Will 'nightshade' vegetables increase joint pain or lupus flares?
The nightshade vegetables include white potatoes, tomatoes, peppers (sweet and hot), and eggplant. There are others, but they are not foods that are typically consumed in the U.S. While there is anecdotal evidence that some of these foods can be related to inflammation, there is no solid scientific evidence to support this concept.
If you have a concern about a particular kind of food in your diet, keep track of when you eat that food, and to look for a pattern between consumption and a worseningof lupus symptoms. You would want to observe a link between a particular food and a flare on multiple occasions -- not just a few times.
If there does seem to be a connection, then by all means, avoid one/all of the nightshade vegetables without creating any nutrient deficiencies. Peppers, for example, are high in vitamin C, but so are plenty of other foods (i.e. citrus), so limiting peppers in your diet isn't going to cause any problems.