Honestly, I don't think that bipolar folk are any more 'Sedentary' then the average person. Its the current lifestyle. Its not convienant to eat healthy or exercise for anyone, really. Internet brings everything to you so its easy to sit on a computer for countless hours.

I could believe that some medications correlates positively to weight gain but I don't think that means the person is inactive. Also lots of comorbid issues that can contribute to certain lifestyles.