White (not sweet) potatoes, tomatoes, sweet and hot peppers, eggplant, tomatillos, tamarios, pepinos, pimentos, paprika, cayenne, and Tabasco sauce are classified as nightshade foods. The ongoing question is, “Do nightshade vegetables increase joint problems such as arthritis?” The controversy stems from alkaloids, a particular compound found in nightshade foods. Whether alkaloids can contribute to joint damage is not clear from current levels of research. Some researchers have speculated that nightshade alkaloids can contribute to excessive loss of calcium from bone and excessive depositing of calcium in soft tissue. For this reason, these researchers have recommended elimination of nightshade foods from the meal plans of all individuals with osteoarthritis, rheumatoid arthritis, or other joint problems like gout.
Because the amount of alkaloids is very low in nightshade foods when compared with other nightshade plants, health problems from nightshade foods may only occur in individuals who are especially sensitive to these alkaloid substances. Non-sensitive individuals may be able to eat these foods, especially in cooked form, without problem. If you are not sure about your sensitivity, but you have an existing joint problem like osteoarthritis, rheumatoid arthritis, or gout, temporary 2-3 week elimination of nightshade foods from your meal plan may be a worthwhile step to determine if these foods could be contributing to your joint problems
"Nightshade" is actually the common name for more than 2,800 species of plants, herbs, shrubs, and trees, many with very different properties and constituents. Nightshades are actually more famous as drugs than as foods for example, mandrake, tobacco, and belladonna.