How does TV watching increase health risk?

Earlier this year I came across a very interesting study on Dr Yoni Freedhoff’s blog Weighty Matters.  Yoni described a new study published in the American Journal of Public Health which suggests that the amount of commercial television (e.g. television with advertisements) that children watch before the age of 6 is associated with increased body weight 5 years down the road, even after adjustment for other important variables including physical activity, socio-economic status and mother’s BMI.  In contrast, watching non-commercial television (DVD’s or TV programs without commercials) showed no association with body weight.  The data was self-reported, but nonetheless these are pretty interesting findings, and suggest that television commercials are likely an important mechanism linking screen time with obesity risk.

Now of course this makes a lot of sense – the more commercial television a child watches, the more junk food ads that they will be exposed to.  And the whole point of junk food advertisements are to get kids to eat more junk food.  So I agree completely with Yoni’s conclusion that we need to keep food advertising away from children (In fact, TV watching in adolescence is associated with poor diet in early adulthood, so maybe we should ban food adverts altogether…).  This makes sense for a whole lot of reasons.  But, if we prevent children from being exposed to food advertising, will the relationship between television watching and health risk completely disappear? Probably not.

Sedentary time (which includes TV time) is linked with all manner of health problems – from abdominal obesity to reduced insulin sensitivity, and even mortality.  And as we have discussed in the past, these relationships are usually found to be independent of physical activity.  In other words, no matter how much time you spend engaging in physical activity, the more time you spend sitting, the greater your health risk.

One mechanism that is likely to link TV time (and overall sedentary time) with increased health risk is the relationship between TV viewing and junkfood ads that we discussed above.  Research also suggests that eating while watching TV may result in greater food intake than eating when not watching TV, which is likely another important mechanism linking TV time with health risk.  But these mechanisms focus on the relationship between screen time and increased body weight, and do little to explain the strong association of sedentary time with numerous metabolic risk factors independent of body weight.  For example, Sardinha and colleagues report that the more time that children spend being sedentary, the greater their risk of insulin resistance, even after control for both total and abdominal fat mass. So, how can sedentary time influence health risk independent of adiposity?

It turns out that engaging in sedentary behaviours like TV watching results in rapid and dramatic changes in skeletal muscle.  For example, in rat models, it has been shown that just 1 day of complete rest results in dramatic reductions in muscle triglyceride uptake, as well as reductions in HDL cholesterol (the good cholesterol).  And in healthy human subjects, just 5 days of bed rest has been shown to result in increased plasma triglycerides and LDL cholesterol, as well as increased insulin resistance – all very bad things.  And these weren’t small changes – triglyceride levels increased by 35%, and insulin resistance by 50%!

These negative changes are likely related to reductions in the activity of lipoprotein lipase, an enzyme which allows muscle to uptake fat, thereby reducing the amount of fat circulating in the blood (it also strongly influences cholesterol levels – the details can be found here).  Animal research has shown that lipoprotein lipase activity is reduced dramatically after just six hours of sedentary behaviour – not unlike a work day for many individuals. Sedentary behaviour may also reduce glucose transporter protein content in the muscle, making it more difficult for glucose to be taken into the muscle, and resulting in higher blood sugar levels.  What is most interesting though, is that these mechanisms have little or nothing to do with the accumulation of body fat.  This means that both lean and obese individuals (and even those with otherwise active lifestyles), are at increased health risk when they spend excessive amounts of time sitting down.

What’s the take-home message?

Although it’s impact on food intake is very important, TV watching (like all forms of sedentary behaviour) is also likely to result in rapid changes in skeletal muscle function, causing dramatic increases in metabolic risk, even for lean or otherwise physically active individuals.  The good news?  Animal research suggests that simply walking at a leisurely pace may be enough to rapidly return these metabolic risk factors to normal levels.

So, let’s work to prevent children from bring exposed to food advertisements, but let’s also focus on reducing all forms of sedentary behaviour.

Travis Saunders

UPDATE: Fellow PLoGster John Rennie offers his thoughts at The Gleaming Retort.

This post was originally published on Obesitypanacea.com in February, 2010.

ResearchBlogging.orgZimmerman, F., & Bell, J. (2009). Associations of Television Content Type and Obesity in Children American Journal of Public Health, 100 (2), 334-340 DOI: 10.2105/AJPH.2008.155119

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11 Responses to How does TV watching increase health risk?

  1. Kate says:

    I’m curious to know exactly how much physical activity, and how often, is required to offset these effects. For example, I spend most of my day in front of the computer, or in a chair or in bed reading. However, I fidget a lot (tapping toes, stretching, etc) while I’m sitting, and I typically get up at least a couple of times an hour for bathroom/tea/water/play-with-the-dog breaks. Those breaks are short (typically 2 minutes or less), but do get me walking around, including up and down stairs. How much more often would I have to get up and walk around to offset the negative muscle effects of sitting the rest of the time? How much of a “leisurely walk” is required to prevent or stop the return those risk factors to normal?

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  2. Travis Saunders, MSc, CEP says:

    That is the $64,000 question! I’m going to be looking at some of these issues during my PhD, but it’s going to be a while before we know anything concrete. Suffice it to say that in general, it’s good to try to limit any extended periods of extended sitting (e.g. work standing up from time to time if possible), and take breaks whenever possible, as you’re already doing.

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  4. Andreas Johansson says:

    How does one vary sedentary time independent of physical activity? Presumably, every extra minute one’s not being sedentary is one extra minute of activity?

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    • Travis says:

      Great question. Sedentary time is time when you are sitting or lying down. So simply standing in place means that you are not being sedentary, but you are not being physically active either. That means that you can be highly active, but also highly sedentary (e.g. you run 90 minutes/day, but spend the rest of your time sitting), or you can have low levels of physical activity, while accumulating very little sedentary time.

      Travis

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      • Andreas Johansson says:

        So how active is active? Does it count to, say, pace about in my appartment (something I tend to do unreasonably much), or is that also non-sedentary non-active behaviour?

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  5. Jonathan says:

    Hmm… given the dangers of sitting, it seems that you will probably die!! while writing your PhD. Unless, of course, you do most of that writing while jogging on a treadmill.

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    • Travis says:

      I do worry about the amount of time I spend sitting at my computer – there are many days where I probably fall into the “highly active/highly sedentary” category that I mentioned above. We have a standing workstation in our lab, and are hoping to build a treadmill station soon as well. The irony of spending all day being sedentary, writing about the health impact of sedentary behaviour, is not lost on me :)

      Travis

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