This paper examined the relationship between time spent in sedentary behaviour and the metabolic syndrome using meta-analysis. Results showed that greater time spent sedentary increased the odds of metabolic syndrome by 73%, thus encouraging people to limit their time spent sitting could reduce the risk of metabolic syndrome. This finding is based on 10 cross-sectional studies of which five are of moderate or high quality. The association was not influenced by sex of participants, the sedentary measure or metabolic syndrome definition employed or by study quality. Furthermore, the relationship between sedentary behaviour and the metabolic syndrome may be independent of physical activity, as demonstrated with the sensitivity analysis. This is important because it suggests that sedentary time could be an independent determinant of metabolic dysfunction distinct to that of physical inactivity. This finding is consistent with those reported for other health outcomes, such as all-cause mortality 
. Moreover, sedentary behaviour, whether measured objectively or subjectively, has been shown to be weakly associated with the amount of time spent in moderate-to-vigorous physical activity 
, confirming one is not simply the inverse of the other. For example, age-adjusted correlation coefficients between TV viewing time and physical activity were as low as −0.11 for women and −0.06 for men in an Australian study 
. Further work is required on the independent and inter-dependent effects of sedentary and physically active behaviours.
The findings of this meta-analysis are important because metabolic syndrome is a large and growing public health problem 
. Furthermore, individuals with the metabolic syndrome have been found to have an increased risk of diabetes 
, all cause and cardiovascular disease mortality, an increased incidence of CVD, CHD and stroke compared with individuals who do not have the metabolic syndrome 
Investigation of potential mechanisms underpinning the association between sedentary behaviour and metabolic health, although still in its infancy, could explain the association between sedentary time and metabolic syndrome reported here. For example, significant reductions in muscle lipoprotein lipase (LPL) activity, a key enzyme regulating lipid metabolism, have been shown to occur during sedentary activity 
. Several studies have prevented weight-bearing activity in the hindlimbs of rats and found a substantial reduction in LPL inactivity in skeletal muscles after relatively short periods of immobilisation of the legs 
; indeed immobilisation has been shown to reduce LPL activity to 10% of its normal function in slow-twitch muscle fibres 
. These low levels of LPL activity were associated with a large decrease in plasma triglyceride uptake locally in the skeletal muscle, a decrease in HDL cholesterol concentration (approximately 20%) and elevated postprandial lipids 
. Of note, exercise training was not associated with any increase in LPL activity above control conditions in fast twitch muscle fibres, supporting the notion that reduced sedentary behaviour may have health benefits that are independent to those associated with moderate- to vigorous-intensity physical activity 
. Although this direct mechanism has not yet been adequately investigated in humans, bed rest studies have confirmed increased sedentary behaviour is associated with a range of deleterious metabolic effects, including deceased lipolysis and marked deteriorations in whole body insulin sensitivity 
, Therefore, although limited in scope, experimental investigation supports the hypothesis that sedentary behaviour may be an independent risk factor for the metabolic syndrome. Sedentary behaviour may also be a risk factor for metabolic syndrome simply on the basis of low energy expenditure resulting in overweight or obesity 
. Moreover, higher levels of sedentary behaviour are associated with poorer diet 
This meta-analysis has several strengths including a broad search on multiple databases, the use of time spent in sedentary behaviour rather than sedentary behaviour as a categorical variable on a physical activity spectrum i.e, defining sedentary behaviour as a lack of physical activity, two independent authors reviewed abstracts and extracted data, and the analysis demonstrated no statistical heterogeneity or publication bias. Nevertheless, the results should be interpreted with some limitations in mind. Most studies (n
8) used self reported television viewing as the surrogate marker of sitting time and this is a limitation because television viewing may not be a good marker of overall sedentary behaviour, particularly in men 
. However, if anything, we would expect the use of this maker of sedentary time to weaken the effect because self reporting generally underestimates the amount of time spent sedentary which, in view of the findings, makes a true association between sedentary time and the metabolic syndrome more likely 
. Future research should aim to measure sedentary behaviour objectively using, for example, accelerometers. A second limitation is that not all studies adjusted for physical activity (n
8/10) and even though the majority of studies did, physical activity was measured and controlled for in a variety ways (Table S5
). For example, some studies entered physical activity in a single step and some in combination with other potential confounders. Finally, all studies in this meta-analysis were cross-sectional, therefore a causal relationship cannot be inferred between sedentary time and metabolic syndrome. Longitudinal and intervention studies are needed to determine the nature of any cause and effect relationship.
In summary, current results, although based on cross-sectional findings, emphasize that it might be important to recommend a reduction in sedentary behaviours, such as TV viewing and time on the computer, for the prevention of metabolic syndrome. However, longitudinal and intervention studies are needed to clarify the nature of any causal relationship between sedentary behaviour and metabolic syndrome.