This is completely untrue. Contrary to popular belief, sitting at a desk is extremely healthy in comparison to manual labour, which destroys your body over time

This is completely untrue. Contrary to popular belief, sitting at a desk is extremely healthy in comparison to manual labour, which destroys your body over time. Standing all day is also worse for your back than sitting all day. Even being a shop assistant is harder on your body, and particularly your back, than sitting at a desk. (Alternating, i.e. an office job with a sit/stand desk, is best.) The primary issue is complete lack of inactivity (a large component just being weight gain), which you can combat by exercising outside of the office.