Sedentary Behaviour in JAMA

In February of this year Dr Ulf Ekelund and colleagues published a study in JAMA examining the combined associations of MVPA and sedentary behaviour with cardiometabolic risk in children and youth:

CONTEXT: Sparse data exist on the combined associations between physical activity and sedentary time with cardiometabolic risk factors in healthy children.

OBJECTIVE:To examine the independent and combined associations between objectively measured time in moderate- to vigorous-intensity physical activity (MVPA) and sedentary time with cardiometabolic risk factors.

DESIGN, SETTING, AND PARTICIPANTS: Pooled data from 14 studies between 1998 and 2009 comprising 20 871 children (aged 4-18 years) from the International Children’s Accelerometry Database. Time spent in MVPA and sedentary time were measured using accelerometry after reanalyzing raw data. The independent associations between time in MVPA and sedentary time, with outcomes, were examined using meta-analysis. Participants were stratified by tertiles of MVPA and sedentary time.

MAIN OUTCOME MEASURES: Waist circumference, systolic blood pressure, fasting triglycerides, high-density lipoprotein cholesterol, and insulin.

RESULTS:Times (mean [SD] min/d) accumulated by children in MVPA and being sedentary were 30 (21) and 354 (96), respectively. Time in MVPA was significantly associated with all cardiometabolic outcomes independent of sex, age, monitor wear time, time spent sedentary, and waist circumference (when not the outcome). Sedentary time was not associated with any outcome independent of time in MVPA. In the combined analyses, higher levels of MVPA were associated with better cardiometabolic risk factors across tertiles of sedentary time. The differences in outcomes between higher and lower MVPA were greater with lower sedentary time. Mean differences in waist circumference between the bottom and top tertiles of MVPA were 5.6 cm (95% CI, 4.8-6.4 cm) for high sedentary time and 3.6 cm (95% CI, 2.8-4.3 cm) for low sedentary time. Mean differences in systolic blood pressure for high and low sedentary time were 0.7 mm Hg (95% CI, -0.07 to 1.6) and 2.5 mm Hg (95% CI, 1.7-3.3), and for high-density lipoprotein cholesterol, differences were -2.6 mg/dL (95% CI, -1.4 to -3.9) and -4.5 mg/dL (95% CI, -3.3 to -5.6), respectively. Geometric mean differences for insulin and triglycerides showed similar variation. Those in the top tertile of MVPA accumulated more than 35 minutes per day in this intensity level compared with fewer than 18 minutes per day for those in the bottom tertile. In prospective analyses (N = 6413 at 2.1 years’ follow-up), MVPA and sedentary time were not associated with waist circumference at follow-up, but a higher waist circumference at baseline was associated with higher amounts of sedentary time at follow-up.

CONCLUSION:Higher MVPA time by children and adolescents was associated with better cardiometabolic risk factors regardless of the amount of sedentary time.

The May 16 issue of JAMA contains further discussion of this important study.  Drs JP Chaput, Ian Janssen, and John Spence write:

Although the authors studied 20 871 children and adolescents and measured sedentary time in an objective way, we believe it is premature to conclude that sedentary time is not associated with adverse cardiometabolic health outcomesin children.

First, different types of sedentary behavior may have different metabolic effects and future studies should therefore investigate specific sedentary behaviors and not only the total accumulated time spent sedentary.

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Second, though the cut point used to define sedentary behavior (100 counts/min on an accelerometer) provides a useful estimate of sitting time, standing still time may also be included as sedentary time.

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Third, it is surprising that children with lower levels of sedentary time combined with lower levels of MVPA had the worst cardiometabolic outcomes. This group of children moves around a lot but at a low intensity.

Finally, food intake was not measured in this study and should be included in future research because of the strong link between poor dietary habits and cardiometabolic health and also because screen-time sedentary behavior has been shown to increase caloric intake in the absence of hunger

Drs Ekelund and colleagues replied to these concerns:

Dr Chaput and colleagues argue that it is premature to conclude that sedentary time is not associated with
cardiometabolic health outcomes in children. Our study concluded that “higher levels of time in MVPA appear to be associated with better cardiometabolic risk factors regardless of the amount of time spent sedentary in youth.” Our conclusion was strongly supported by the data, but we do not dismiss a potential role of specific sedentary behaviors in relation to cardiometabolic risk.

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We only examined the associations with total sedentary time. Specific sedentary behaviors, such as TV viewing, may have different associations with cardiometabolic health outcomes. We stated that: “decreasing TV time in youth may still be an important public health goal as TV viewing may be associated with other unhealthy behaviors such as snacking and soft drink consumption. Further TV viewing is also associated with exposure to advertisements that often promote unhealthy dietary habits.”

In their conclusion, Drs Ekelund and colleagues suggest that both groups have reached a common conclusion:

Finally, Chaput et al state that “the take-home message should be to increase children’s participation in MVPA and reduce their screen-related sedentary time”; this is exactly what we proposed in our article.

The full article and the letters are available through the JAMA website.