I remember doing the iodine test for starch on potatoes and sweet potatoes. Potatoes turned purple but the sweet potatoes didn't. Obviously Sweet potatoe is a better carb. From memory the most nutrious part of a plant is usually just under the skin. I prefer to scub them and bake them. Microwave if the time is short with the skin on. It is a bit of an acquired taste though. It took me 15 year in New Guinea to get used to it.