Though nightshades don't cause arthritis, in some people they seem to exacerbate it, according to About.com. This is because nightshades, such as tomatoes and eggplants, contain alkaloids.
These plants have four kinds of alkaloids, says About.com. They are steroid, tropane, pyrrolizidine and indole alkaloids. Steroids, especially solanine, are the most abundant type of alkaloids found in nightshades.
One theory posits that these alkalines take calcium out of the bone and put them in soft tissue, states About.com. Another theory says they cause inflammation and stimulate the immune system. To test whether a person's arthritis is worsened by nightshade, doctors recommend going on a two- or three-week fast and seeing if the arthritis symptoms are eased.