There is a place with a yellow sign on Gurrachaga near the corner of Honduras - "Flores of something" I can't remember the name but its a dietetica and does organic/healthy/yummy food. They also do take away. Suerte!
Undoubtedly the advise of Napoleon is excellent. If I wanted to lose weight, I´d stay away from away restaurant. Forget rice, pasta and bread. Eat foods that contain fiber. I´ve found that the best produce is sold on the streets by (mostly Bolivian) woman. Join a gym. If your destination is too far go by bicycle