Screen time and sedentary behaviour in the early years (Part 2): Six physical health risks thought to be related to sedentary and screen time

Dr Breanne Kunstler (BBiomedSci, BHealthSci, MPhysio, PhD).

Last week I wrote an introduction to sedentary and screen time. This is the second post in a series of posts on children’s screen time and will involve examining some of the physical health risks associated with screen time.

Parents often worry that their children are spending too long watching screens. Screen time involves time spent with various devices, such as laptops, desktop computers, game consoles, smartphones, tablets, TVs and anything else I have forgotten to mention and that you are probably holding right now. Screens dominate our lives, making life both easier and more entertaining. However, there might be risks associated with using them too much, making them dangerous for the health of our kids.

Image 1: Are TVs really dangerous? (credit: The Imaginative Conservative)

I did a quick search and found hundreds of studies on screen time and child health, dozens of which were review-level (some of the best evidence you can get). I looked at some of the highest quality review-level evidence to inform this post. Basically, if I found high-quality review-level evidence to support relationships between screen time and health then I was pretty confident that the relationships exist.

Let’s have a look at what this research has to say about the physical health risks associated with screen time for kids. I will use a three part system ('confident', 'hmm maybe' and 'unlikely') to indicate how confident I am in the findings of the studies I came across.

High sedentary time is related to poor fitness: Confident

High level studies and opinion pieces have found that high sedentary and screen time is related to lower fitness in kids (Carson et al, 2016; LeBlanc et al, 2017; Saunders et al, 2017; van Ekris et al, 2016). Another review reported that that low sedentary time is an important component of the ‘ideal behaviour’ triad (low sedentary time, sufficient sleep, high activity levels). This means that the healthiest kids are considered to be active, not sedentary and sleep well. Kids who don't meet all three and are instead highly sedentary have greater risk of being unfit (Kuzik et al, 2017).

Due to the high level of evidence on this topic, I am confident that high sedentary time is related to poor fitness. It kinda makes sense really, you don’t often see fit couch potatoes.

Image 2: I stand corrected, but at least I'm standing (credit: Pinterest)

High screen time is related to poor cardiometabolic health: Confident

Cardiometabolic health relates to the absence of diseases of the heart (e.g. ischaemia), blood vessels (e.g. atherosclerotic plaques in arteries, high blood pressure) and metabolic impairments that can impact heart health (e.g. diabetes, poor cholesterol).

High screen time and TV viewing is considered related to poor cardiometabolic health in kids aged 5 to 17 years old. Specifically, excessive screen time is associated with greater risk of high blood pressure, insulin resistance (a marker for diabetes) and low HDL cholesterol (the good cholesterol, or the one we want more of).

One study found that there is no relationship between sitting down for long periods and poor cardiometabolic health; the relationship only appears once screens are added to the mix. So the risk for poor cardiometabolic health in kids who are sedentary relies a lot on the addition of screens, like TV.

The high level of evidence supporting these associations gives me confidence that high levels of screen time can increase kids' risk of poor cardiometabolic health.

High screen time is associated with obesity: Hmm maybe

Oooooooh controversial! Someone is going to call me nuts for this one, but I'm not yet convinced that there's a link.

Image 3: Can we fit 'screen time and childhood obesity' in here somewhere? (credit: getnclass.com)

If you think that there’s a relationship between screen time and having a poor diet, then you probably expect there to be a relationship between screen time and childhood obesity. However, the research is very mixed in this area.

Several small studies and some review level studies have reported that children with high sedentary time are more likely to be obese, have a high fat percentage or poor body composition (Dumuid et al, 2016; LeBlanc et al, 2017; Oliver, 2017; van Ekris et al, 2016).

More specifically, several other studies and reviews have drawn a link between children with high screen time, like TV viewing, and being more likely to be obese, have a high fat percentage or poor body composition (Bai et al, 2016; Carson et al, 2016; Domingues-Montanari, 2017; Lissak, 2018; Oliver, 2017; Saunders et al, 2017; Tripathy et al 2019; Wolf et al, 2018).

Other high-quality, review-level studies have concluded that there’s no convincing relationship between sedentary time or screen time and the likelihood that a child or adult will be obese. A high-quality review found only small to very small relationships between screen time and adiposity (fatness) and this relationship was lost when the researchers used different tools to collect their data, suggesting the research methods could explain the findings.

