"Extensive periods of sitting contribute to bad cholesterol, weight gain and limit muscle contractions. A study led by epidemiologist Steve Blair demonstrated a strong link between sedentary activity and death from heart disease among adult men. According to researchers at the University of North Carolina at Wilmington, people gain 16 pounds on average, within 8 months of starting sedentary office work." (Source)
The good news is that we can do some things during the day to help reduce our sitting-related risks. From taking more frequent walking breaks to stretching regularly and improving posture - everything adds up.
Here's a great, simple stretching video from SparkPeople that you can do multiple times a day:
Another option if you're tied to a computer but have a laptop: take breaks and stand up while your laptop is on a counter top. Simply standing versus sitting means that you have larger muscles such as your back and leg muscles engaged and people find that they are more productive and alert while doing their work. Start slow though - transitioning from sitting to standing takes time and patience while you retrain your body how much work it needs to do during the day.