Solanine poisoning

[T]he potato is the most common cause of solanine poisoning in humans. But how do you know when solanine is present in a potato? The tuber is turning green.

Though the green color that forms on the skin of a potato is actually chlorophyll, which isn’t toxic at all (it’s the plant’s response to light exposure), the presence of chlorophyll indicates concentrations of solanine. The nerve toxin is produced in the green part of the potato (the leaves, the stem, and any green spots on the skin).

The reason it exists? It’s a part of the plant’s defense against insects, disease and other predators. If you eat enough of the green stuff, it can cause vomiting, diarrhea, headaches, paralysis of the central nervous system... but in some rare cases the poisoning can cause coma—even death...

Fatal cases of solanine poisoning are very rare these days. Most commercial varieties of potatoes are screened for solanine, but any potato will build up the toxin to dangerous levels if exposed to light or stored improperly. Often, the highest concentrations of solanine are in the peel, just below the surface and in the sprouted “eyes”—things that are typically removed in cooking preparation...

More at the Smithsonian's Food & Think blog.  Photo credit unknown.