Some members of the deadly nightshade family, Solanaceae, produce chemicals called glycoalkaloids, which are toxins.
Normal amounts of the parts of these plants that are normally consumed are not ordinarily toxic to human beings, though the potato plant itself is very toxic, as is the tomato plant.
Consuming very large quantities of potato skins, 2.4 pounds for an adult or 1.4 pounds for a child, can cause severe illness.
In some cases the potato can become toxic because of the “greening effect,” in which disease, damage or exposure to light causes synthesis of harmful chemicals to occur. When this happens, the toxin levels increase about twelvefold.
For potato varieties with a naturally high level of glycoalkaloids, it would take 3 pounds of potatoes, about 6.4 baking potatoes, to make an adult ill and about 1.5 pounds for a child.
But if the same potatoes were green, the toxic amount would shrink to one-twelfth that amount.
About twenty glycoaldehydes have been found in potatoes, though only two, alpha-solanine and alpha-chaconine, are a significant problem.
They cause severe digestive distress and are neurotoxins, interfering with nerve transmissions. For some potato varieties, less than an ounce of the potato shoot emerging from the eye would be toxic for adults.
Tomatoes contain a glycoalkaloid called alpha-tomatine, and eggplant and red and green peppers contain solanine.
However, it has been estimated from animal studies that an adult would have to eat 4.5 pounds of eggplant or pepper or 150 small green tomatoes to reach a potentially lethal dose, and fully ripe tomatoes have virtually no toxin.
In contrast, less than two ounces of tomato leaves is likely to be lethal for an adult.