While that may sound a bit dramatic to phrase it so severely, the fact is, it’s true.
Quite true, actually, in the literal sense.
People who sit a lot die much, much younger than people who don’t. The numbers vary a bit, but according to one study, those who sit 6+ hours per day are likely to die 15 years before those who sit 3 or fewer hours per day.
Yeah. That wasn’t a misprint.
Another study found that people who sit for most of the day are 54% more likely to die of a heart attack than those who do not.
There are all sorts of reasons for this, but researchers haven’t been able to pinpoint the specific cause. Obviously, a big part of it is decreased caloric expenditure—standing for an hour burns 60 more calories than sitting for that length of time. But that’s just one aspect. Sitting too much causes gluteal amnesia (basically, makes the glutes less likely to fire during both exercise and day-to-day activity), which decreases the amount of calories you burn.
Now, you might think that this only applies to people who are sedentary overall; that you’re exempt from all of the consequences of sitters because you exercise and eat well. These things help, but they don’t make you immune.
One study found that even among people who were otherwise active, sitting for more than 4 hours per day had deleterious effects.
In terms of your actual fitness, sitting too much is going to mess up your workouts in one very simple way: it’s going to screw up your posture. When you sit, your hip flexors are in a shortened position; sitting too often creates perpetual shortness (or “tightness”), to the extent that it can pull your pelvis into posterior tilt. This will make you primed for injuries.
Similarly, sitting too much generally means you’re looking down at a computer, and typing at one—the result of this behavior is chronically shortened/tight pecs, creating a round, forward-rolled shoulder effect.
In addition to just looking terrible, this has implications for shoulder and back health.
All of which is to say, sitting too much is going to make you more likely to get injured from your workouts, and cause you to get less out of them even if you’re not injured in any acute way.
So, what to do? The easiest answer is to sit less and stand more.