Watching a favorite television show may feel relaxing, but in a new study, the longer 30-somethings spent in front of the TV, the stiffer their arteries - a sign of likely heart disease in the future.

“The fact that your arteries aren’t elastic, it predisposes you to develop hypertension in later age and cardiovascular disease,” Isabel Ferreira, senior epidemiologist at Maastricht University in the Netherlands, said.

Previous studies have linked TV watching to increased weight, cholesterol, blood pressure and diabetes, she and her colleagues write in the British Journal of Sports Medicine.

It’s thought that people who spend more time in front of their TVs are less likely to get up and be physically active throughout the day, leading to a variety of problems associated with a sedentary lifestyle.

For the new study, researchers wanted to see whether early signs of damage caused by too little activity could be detected in younger adults.

They used data collected from 373 women and men, who filled out questionnaires about their TV viewing, exercise and other habits at age 32 and then again at age 36.

At age 36, each participant also had an ultrasound measurement of the stiffness of several major arteries in the body.

The researchers found those with the stiffest carotid artery, which is the main blood vessel in the head and neck, spent an average of about 20 more minutes per day watching TV, compared to people with the most elastic carotid artery.

Similar results were seen for stiffness of the femoral arteries in the legs.

Ferreira said the “critical cutoff” was about two hours per day of sitting. That’s in line with current recommendations from the American Academy of Pediatrics for maximum screen time for children.

What’s more, the negative effects of sitting did not appear to be offset by exercising.

“The funny thing is even if they do physical activity… that doesn’t correct the bad effects of sedentary time,” Ferreira said.

Joel Stager, a professor at the Indiana University-Bloomington School of Public Health, told Reuters Health that those with stiff arteries wouldn’t face immediate health problems. But it raises their risks later on.

“To be honest about this particular measure, it’s more of an association of future problems,” he said. “In other words, it’s predictive of cardiovascular disease down the road.”

Stager was not involved with the new study, but has researched arterial stiffening among college-age people.

“We are catching the early stages of this process,” Ferreira said.

Stager also added that the new study cannot prove watching TV is what caused people’s arteries to stiffen. It could be some other factor that goes along with TV watching, for instance, or young people with stiff arteries might be more likely to stay in and watch TV.

Ferreira told Reuters Health that more research into how watching TV may be tied to arterial stiffness is needed. But she said there is a take-home message for the average person.

“To put it simply, be active,” she said. “And on top of that don’t spend more than two hours sitting in front of your television, computer or laptop per day.”

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SOURCE: British Journal of Sports Medicine, online October 7, 2013