The potato is a valuable source of easily digested starch, but the main other nutrients are in the outer layer, immediately below the skin. Peeling a potato destroys its nutritional value, as does baking a potato if the skin is then burnt or discarded. Potatoes should be scrubbed clean and then boiled until just soft or baked without burning the skin. The skin should be eaten as well as the starchy middle. The green skin and flesh that develops when a potato is exposed to the light should be discarded as it is poisonous.
Sweet potatoes include more sugar and twice as much vitamin C as ordinary potatoes but do not keep well. They are usually cooked by boiling and mashing, but are also excellent when baked.
Jerusalem artichokes taste sweet because of a sugar called inulin. They can be cooked in the same way as potato.
Yams are the tubers of tropical climbing plants, an important source of starch in parts of Africa, but they contribute little more to the diet.