Couch potatoes: Where the risks lie

Several new studies finger television viewing as a potentially unhealthy pastime. I know, that hardly sounds surprising. For years, research has been linking hours in front of TV screens with an elevated risk of developing heart disease and diabetes, not to mention obesity. But what makes the recent spate of analyses different, researchers argue, is that they’re finally homing in on consistent estimates of the magnitude of risks — and hints at what underlies them.

For instance, the conventional wisdom has been that TV sickens by enticing people to become increasingly sedentary. And it certainly does that. But if just sitting still were the problem, knitting or reading should be equally toxic. And they don’t appear to be, says Frank Hu of the Harvard School of Public Health in Boston, who coauthored the most recent analysis of TV’s threat to health in the June 15 Journal of the American Medical Association. 

“There is something special about watching TV,” Hu contends, “because when people watch they tend to eat — and are bombarded with junk food commercials.” That doesn’t happen when you read, he points out. And at least in the United States, he notes, “the amount of time spent watching TV is just incredible. Could you read for 5 hours nonstop? Could you knit for 5 hours nonstop?” Yet he cites survey data indicating Americans now plant themselves in front of TV screens an average of 5 hours daily.

Even if other activities were as sedentary as TV viewing, they don’t appear to be as prevalent, according to nationally representative U.S. data reported in a 2008 study by Charles Matthews (then at Vanderbilt and now at the National Institutes of Health) and his colleagues. More than 6,000 Americans wore activity monitors for up to one week. These showed that most people daily spent at least 7.7 waking hours sedentary. The least active segments of society: older adolescents and the elderly, who spent some 60 percent of their daytime largely immobile.

Athletes aren’t immune
Last summer, Alpa Patel of the American Cancer Society, in Atlanta, and her colleagues evaluated how much of their non-working day 123,000 Americans spend sitting. Although most of this time was probably spent watching TV, Patel says, some of it also would have included time reading, riding in a car or bus — even, presumably, seated in the bath or at the dinner table (although increasingly, most Americans spend only short periods dining).

After 14 years of followup, a clear trend emerged. People who typically spent more than 6 hours seated each day were substantially more likely to die prematurely from chronic disease — conditions that included heart disease, diabetes, stroke and cancer — than were active individuals who spent their day on the move all but three hours or less.

Moreover, Patel notes, this analysis didn’t just focus on sedentary time. It also attempted to quantify the intensity of energy expenditures when people were moving. The researchers describe this facet in terms of metabolic — or MET — rates; they’re characterized as multiples of rate at which the body uses energy while sitting. Walking and dancing got MET scores of 3.5, jogging or lap swimming were a whopping 7, shopping was a more leisurely 2.5, and gardening or mowing the yard came in at 3. These MET scores were then multiplied by the number of hours each week people engaged in associated pastimes.

In the August 15, 2010, American Journal of Epidemiology, Patel’s group reported that people who spent more than 6 hours of their leisure time each day on their butts — and who racked up fewer than 24.5 MET-hours per week in mobile activities — were substantially more likely to die early than were individuals who averaged fewer than 3 hours sitting each day and who also racked up at least 52 MET-hours per week. How much higher the risk of premature death was varied dramatically by gender: The least active women experienced a 94 percent spike, their male counterparts just a 48 percent increase.

Patel and her colleagues say their evaluating risk in terms of both sitting time and physical activity is novel. And also important. Because using these data, they can now argue that even for people who are quite active during the week, there remain detrimental effects of being sedentary for hours each day. Going to the gym regularly helps, Patel says – but does not give athletic types license to veg out the rest of the time with impunity.

Emmanuel Stamatakis of University College London, in England, and his colleagues performed a related analysis of heart- and death risks (from all causes) in a study of more than 4,500 Scottish adults. Instead of focusing on time sitting, they correlated these health outcomes with average daily leisure time spent in front of a “screen” — be that a television, computer, smart phone or something else.

Here they divided daily screen time into: less than 2 hours per day, more than 2 but fewer than 4 hours, or at least 4 hours. People averaging the most screen time were 50 percent more likely to die prematurely during the roughly four years they were followed up than were people who watched screens the least each day, they reported January 18 in the Journal of the American College of Cardiology. Risk of developing a cardiovascular event (such as heart arrhythmia or heart attack) during that followup period more than doubled for people who were in the highest screen-viewing group (versus the lowest).

As had Patel’s group, this one adjusted its analyses to account for how active their participants typically were when they weren’t partaking of screen entertainment. And this diminished “only slightly” the apparent risk associated with viewing time, the reported.

Homing in on mechanisms
Such findings raise the obvious question: What’s going on?

“Our work suggests that inflammation may explain some of the risk,” Stamatakis says. C-reactive protein, or CRP, is a blood marker of systemic inflammation, which is associated with a host of chronic diseases, including diabetes, cancer and heart disease. And among the Scots that his group studied, there was a steady average increase in CRP as daily viewing time increased. Among people watching screens at least four hours a day, the CRP average was almost double that measured in people watching screens fewer than two hours per day.

In addition, people who engaged in screen entertainment the most also tended to be heavier and to have lower concentrations of high-density-lipoprotein cholesterol, the so-called good cholesterol.

Stamatakis points to animal data as indicating yet another possible risk factor. Being sedentary reduces the activity of lipoprotein lipase — an enzyme that breaks down fats (such as triglycerides) in the blood so that they are available to fuel the energy needs of muscle. In animals, he says, sitting reduces this enzyme’s activity up to 90 percent compared to when they are standing or moving. If the same occurs in humans, it could foster heart disease.

“Humans are simply not made to sit down for long periods,” Stamatakis contends. He recommends that people put less of a premium on labor-saving devices and processes. Go for a walk, stand instead of sit and look for screen entertainment that invites action. Society would be healthier, he argues, if it evolved back to where “body movement is the default state and sitting a necessity that should be avoided when possible.”