How you live when you're young has a lot to do with how healthy you'll be when you get older. A new study paints a rather grim picture of what happens to the minds of young adults who spend too much time in front of the TV.

Twenty-five years later, their brains had practically turned into tapioca. Well, actually the effects weren't quite that bad. The TV viewers had slower mental processing speed and were worse at executive functions such as planning ahead and solving problems. Their verbal memories, however, were as good as those who didn't watch much TV.

The study strongly suggests that almost anything you can do is better for your mind than sitting in front of the TV.

Over 25 years, on at least three separate visits, 3,000 people, aged 18-30 at the start of the study, filled out questionnaires describing their TV viewing and how physically active they were. High television viewers were people who reported watching more than three hours a day on over two-thirds of their visits.

After 25 years, participants' mental ability was measured by three different tests: the Digit Symbol Substitution Test (DSST) for processing speed, the Stroop test for executive function, and the Rey Auditory Verbal Learning Test for word memory.

Executive function has been described as the brain behind the brain. It's an umbrella term for skills that involve mental control and self-regulation. These include planning, problem solving and overall mental flexibility.

People who were high television viewers during the 25 years (10.9%) were more likely to test poorly for processing speed and executive function, while people with low physical activity (16.3%) during the 25 years were likely to test poorly for processing speed. And the people with both low physical activity and high television viewing over the 25 years (3.3%) were the likeliest to test poorly for processing speed and executive function. Their odds of poor performance were almost twice as high as people with both low TV viewing and high physical activity.

The study also implicates lack of physical activity in mental decline. Most everyone has heard by now that sitting is bad for the body. This study also suggests that it's bad for the mind.

People looking for loopholes might point out that the study can't say whether it was actually TV viewing or sitting (presumably the people sat while they watched TV) or even some unknown factor that was responsible for people's later mental decline. Or perhaps they were already mentally vulnerable. Nevertheless, the study strongly suggests that almost anything you can do is better for your mind than sitting in front of the TV. And if too much sitting and tube time don't get you now, they will in a few years.

A few simple alternatives to tube time: take the dog for a walk, play with your kids or sign up for a softball team or yoga class. Most people can easily cut their TV viewing time by an hour a week. More is better.

If too much TV is bad for the mind, could cutting down on TV viewing right now actually sharpen it? It's an interesting idea that people might want to put to the test. Even if it doesn't net you a Nobel Prize 25 years down the road, it might make your mind more nimble. Some people would call it a no-brainer.

The study is published in JAMA Psychiatry.