement error. Firstly, weekday TV viewing is suggested to be a reasonable indicator of overall TV viewing in children [9
]. Secondly, measurement error could only explain the observed associations if there was a systematic bias, such as if those who were overweight over-reported their time spent viewing TV, and if those who were of normal weight underreported their time spent viewing TV. It is, however, plausible that overweight children underreport the amount of time spent viewing TV, similar to that which has been observed for food intake [23
]. If this is the case, it may then underestimate the true association between TV viewing and individual and clustered metabolic risk. Finally, our exposures are likely to be measured with different degrees of error, which will differently attenuate the true associations. Unfortunately, repeated measurements of our exposures are not available, which precludes the possibility of correcting our analyses for measurement error.
The strengths of our study include our large population-based sample, our validated method for measuring PA by accelerometry, the collection of fasting blood samples in a large group of children, the use of skin-fold measurements for assessing adiposity, and the use of a computer-based questionnaire for assessing self-reported variables. Finally, few if any previous studies have examined the joint association of objectively measured PA and TV viewing with regard to metabolic-risk factors in children, taking into account the potential confounding effect of obesity as well as other known confounding factors.
Other researchers have found TV viewing to be associated with obesity and some metabolic-risk factors [4
], and suggested that TV viewing maybe an indicator of sedentary behaviour. However, we did not observe any association between TV viewing and PA, and the association between TV viewing and adiposity was independent of PA. This suggests that TV viewing does not displace PA and that other factors, such as dietary behaviour and quality while viewing TV, may influence energy balance and thereby body weight. It has also been suggested that TV viewing has a lowering effect on the metabolic rate in children [26
], but the data are not conclusive [27
]. In post hoc analyses, we observed that the self-reported frequency of eating meals while viewing TV attenuated the association between TV viewing and adiposity. Furthermore, self-reported eating frequency while viewing TV was associated with adiposity independently of gender, age group, study location, and PA. Decreasing targeted sedentary behaviour, including TV viewing, significantly decreases energy intake in youth, whereas increasing sedentary behaviour did not affect energy intake [29
]. Furthermore, eating between meals is consistently associated with TV viewing [30
] and snacking, while watching TV is associated with increased total energy intake and energy intake from fat in particular [31
]. Taken together, this suggests that TV viewing is associated with increased energy intake, which may affect energy balance and subsequent weight gain in children. However, it is also possible that TV viewing is increased as a result of being more overweight.
PA explains a low amount of the variance in obesity in youth [16
], and PA is weakly associated with weight gain [33
]. Reducing TV viewing is likely to prevent weight gain either directly or indirectly. The American Academy of Pediatrics suggests that TV viewing should be limited to 1–2 h d−1
in children [34
], although data suggest that less than 1 h is even better [9
]. Our data suggest that TV viewing, but not PA, is associated with adiposity, whereas PA is associated with other metabolic-risk factors. Preventive strategies may therefore need to target these two behaviours separately.
We have previously observed a significant inverse association between objectively measured PA and clustered metabolic risk in a subgroup of our study participants [19
]. Our current findings extend these observations to also include older children and children from different socio-cultural and geographical locations. The observed associations between PA and metabolic risk were strong and independent of TV viewing, adiposity, and other confounding factors. It is biologically plausible that PA improves the metabolic-risk profile without influencing adiposity. Firstly, PA improves insulin action and glucose transport [35
]. Secondly, PA increases blood flow and oxygen supply through increased capillarization and vasodilatation by nitric oxide, which improves fat metabolism [36
]. Thirdly, PA may affect sympathetic tone and thus blood pressure may decrease through a more efficient recruitment of the motor units in the muscle [38
The individual risk factors assessed in this study are established risk factors for CVD in adult life. Therefore, identifying the associations of sedentary and PA behaviour with these risk factors in children may be essential for the primary and secondary prevention of all diseases that result from sedentary behaviour. In the present study, moving from one quartile of PA to the next (an increase of approximately 140 cpm d−1
) equates to improvements in metabolic risk of about 0.053 standard deviation (). Based on doubly-labelled water data [17
], this amount of activity equates to energy expenditure through PA of about 30 kJ kg−1
. For the average child in our study, this is approximately 1.5 MJ d−1
, or about 50–60 min d−1
of moderate-intensity activity. This amount of activity can be accumulated through various activities and does not necessarily include structured exercise. A change of 0.053 standard deviation corresponds, for example, to a change in blood pressure of less than 0.1 mm Hg, a change in insulin of about 0.04 mmol l−1
, or a change in triglycerides of about 0.003 mmol l−1
. Some could regard this as clinically insignificant. However, this dose-response relationship was observed in a population of healthy children and, as metabolic-risk factors track over time, they may result in substantial reductions of the occurrence of disease later in life [3
In conclusion, in relation to metabolic risk in children, TV viewing and PA should be considered as separate entities as they are differentially associated with individual and clustered metabolic-risk factors. Our data suggest that the association between TV viewing and clustered metabolic risk is mediated by adiposity, whereas PA is associated with individual and clustered risk independently of TV viewing and adiposity. These observations may provide separate opportunities for prevention against obesity and metabolic risk in children.