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.
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.
UPDATE: Fellow PLoGster John Rennie offers his thoughts at The Gleaming Retort.
This post was originally published on Obesitypanacea.com in February, 2010.
Zimmerman, 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