Do potatoes and rice lose significant amount of fiber when overcooked? I overcooked both brown rice and potatoes today, both boiling. The skin on the potatoes was intact even though I overcooked it but too soft. Does that mean the fiber is significantly reduced in both foods?  Does the fiber leech out into the water?
No, fiber is a very hardy beast chemically. There is no fiber lost at all, no matter how much you overcook them in a dish. You would have to throw them into a furnace and take out crisps of carbon no longer recognizable as food to change the fiber.