Some authors have also found that participating in high levels of physical activity (Bai et al, 2016; Dumuid et al, 2016) and sleep (Kuzik et al, 2017) can minimise any relationship between screen time and obesity. This suggests that active and well rested kids experience less deleterious effects from being sedentary compared to inactive and sleepy kids.

Adding screen time (e.g. TV) to being sedentary (e.g. lounging around) appears to increase the chance a child will be obese, suggesting that it’s the addition of screen time that really increases the chance of obesity, and not just the sitting. However, another high-quality review found that there is no increase in obesity risk as the time spent watching TV or on the computer increases.

Some researchers have even suggested that high screen time actually causes obesity in children, as opposed to simply being related to it. However, two high-quality and robust studies (Biddle et al, 2017; Ross et al, 2015) state that there is currently no evidence to suggest that being sedentary causes obesity in children.

I’m going to continue to believe that sedentary time does not cause obesity. Also, I don’t think we can confidently say that there’s a clear relationship between screen time and obesity. However, I do believe there is some kind of relationship between the two, where screen time might make childhood obesity more likely when other behaviours are taken into account. For example, I think any increase in obesity risk might be due to kids participating in other behaviours while being sedentary, like sleeping poorly and eating mindlessly (especially junk food) in front of the TV while being exposed to junk food marketing.

High screen time is related to having a poor diet: Confident

As I mentioned, I’m not convinced that only participating in sedentary time or screen time increases the chance that kids will become obese. However, I do think that the things kids do while on screens, like eating junk food, could increase their chance of obesity. Let’s see if there's any evidence to support this idea.

Image 4: A mix between these two kids could mark a dangerous susceptibility to childhood obesity (credit: me.me)

High-quality research has found a relationship between screen time and diet choices. Children under 12 years old with high screen time are more likely to consume fewer fruits and vegetables, and eat more unhealthy foods, compared to their peers. Furthermore, children aged 1 to 18 years with high TV viewing time are more likely to consume pizza, fried foods, sweets and snacks and are less likely to eat fruits and vegetables compared to children with low TV viewing time. The evidence for these relationships comes from review-level evidence, which gives me confidence that these relationships actually exist and that screen time is indeed related to having a poor diet.

The typical child that fits the above description would be one who comes home from school and sits in front of the TV with a big bag of chips (Image 5). Let’s be real here: which one of us hasn’t come home, plonked ourselves on the couch and watched our favourite TV show while enjoying some food or drinks we like? I guess the issue here is that children might form a lifelong habit of doing this, especially if they have nothing else to do after school or on the weekend.

Image 5: Does this child have a high risk of becoming obese? Probably (credit: alamy.com)

Children from poorer areas who have parents with lower incomes (i.e. low socioeconomic background) are more likely to eat while they watch TV and be overweight or obese. Children from culturally and linguistically diverse (CALD) backgrounds have been reported to have poorer quality diets compared to non-CALD children. These groups of children might be the unfortunate ones who cannot afford organised or structured after-school activities. This might increase their chance of mindless snacking while watching TV and, therefore, their obesity risk. However, I think more research is needed before I can draw this conclusion.

High screen time is related to poor growth and development: Hmm maybe

As I mentioned above, getting sufficient sleep, not participating in too much sedentary time and being physically active is considered the ‘ideal’ behaviour triad for kids to achieve good health. Kids who have this triad have a larger chance of developing well physically. Conversely, those excessively exposed to screens in the early years have a greater chance of decreased growth. Some researchers have also suggested that exposure to screens can impair child eye development, increasing myopia (short-sightedness) risk. However, this is unlikely.

I didn't find a lot of high-quality or review-level evidence to support these findings, so I'm not yet convinced that there's a relationship between sedentary or screen time and poor growth and development. I need to do some more searching.

High screen time is related to poor bone health: Hmm maybe

There's conflicting findings for the relationship between screen time and bone health. A high-quality review found that there’s unlikely to be a convincing relationship between sedentary time and poor bone health in children and adolescents. In contrast, a lower quality review found that excessive screen time is associated with reduced bone density in children and adolescents.

I need to see more high-quality evidence before I'm convinced that screen time is related to bone health.

In summary

It is clear that there’s a relationship between sedentary or screen time and physical health. I am most confident about the relationships between screen time and poor diet, screen time and cardiometabolic health, and sedentary time and poor fitness. I'm not so confident about there being a relationship between screen time and childhood obesity, poor growth and development, and bone health.

In my next post I will explore the relationships between screen time, psychological health and academic achievement. 

Happy reading!