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WEDNESDAY, March 3 (HealthDay News) -- People who watch more television in their 20s and 30s are more apt to develop heart disease risk factors by the time they reach their mid-40s than people who spent less time in front of the screen, a new study finds.
And while that's worrisome enough for many, the worse news is that you can't exercise the risk away.
The findings are to be presented Wednesday at the American Heart Association's Joint Conference: Nutrition, Physical Activity and Metabolism, in San Francisco.
"When we took into account physical activity, the negative effects of TV viewing persisted," confirmed Emmanuel Stamatakis, lead author of the paper and a senior research associate at University College London. "A likely explanation is that the harmful effects of prolonged sitting cannot be simply compensated by doing some physical activity. In other words, the mechanisms of action of the harms of sitting are not the opposite of the benefits of exercise."
And there may be another negative twist to staying glued to a screen.
"We also would expect that, to some extent specifically, TV viewing harms through increased caloric intake [of unhealthy foods]. There is some evidence to support such an explanation but our study could not take into account food intake," Stamatakis said.
Dr. Eugene Storozynsky, assistant professor of medicine at the University of Rochester Medical Center, believes the association might come from behaviors other than sitting.
"I suspect it's not so much the TV watching but other behaviors that go along with TV watching -- specifically, were the study subjects eating lots of carbohydrate-rich foods or drinking carbohydrate-rich drinks at the time they're TV watching?" he said.
"It would be interesting to see if there is a similar association with other sedentary lifestyles with the caveat that these individuals are not able to eat or drink all the time," Storozynsky added. "Has anyone done a study on people who spend a lot of time in libraries and just read and then exercise? After all, every elliptical trainer and treadmill in the gym has a TV above it."
The study was also unusual in that it looked at people in post-adolescence and early adulthood, he added, and it also focused on a specific type of sedentary behavior, said Dr. Robert Scott III, assistant professor of internal medicine at Texas A&M Health Science Center College of Medicine and senior staff cardiologist at Scott & White in Temple, Texas.
The researchers looked at the TV watching and exercise habits of more than 5,600 men and women born in Britain in 1958. Initial measurements were taken in 1981, when the participants were 23 years of age, with additional follow-up done when they reached age 44.
The researchers unearthed three factors that, together, explained 57 percent of the risk difference between those who watched more TV and those who watched less.
One had to do with metabolism -- triglycerides, HDL ("good") cholesterol, body mass index, waist circumference and blood pressure -- which explained 28 percent of the variance.
The second was an inflammatory component including levels of (inflammation-linked) C-reactive protein, which explained 16 percent of variance. The third component involved total cholesterol and LDL ("bad") cholesterol, which explained 13 percent of the variance.
The associations remained evident even after adjusting for physical activity, the team found.
"It was still better if they exercised versus not exercising but the more you watched TV, even if you exercised, increased these risk factors," Scott said.
But it still comes back to the same bottom line people have been told before.
"Sitting in front of the TV for long periods of time should be avoided. In the U.K. and U.S., adults watch TV for around three to four hours a day on average," Stamatakis said. "This is simply too much, given that most of us have sedentary jobs, drive or commute by motorized transport and have lots of other opportunities (that we usually do not miss) to sit," he explained.
"Humans are not made to be sitting for the 10 to 11 hours that, on average, we do (and these 10 to 11 hours are in addition to sleeping)," he added. "Our evolution teaches us that our bodies are made to hunt and run away from our predators, not sit in front of a screen. We have completely ignored this side of our nature; therefore, it is not surprising that our bodies accumulate too much fat, get heart disease, cancer and type 2 diabetes. All these are direct consequences of too much sitting and not enough physical activity -- both, not either/or."
There's more on heart disease at the U.S. Centers for Disease Control and Prevention.