1. In an American cohort of adolescents, a combination of high screen time and low physical activity was associated with increased rates of participants being overweight and/or obese.
2. High step count may not offset risk of high screen time and low screen time may not offset risk of low step count in terms of risk for higher BMI percentiles
Evidence Rating Level: 2 (Good)
Study Rundown: Current American physical activity guidelines recommend 60 minutes of moderate to vigorous intensity physical activity daily for adolescents. Over the last 5 years it was found that less than 25% of adolescents were meeting these guidelines, with a further drop to less than 10% during the COVID-19 pandemic. It is generally known that more physical activity and less screen time are favourable factors for the prevention of obesity, however gaps exist in the current guidelines. There is lack of sufficient evidence for recommendations of sedentary behaviours, as well as a gap in knowledge of the understanding of dose response associations between physical activities and health outcomes using objective measures. Additionally, there are limited studies examining the interactive effects of sedentary behaviour and physical activity on adolescent health outcomes. A previous Canadian study looked at the optimal combination of physical activity and screen time needed to minimize the risk of overweight and obesity, this study concluded that meeting both physical activity and screentime recommendations was correlated with lower odds of overweight or obesity but was limited in its use of self-report measures for physical activity. The goal of this study was to use step counts as a measure of physical activity and examine screen time to determine the interactions between physical activity and screen time with BMI percentile and overweight and obesity. The authors found that a combination of high screen time and low physical activity was associated with overweight and obesity and suggested that a high step count may not offset the risk for adolescents with high screen time and low screen time may not offset risk for those with low step count. These findings elucidate further insights into the risk factors of adolescent obesity, and may be used to inform future guidelines.
Click here to read the study in JAMA Network Open
Relevant Reading: Combinations of physical activity and screen time recommendations and their association with overweight/obesity in adolescents
In-Depth [cross sectional study]: This study included a total of 5797 adolescents from the ages of 10-14 from 21 racially and ethnically diverse sites across the US. Total step counts were categorized into low (1000 to 6000 steps per day), medium (>6000 to 12000 steps per day) and high (>12000 steps per day). Step counts were measured using a Fitbit. Screen time was self-reported and categorized into low (0 to 4 hours per day), medium (>4 to 8 hours per day) and high (>8 hours per day). The main outcome was BMI which was calculated by using weight in kilograms divide by height in meters squared. Results were converted into sex and age specific percentiles. Those with a BMI of 85th percentile or higher for sex and age were classified as being overweight or obese. The mean screen time was 6.5 hours per day and mean overall step count was 9246.6 steps per day. Models examining screen time only found that medium and high screen time categories were associated with higher BMI percentile (medium: β = 4.21; 95% CI, 2.18-6.23; high: β = 6.97; 95% CI, 4.73-9.28) and higher risk of obesity (medium: RR, 1.25; 95% CI, 1.14-1.39; high: RR, 1.32; 95% CI, 1.19-1.47) compared to low screen time. Models examining step count only found the low step count category was associated with higher BMI percentile (β = 5.98; 95% CI, 2.79-9.16) and low and medium step count categories were associated with higher risk of overweight or obesity compared to high step count. Models including both screen time and step count, had similar findings. Effect modification was seen between screen time and step count, for example for those with low screen use medium step count as associated with a 1.55 higher BMI percentile and low step count was associated with 7.48 higher BMI percentile. For those with high screen use, step count categories did not significantly change the association with higher BMI. Similarly, for those with low step count, screen time categories did not significantly change the association with higher BMI. Limitations to this study include the fact that Fitbit device data was only collected for 21 days, which may not be representative of levels of physical activity in the long term. Additionally, screen time measures were self-reported, and therefore subjected to recall and reporting bias. Overall, this study found that a combination of less than 4 hours of screen time per day and more than 12 000 steps per day is beneficial to minimize the risk of high BMI. It also found that lower screen time in less physically active individuals and higher step counts in those with more screen time did not offset the risk for high BMI. Futures studies that occur over a longer time period and those that examine other factors for disease such as sleep may help inform future physical activity and screen time guidelines for adolescents.
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